Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: caldera on March 13, 2011, 01:08:15 PM
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1. Were the 4 blade props "better" than three blades?
2. Why did The P-51 and P-47 have 4 blade props, while P-38, P-39, P-40 and the blue planes have three blades?
3. Was there a shortage of 4 blade props or were they just considered to be not worth upgrading to?
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OK - this is a complex one. Basically, propeller design varies by aircraft because you are trying to balance a bunch of conflicting design limits and factors. For example - you get more thrust from longer blades (larger swept area), but you have have to take into account ground clearance, and a limit on the prop tip speed which cannot exceed the speed of sound. While you get more thrust from more blades, the air vortex created by each blade can interfere with the next blade, so that sets an upper limit as to how many blades you can have (and yes, there have been 5 and 6-bladed props, and counter-rotating ones as well, but we are talking WW2 era aircraft here.
Now, also realize that the weight and drag of the prop (which is a factor of how many blades, blade length, width, and pitch) has to be balanced against the power of your engine. Aces High notwithstanding, most of the time the engine would be running at a cruise power setting to save fuel - engines have a particular power and rpm setting at which they are going to be most efficient, so what you are looking to do is get as much thrust from your prop as you can at that power and rpm. Balance that against the altitude you are likely to be operating (higher up means thinner air, which means you want bigger, wider props so you can get more thrust up there), and the performance you need while in combat at military / emergency power and it all becomes quite an interesting engineering problem.
Give a big <S>! to the aeronautical engineers of the day folks. They figured this all out with paper, pencil and a slide rule...
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another question: what determines where the propeller is placed?
what i mean is, i know that putting the propeller infront of the plane "pulls" the plane through the air, while putting it in back "pushes".
so what determines wether the engine gets put in front or back?
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Eagle, I agree with all except the wider up high statement. Props loose efficiency when you make them wider (I.E. worse aspect ratio) but if you can not turn the HP into thrust due to lack of area you make them wider, this is not normally a problem at high alt & high speeds but is an issue at best climb speeds.
Making blades wider (same engine) normally would lower top speed slightly but gain climb rate.
HiTech
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another question: what determines where the propeller is placed?
what i mean is, i know that putting the propeller infront of the plane "pulls" the plane through the air, while putting it in back "pushes".
so what determines wether the engine gets put in front or back?
There are a lot of design factors you can think about to determine whether or not you want the engine in the front or back. First, there is a center of gravity issue - you are trying to be relatively balanced (front to back weight) on a point in your wing where that main spar will be imparting lift to your airframe. That balance has to include the engine, fuel, ordnance in both a fully loaded and nearly empty configuration (and this is why some rides get squirrelly if you don't empty the fuel tanks in the right order).
Next (on my list at least) is that you have to COOL your engines, and the KISS principle is always a good one to keep in mind when designing aircraft. If you are using radial engines then it is a lot easier to keep them cool with a lot of air being rammed in from the front of the aircraft in flight. If you are using inline engines, then you have radiators and coolant circulating around, which gives you some other options - some of the notable exceptions to the 'engine in front' design were the P-39, and one my favorites the Do-335. You can see the engineering difficulties and trade-offs in both designs vs a 'normal' fighter - in a Do-335, having a prop in the tail and in the front of the plane made them do some tricks with the landing gear and a underneath vertical stab to keep the pilot from chewing up his aft prop, and they had to put a big air scoop underneath to cool the aft engine. In a P-39, they made room for a monster cannon up front, but the center of gravity is ' a bit off ' and this made the plane unpopular with a lot of pilots. Also on the 'problems with rear mounted engines' list is that a prop in the tail makes bailing out a big problem. In the Do-335 they invented an ejection seat to prevent the pilot from being chopped into burger, and had a system of explosive bolts to take off the rear prop.
You see some postwar aircraft with rear-facing engines (like the B-36), but I think the engineering problems vs little if any performance gains is what kept the engines (mostly) in the front during the war. With more development time, we might have seen the Do-335 go operational (and thus my wish-list postings to HTC could be realized).
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Eagle, I agree with all except the wider up high statement. Props loose efficiency when you make them wider (I.E. worse aspect ratio) but if you can not turn the HP into thrust due to lack of area you make them wider, this is not normally a problem at high alt & high speeds but is an issue at best climb speeds.
Making blades wider (same engine) normally would lower top speed slightly but gain climb rate.
HiTech
I see what you mean. I was thinking of the 109K-4 and its wider than normal 109 prop. I had always thought that they increased area to make up for less air up high, but your post suggests to me that they widened the blades to give it a better climb rate with all that extra HP.
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Eagle, I agree with all except the wider up high statement. Props loose efficiency when you make them wider (I.E. worse aspect ratio) but if you can not turn the HP into thrust due to lack of area you make them wider, this is not normally a problem at high alt & high speeds but is an issue at best climb speeds.
Making blades wider (same engine) normally would lower top speed slightly but gain climb rate.
HiTech
Also note, that you can't have the blades too 'long' as the tips might approach super sonic speeds and become very inefficient. As a result designers are forced too
1) have more blades
2) make them wider to push more air
or some combo of the two.
All of the ways to 'cope' has issues too, as Hitech pointed out, wide blades are less efficient.
I see what you mean. I was thinking of the 109K-4 and its wider than normal 109 prop. I had always thought that they increased area to make up for less air up high, but your post suggests to me that they widened the blades to give it a better climb rate with all that extra HP.
As the HP increased on the 109 series airplanes, they had to find a way to transfer that HP into thrust. They couldn't easily make the blades longer as that would force a greater redesign of the aircraft as it would require more clearance, so they increased the width of the propeller (I don't know why that was chosen over adding a fourth blade... anyone?). Also note that different 109ks had different props, and almost at the end of the war, there was even an 'experimental' swept prop that improved top speed significantly.
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I remember some of the experiments with swept blade props - NASA did some testing on that as well back in the 80s but it never seemed to go anywhere. I also wondered why the Germans stuck with 3-bladed props on the 109K4 as well as on the 190D9 and TA152s.
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I remember some of the experiments with swept blade props - NASA did some testing on that as well back in the 80s but it never seemed to go anywhere. I also wondered why the Germans stuck with 3-bladed props on the 109K4 as well as on the 190D9 and TA152s.
They wanted a 4 bladed prop for the 152. To save on materials they went with the 3 IIRC. There is a pic of a 4 bladed 152 out there.
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This topic makes me wonder if the profile of the planes we have take into account "Variable pitch"? Some planes could pitch the angle of the props for better performance. I dont know if this is even accounted or even if it is able to be accounted for. Was just mainly curious to see if the guys who have been around for a long time may know? If it is and can be used I have no idea how and would love to gain the knowledge.
BigKev
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Very interesting topic.
BTW
Why the Contra-rotating propellers are much more noisy? IIRC the post war pilots were complain about it.
http://en.wikipedia.org/wiki/Contra-rotating_propellers
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So the answer to question #2 is? Anyone? Anyone? Bueler? Bueler?
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This topic makes me wonder if the profile of the planes we have take into account "Variable pitch"? Some planes could pitch the angle of the props for better performance. I dont know if this is even accounted or even if it is able to be accounted for. Was just mainly curious to see if the guys who have been around for a long time may know? If it is and can be used I have no idea how and would love to gain the knowledge.
So far as I know, all of our planes have constant speed propellers. Your throttle sets manifold pressure, your RPM control (whatever you use!) sets the propeller pitch at that particular moment, and the propeller automatically adjusts its pitch to maintain that RPM as you do things like climb and dive.
- oldman
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IRL, the 109 has manual and automatic pitch control for the prop.
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IRL, the 109 has manual and automatic pitch control for the prop.
That is true, in AH if you look at the cockpit, there is a thing that looks kinda like a clock, thats the pitch control. You can't actually change the pitch, but the dial is there.
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2. Why did The P-51 and P-47 have 4 blade props, while P-38, P-39, P-40 and the blue planes have three blades?
Remember that the P-51A uses an Allison engine, same as the -38,-39, and -40. All have 3 bladed props. I'd say the reason had to do with available horsepower.
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So far as I know, all of our planes have constant speed propellers. Your throttle sets manifold pressure, your RPM control (whatever you use!) sets the propeller pitch at that particular moment, and the propeller automatically adjusts its pitch to maintain that RPM as you do things like climb and dive.
- oldman
Thanks! I was just curious. I do adjust manifold pressure and rpm's but I had no idea it was factored into the propeller pitch. Good to know info as an FYI.
BigKev
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Remember that the P-51A uses an Allison engine, same as the -38,-39, and -40. All have 3 bladed props. I'd say the reason had to do with available horsepower.
What about the 2,000 hp P-47 (4 blades) vs the 2,000 hp F6F and F4U (3 blades)? All three had the R2800 IIRC.
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What about the 2,000 hp P-47 (4 blades) vs the 2,000 hp F6F and F4U (3 blades)? All three had the R2800 IIRC.
I've always wondered about that as well. Specially with the F4U-4 swinging a four bladed prop. Wouldn't the earlier corsair marks have benefited from a four bladed prop?
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So far as I know, all of our planes have constant speed propellers. Your throttle sets manifold pressure, your RPM control (whatever you use!) sets the propeller pitch at that particular moment, and the propeller automatically adjusts its pitch to maintain that RPM as you do things like climb and dive.
- oldman
All Mustangs (and I'm pretty sure most a/c using any variation of Hamilton Standard Constant Speed Prop systems) had a manual overide capability to vary between Coarse (aft) and Fine (forward) Pitch.
The P-51 pitch control was automatically moved forard with a forward movement of the throttle, but the pilot could maintain high boost and then manually (left hand directly under the throttle control) move the pitch control back toward Coarse.
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All Mustangs (and I'm pretty sure most a/c using any variation of Hamilton Standard Constant Speed Prop systems) had a manual overide capability to vary between Coarse (aft) and Fine (forward) Pitch.
The B-17 and B-24 have Hamilton Standard props...all we could do was select RPM with the prop control. The only pitch control we had was to feather the prop.
The P-51 pitch control was automatically moved forard with a forward movement of the throttle, but the pilot could maintain high boost and then manually (left hand directly under the throttle control) move the pitch control back toward Coarse.
When I flew Crazy Horse it didn't work as you described. It worked as any constant speed prop I've flown with..RPM is set with the prop control (then governed by varying the prop pitch). I can't see why you'd want the pitch to change with throttle change except to prevent overboost, but then at times that you're likely to push the throttle up you've already increased prop RPM since you're wanting more power.
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All Mustangs (and I'm pretty sure most a/c using any variation of Hamilton Standard Constant Speed Prop systems) had a manual overide capability to vary between Coarse (aft) and Fine (forward) Pitch.
The P-51 pitch control was automatically moved forard with a forward movement of the throttle, but the pilot could maintain high boost and then manually (left hand directly under the throttle control) move the pitch control back toward Coarse.
Yes and no, fine pitch is really just increasing rpm, coarse pitch low rpm. I do not know of any wwii planes that had any other system then this one. The control just sets the rpm on the Governor. It then changes the pitch.
Most pilots use the term interchangeably, pitch control is the lever that sets RPM, but it is indirectly controlling pitch.
So if a plane sais it has pitch control,or prop control, just assume it is an normal rpm setting.
When doing my checks, I always call the lever prop. I.E. Prop,Mixture ,boost pump, gear down and bolted, tank on the fullest.
Same with start, prop forward, mixture ritch, boost pump to 20 and off, hot start, thottle full mixture ideal cut off, crank until fire or 3 secs.Wait 10 and repeat. Then do the fun mixture throttle shuffle while pinching the stick between your legs.
Cold start is easy, mixture rich thottle 1/8 inch ,crank for 1 blade and watch it fire.
HiTech
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hitech have you flown a P-51? Iirc drgondog has about 50 hrs in the P-51.
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hitech have you flown a P-51? Iirc drgondog has about 50 hrs in the P-51.
Yes, have 1 hour dog fighting Bob Shaw.
(http://www.hitechcreations.com/images/dalep51_s.gif)
Is a photo taken by Bob of me coming threw the vertical a formation loop.
HiTech
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Yes, have 1 hour dog fighting Bob Shaw.
(http://www.hitechcreations.com/images/dalep51_s.gif)
Is a photo taken by Bob of me coming threw the vertical a formation loop.
HiTech
who won? :noid :bolt: :D
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who won? :noid :bolt: :D
they collided, but only Bob got the message and damage :D :bolt: :bolt:
Jokes aside, I bet it was fun as fk!!, I'm tottaly jealous. From what I hear, Bob is a compact built guy, I bet he can handle more 'g's than most people.
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they collided, but only Bob got the message and damage :D :bolt: :bolt:
Jokes aside, I bet it was fun as fk!!, I'm tottaly jealous. From what I hear, Bob is a compact built guy, I bet he can handle more 'g's than most people.
Thats why we call him mouse.
One day we were flying my RV together and decided to go find 6g corner speed. Bobs flying from the back. Anyway we start at about 8k. After about 1 min I finally go lights out.
The same day we found min loop speed of about 112 knots.
Did I mention he also broke my plane 1 day?
HiTech
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I see what you mean. I was thinking of the 109K-4 and its wider than normal 109 prop. I had always thought that they increased area to make up for less air up high, but your post suggests to me that they widened the blades to give it a better climb rate with all that extra HP.
This is why the P-47 was changed to the "paddle-blade" prop. When originally designed, the prop was optimized for high-altitude cruise performance. In wartime service, the need to increase the climb rate was realized, and they added the broader chord prop to increase the rate of climb, which actually knocked a few mph off the top speed.
Generally speaking, the fewer blades and the larger the diameter the better, but then you run into all the other design problems previously mentioned--keeping tip speeds subsonic, ground clearance, power absorption, etc., you sometimes need to add blades, make them shorter, or have thinner or thicker chords.
Propeller theory is pretty mind-bending when you get into the more technical/advanced aspects of it. It is probably a bigger example of "everything's a trade off" in aircraft design than some of the airframe issues. And unfortunately, without some very in-depth discussion, its very difficult to boil down into simple, generalized statements. Its one of the reasons computing thrust equations on propeller aircraft is so difficult.
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How did he break your plane?
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Did I mention he also broke my plane 1 day?
HiTech
sounds like an expensive date... :D
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How did he break your plane?
He did a high speed taxi off the runway, threw a ditch, onto taxi way with me in the back seat.
Ended with bent main gear, and rear 2 bulk heads bent from when the tail wheel hooked the taxi way.
Was a bad day flying.
The flight home from Dayton that trip, I lost a mechanical fuel pump on climb out and starter didn't engage with out much help on the fuel stop. DFW approach was nice that day and let me fly right over the center of DFW at 9k. (i didn't feel like being low with only electric fuel pump working).
HiTech
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He did a high speed taxi off the runway, threw a ditch, onto taxi way with me in the back seat.
Ended with bent main gear, and rear 2 bulk heads bent from when the tail wheel hooked the taxi way.
Was a bad day flying.
The flight home from Dayton that trip, I lost a mechanical fuel pump on climb out and starter didn't engage with out much help on the fuel stop. DFW approach was nice that day and let me fly right over the center of DFW at 9k. (i didn't feel like being low with only electric fuel pump working).
HiTech
that sucks.... I'm glad you made it back to the airstrip, I'm guessing once the fuel pump stopped, your engine died.
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that sucks.... I'm glad you made it back to the airstrip, I'm guessing once the fuel pump stopped, your engine died.
He had an electric pump as backup, probably.
- oldman
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That stinks HT. Glad you got everything back in one piece.
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I've always wondered about that as well. Specially with the F4U-4 swinging a four bladed prop. Wouldn't the earlier corsair marks have benefited from a four bladed prop?
I think the simple explanation is that there wasn't enough horsepower. Yes, 2000 hp is a lot, but the blades they used were massive and they could have opted for shorter by more blades (which is eventually what they did; the F4U-4's prop diameter is 13' 2" whereas F4U-1's have 13' 4"), but when they did their calculations, I suppose they came to the conclusion that 3 blades would be better. Then the R-2800-18W became available and it spun 4 blades.
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I remember some of the experiments with swept blade props - NASA did some testing on that as well back in the 80s but it never seemed to go anywhere.
I remember seeing the C-130 "Hurricane Hunters" when i was stationed at Keesler AFB and they have a swept prop.
http://upload.wikimedia.org/wikipedia/commons/6/69/Lockheed_Martin_WC-130J.jpg
I also read that the AF is testing a similar configuration but with eight blades instead of the traditional four bladed paddle props.
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Generally speaking, the fewer blades and the larger the diameter the better, but then you run into all the other design problems previously mentioned--keeping tip speeds subsonic, ground clearance, power absorption, etc., you sometimes need to add blades, make them shorter, or have thinner or thicker chords.
Or make other design decisions with the airframe, which gave us the F4U Corsair -- the inverted gull wing was necessary to keep the landing gear short enough so that making them strong enough for carrier landings didn't impose a weight penalty with the propellor size they had already decided on. That it was aerodynamically superior was a bonus.
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Hm, I'm curious here. Why would one not want the tip of the prop to breach the
speed of sound? Would it have something to do with the rest of the prop not
spinning as fast? What would happen if it did?
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Hm, I'm curious here. Why would one not want the tip of the prop to breach the
speed of sound? Would it have something to do with the rest of the prop not
spinning as fast? What would happen if it did?
I assume that it is a drag issue. I'm just guessing here but, I would assume that a shockwave starts to built at the tip and would cause some serious turbulence and drag. Anyone?
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Hm, I'm curious here. Why would one not want the tip of the prop to breach the
speed of sound? Would it have something to do with the rest of the prop not
spinning as fast? What would happen if it did?
Supersonic prop tip speeds do occur. T-6 Texan props are one example and if you've ever been to Reno and heard the T-6 class race, you know that they have a very loud prop noise. Some Cessna 180 owners have the pitch on their props so high (in order to generate a lot of static thrust that shortens takeoff distance) that the prop tips will go supersonic resulting in a lot of noise on takeoff. The Russian "Bear" bomber reportedly has very loud prop noise as a result of its props going supersonic.
Again, generally speaking, supersonic prop tips can create three conditions: (1) Extremely loud noise. (2) Flow over the propellor blade separates, greatly reducing prop efficiency and creating drag and (3) introduction of vibration/stresses on the propeller blades that can damage the propellor. Generally speaking, the aircraft won't gain any advantages from the increased prop speed/diameter if the blade tips are supersonic, and therefore there's no reason to design a prop that will regularly have supersonic tip speeds. There are exceptions, as I mentioned above.
Not the best example, but watch this 180 video and you can hear the prop tips going supersonic during the takeoff and climb out.
http://www.youtube.com/watch?v=_BI0c_0uupQ
[EDIT] Here's a video of the T-6 class at Reno. A better example of supersonic tip speed noise. http://www.youtube.com/watch?v=nen94IP8mNg
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Some Cessna 180 owners have the pitch on their props so high (in order to generate a lot of static thrust that shortens takeoff distance) that the prop tips will go supersonic resulting in a lot of noise on takeoff.
If the pitch was set "high" the prop would be limited in RPM keeping the tip speed low. The 180/185 prop noise is more from the length of the 2-blade prop the engine turning up to 2850 RPM for takeoff power.
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Supersonic prop tip speeds do occur. T-6 Texan props are one example and if you've ever been to Reno and heard the T-6 class race, you know that they have a very loud prop noise. Some Cessna 180 owners have the pitch on their props so high (in order to generate a lot of static thrust that shortens takeoff distance) that the prop tips will go supersonic resulting in a lot of noise on takeoff. The Russian "Bear" bomber reportedly has very loud prop noise as a result of its props going supersonic.
Again, generally speaking, supersonic prop tips can create three conditions: (1) Extremely loud noise. (2) Flow over the propellor blade separates, greatly reducing prop efficiency and creating drag and (3) introduction of vibration/stresses on the propeller blades that can damage the propellor. Generally speaking, the aircraft won't gain any advantages from the increased prop speed/diameter if the blade tips are supersonic, and therefore there's no reason to design a prop that will regularly have supersonic tip speeds. There are exceptions, as I mentioned above.
Not the best example, but watch this 180 video and you can hear the prop tips going supersonic during the takeoff and climb out.
http://www.youtube.com/watch?v=_BI0c_0uupQ
[EDIT] Here's a video of the T-6 class at Reno. A better example of supersonic tip speed noise. http://www.youtube.com/watch?v=nen94IP8mNg
Good stuff, thanks!
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Some Cessna 180 owners have the pitch on their props so high (in order to generate a lot of static thrust that shortens takeoff distance) that the prop tips will go supersonic resulting in a lot of noise on takeoff. T
You sure you have this correct? Unless you could previously over rev the engine standing still, higher pitch would = less thrust.
And are you sure pitch changes tip speed much, unless rpm is changed?
HiTech
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Again, generally speaking, supersonic prop tips can create three conditions: (1) Extremely loud noise. (2) Flow over the propellor blade separates, greatly reducing prop efficiency and creating drag and (3) introduction of vibration/stresses on the propeller blades that can damage the propeller. Generally speaking, the aircraft won't gain any advantages from the increased prop speed/diameter if the blade tips are supersonic, and therefore there's no reason to design a prop that will regularly have supersonic tip speeds. There are exceptions, as I mentioned above.
Yes, that's it in a nutshell. You can make a prop go supersonic but there really isn't much advantage to it. You take a heavy duty (or specially designed) prop and mate it to a big honkin' powerplant and the tips go supersonic - which creates a huge noise problem (and shockwave hazard - do a quick search on the XF-84H trials and see what I mean about that). Here's an article on it if you want some supplemental reading:
http://www.me.wustl.edu/~aiaa/November_1996.pdf (http://www.me.wustl.edu/~aiaa/November_1996.pdf)
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You sure you have this correct? Unless you could previously over rev the engine standing still, higher pitch would = less thrust.
And are you sure pitch changes tip speed much, unless rpm is changed?
HiTech
Sorry, poor terminology on my part, but an A&P friend of mine once told me that a lot of the 180 community purposefully generates those types of tip speeds to get those short take-off runs.
[Edit] Dale, do I remember correctly that you had a 180 at some point?
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[Edit] Dale, do I remember correctly that you had a 180 at some point?
'57 182 which at that time was a 180 with a nosewheel. The 180 in the video is the later model 180 that was pretty much an underpowered 185. It has the big tail and an extra window/seat row but IIRC still had the same 230HP Continental of the 182/180 series. The 185s have the 285 to 315 HP engines with the long 2-blade props that really make a racket. Something I have seen quite often is guys on floats putting longer props on the 180s to help get them off the water -- the longer prop makes more noise.
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Quick semi-off topic question: Why does a cirrus use a 3 bladed prop with an IO360, but 172s running an IO360 only use a 2 blade?
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Quick semi-off topic question: Why does a cirrus use a 3 bladed prop with an IO360, but 172s running an IO360 only use a 2 blade?
Perhaps a matter of ground clearance?
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Quick semi-off topic question: Why does a cirrus use a 3 bladed prop with an IO360, but 172s running an IO360 only use a 2 blade?
Different design priorities. Perhaps Cirrus wanted a better rate of climb, or more ground clearance, or needed shorter blades for tip speeds, or needed more blades for better power absorption, etc. Maybe they thought it looked better...
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I remember some of the experiments with swept blade props - NASA did some testing on that as well back in the 80s but it never seemed to go anywhere. I also wondered why the Germans stuck with 3-bladed props on the 109K4 as well as on the 190D9 and TA152s.
Actually, I was a wind tunnel test engineer at Ames in the late 80's. I remember an internal (principal investigator aws another NASA division) test we did on an unducted fan in the 14' TWT. That was basic research where we were mapping out the pressure distribution on the nacelles. Otherwise, IIRFC, the use of the fan is all about fuel. It's kind of ultra high bypass, if you like.
As for the swept blades, my recollection is that it's the same issue as with swept wings - delaying the onset of transonic wave drag - which goes something like the thickness/chord ratio. Apparent chord rises with sweep, thus t/c decreases. Also, and as an interesting aside, IIRC, the 262's sweep was all about c.g. - any other benefits were unintended.
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Also, and as an interesting aside, IIRC, the 262's sweep was all about c.g. - any other benefits were unintended.
I never knew that--very interesting...
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I never knew that--very interesting...
The kommet had swept wings too, and was design in '41. At what point did they discover that property of swept wings?
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Maybe they thought it looked better...
That's the only thing I think it might be, maybe Wolfala has something to say about it, i know he owns a cirrus.
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Also, and as an interesting aside, IIRC, the 262's sweep was all about c.g. - any other benefits were unintended.
I'd heard that several times and I guess I never researched it. To my eye it looks fantastic but I'd wondered if any significant advantages to that (relatively small) amount of sweep would be realized.
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I never knew that--very interesting...
I said that from memory. I took a class i n advanced design with Ilan Kroo at SU back in '87. He cited it. I was able to find this very quickly in Wiki so perhaps, per usual, Kroo knew what he was talking about:
Although it is often stated the Me 262 is a "swept wing" design, the production Me 262 had a leading edge sweep of only 18.5°, too slight to achieve any significant advantage in increasing the critical Mach number.[15] Sweep was added after the initial design of the aircraft, when the engines proved to be heavier than originally expected, primarily to position the center of lift properly relative to the centre of mass.
<end snip>
As for the 163, I can make no claim but note that the genesis of the two designs is separated by a number of years of intense "learning" and "incentive".
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I'd heard that several times and I guess I never researched it. To my eye it looks fantastic but I'd wondered if any significant advantages to that (relatively small) amount of sweep would be realized.
According to Raymer, sweep contributes a few advantages to different aspects of high-speed flight. For supersonic flight, loss of lift associated with supersonic flow can be reduced by sweeping the wing leading edge aft of the Mach cone angle, where the mach cone angle = the arcsin(1/Mach #). Since the 262 wasn't designed for supersonic speed, it gained no advantage from the designed sweep since at a Mach number of 1, no sweep is required.
On the other hand, wing sweep in the sub-sonic/transonic range increases the critical mach number of a wing, so it may have extended its critical mach number with the wing sweep that was designed. According to Wiki, compared to the P-80 (which had basically no wing sweep), the USAF tested both aircraft and determined the 262 had a higher critical mach number, so perhaps the wing sweep created the advantage, although airfoil thickness and profile has an affect as well, and I don't know how those two aircraft compared in those respects.
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According to Raymer, sweep contributes a few advantages to different aspects of high-speed flight. For supersonic flight, loss of lift associated with supersonic flow can be reduced by sweeping the wing leading edge aft of the Mach cone angle, where the mach cone angle = the arcsin(1/Mach #). Since the 262 wasn't designed for supersonic speed, it gained no advantage from the designed sweep since at a Mach number of 1, no sweep is required.
On the other hand, wing sweep in the sub-sonic/transonic range increases the critical mach number of a wing, so it may have extended its critical mach number with the wing sweep that was designed. According to Wiki, compared to the P-80 (which had basically no wing sweep), the USAF tested both aircraft and determined the 262 had a higher critical mach number, so perhaps the wing sweep created the advantage, although airfoil thickness and profile has an affect as well, and I don't know how those two aircraft compared in those respects.
You could check it quickly and dirtily by comparing the relative increase in t/c for an 18.5 degree sweep.
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To my eye it looks fantastic
Indeed! everytime I fly a 262 it reminds me of a boeing 737 wing. however a quick google search says the 737 is 25 degree sweep.. still looks the same to me!
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I wonder what its quarter chord sweep is compared to the leading edge sweep. Does anyone have a scale drawing of the wing that measurements could be made off of?
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So.. do any AH props occasionally go supersonic?
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I wonder what its quarter chord sweep is compared to the leading edge sweep. Does anyone have a scale drawing of the wing that measurements could be made off of?
I found this ( http://www.albentley-drawings.com/me262.htm ) and also fired off a request to the SturmVogel people...
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The reference sweep angle quoted is usually the sweep angle for the 1/4 chord line..
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The reference sweep angle quoted is usually the sweep angle for the 1/4 chord line..
Yeah, but all the "soft" information sights like Wiki are showing the leading edge sweep angle, which is only good for analyzing the mach cone sweep. I haven't imported those .jpg that PJ linked into my CAD program, but it looks like the 1/4 chord is less than the leading edge.
[EDIT] From my CAD program, it looks like the 1/4 chord sweep is 16 degrees...
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So.. do any AH props occasionally go supersonic?
Well, its possible. There is two components to the "helical" prop tip speed. One is the rotational velocity alone, which is a function of rpm and diameter. To add the forward speed of the aircraft, we have to use a formula:
Vtip = Sqrt(V^2 + Vtip(static)^2)
Where V = forward velocity of the aircraft and Vtip(static) = the rotational velocity of the tip.
Vtip(static) = pi*rps*diameter
rps = revolutions per second
*velocities for V need to be expressed in feet per second
So, if we know the diameter and RPM of a prop, and the forward velocity of the aircraft, we can compute the helical tip speed to see if it approaches or exceeds Mach 1. Remember though, that the speed of sound changes with temperature and altitude. Luckily, in AH, we use standard temperatures and standard lapse rates, so mach is easily predicted using standard dynamic pressure tables.
For the P-47M at 2700 engine RPM (equal to 1350 propeller RPM) and 300mph at sea level using the above equations, we find (if I have my prop reduction gear numbers correct):
Vtip(static) = 3.14*22.5*12 ~ 848 fps
V = 440 fps
Then
sqrt(440^2 + 848^2) = 955 fps. Mach 1 at sea level/standard conditions = 1116 fps. Vtip = .86 Mach
Now if that same plane comes out of a dive and is carrying 500 mph then:
V = 734 fps
and
sqrt(734^2 + 848^2) = 1121. 1121/1116 ~ Vtip of 1.005 Mach
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Remember though, that the speed of sound changes with temperature and altitude.
root-gamma-are-tee, if I recall correctly...
Still, per usual, you've calc'ed with aplomb.
Not to highjack but you might be the guy to ask (actually, probably a COADER for this one)... and it came up in FSO last week so I said i'd check. I kind of assume the AH flight models use lumped masses for several of the main lumps of each aircraft. One of those is, I assume also, the fuel tanks. Does the AH model update, in time steps, the mass in the fuel tanks? I'm assuming it must certainly calc the overall mass but also the location associated with each discrete lump (for the inertial props).
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root-gamma-are-tee, if I recall correctly...
Still, per usual, you've calc'ed with aplomb.
Not to highjack but you might be the guy to ask (actually, probably a COADER for this one)... and it came up in FSO last week so I said i'd check. I kind of assume the AH flight models use lumped masses for several of the main lumps of each aircraft. One of those is, I assume also, the fuel tanks. Does the AH model update, in time steps, the mass in the fuel tanks? I'm assuming it must certainly calc the overall mass but also the location associated with each discrete lump (for the inertial props).
I have no idea... With some luck, this discussion will be compelling enough for HiTech to make an appearance, and he can educate us.
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Actually, I made a mistake above, as I used 12 feet for the diameter of the P-47M prop. The correct diameter is 13 feet.
So:
Vtip(static) ~ 918 fps versus 848 as I used originally. At 500 mph and sea level Vtip would then = 1175 fps or Mach 1.05
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Actually, I made a mistake above, as I used 12 feet for the diameter of the P-47M prop. The correct diameter is 13 feet.
So:
Vtip(static) ~ 918 fps versus 848 as I used originally. At 500 mph and sea level Vtip would then = 1175 fps or Mach 1.05
I jinxed you. But, of course, I wasn't checking the physicals of the ac anyway.
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Somehow didn't expect the math to be so straight forward.. Thanks Stoney.
With some luck, this discussion will be compelling enough for HiTech to make an appearance, and he can educate us.
And maybe add supersonic prop sounds to the game :D
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Somehow didn't expect the math to be so straight forward.. Thanks Stoney.And maybe add supersonic prop sounds to the game :D
Well, determining the speed is pretty straightforward. What would be very difficult is to model the effects. Prop efficiency is such a ethereal thing sometimes I'm not even sure what kind of performance penalty would be incurred, or if it would even be worth the trouble to determine/model. The other thing is that we have rock-solid RPM in-game, with no potential for overspeed. I don't know if a P-47 could maintain 2700 RPM in a 500 mph dive in real life.
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But the props' mach sound would only depend on its speed, right?
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root-gamma-are-tee, if I recall correctly...
Still, per usual, you've calc'ed with aplomb.
Not to highjack but you might be the guy to ask (actually, probably a COADER for this one)... and it came up in FSO last week so I said i'd check. I kind of assume the AH flight models use lumped masses for several of the main lumps of each aircraft. One of those is, I assume also, the fuel tanks. Does the AH model update, in time steps, the mass in the fuel tanks? I'm assuming it must certainly calc the overall mass but also the location associated with each discrete lump (for the inertial props).
Yes for consumables, I.E. weapons and fuel.
No for parts of the airplane. I.E. loose a wing tip neither the mass nor the CG change.
HiTEch
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The other thing is that we have rock-solid RPM in-game, with no potential for overspeed. I don't know if a P-47 could maintain 2700 RPM in a 500 mph dive in real life.
I'm thinking the potential for overspeed does exist. I've noticed in a high-speed power off descent in the P47 (either D40 or M, can't recall which it was) the RPM was up around 2000 or so even though I had rolled the RPM back to minimum, once I had slowed the RPM dropped down to around 1000 so it appears that airflow is "pushing" the prop above where I had it set.
I would think you'd have to get well over 500IAS to overspeed the prop. The minimum governing RPM on the Ham Standard is down at 1200 so the prop is able to put quite a bit of pitch in to slow it....no practical experience diving a Jug at 500IAS...just guessing. In real life I've never had a prop gain RPM. When hauling jumpers we used a low RPM setting with high airspeed (well, high for a Cessna -- 170 +/-) and didn't notice any RPM change.
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Yes for consumables, I.E. weapons and fuel.
No for parts of the airplane. I.E. loose a wing tip neither the mass nor the CG change.
HiTEch
That's interesting. What changes then just the lift and drag?
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That's interesting. What changes then just the lift and drag?
Not just that. While it's true that weight would decrease such that lift to counter gravity would decrease (as would any induced drag), the other significant impact of changing masses is the inertial properties. Recall those as the rotational analog of mass - Tau = I*Omega just like F=M*A. These are simplified reps of vector equations, of course. In any case, Inertia is a function of mass and it's distribution. Hence, all other things equal, you're likely to get more pitch authority if you minimize your pitch axis inertia. Anyway, you can look at all the equations of motion here ( I pull a more or less random source, there are many good texts on this): http://books.google.com/books?id=AUWCXVepMKEC&pg=PA107&lpg=PA107&dq=aircraft+equations+of+motion+linearized&source=bl&ots=sN1j0dlEoH&sig=ajyS9c3YsxbBZUu905sSTYXpJ1Y&hl=en&ei=SwiNTaiHCejp0gH2xJ2-Cw&sa=X&oi=book_result&ct=result&resnum=6&sqi=2&ved=0CDUQ6AEwBQ#v=onepage&q=aircraft%20equations%20of%20motion%20linearized&f=false
and you can see the inertia terms by their I notation. They go through a nice linearization in this text I searched up at random and arrive, further down, at some linearized forms (see 4.38). It's easier to get an intuitive feel for the impact of inertia terms from those.
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Thanks PJ but I meant it was interesting that the weight loss and CG change weren't modeled in AH. I was asking if, in the case of a wingtip loss, only the drag and lift difference was modeled.
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I'm thinking the potential for overspeed does exist.
I'm sure it does. Governor can take care of a lot, but I'm sure that it can't take care of everything, particularly in Jug Dives.
- oldman
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Thanks PJ but I meant it was interesting that the weight loss and CG change weren't modeled in AH. I was asking if, in the case of a wingtip loss, only the drag and lift difference was modeled.
Okay. I wasn't sure of your basis. There seems to be a wide range of technical knowledge in this place. Of course, I know you're training staff so I figured you're technically clued in anyway but might like to see those EoMs.
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In real life I've never had a prop gain RPM. When hauling jumpers we used a low RPM setting with high airspeed (well, high for a Cessna -- 170 +/-) and didn't notice any RPM change.
My Grumman had a climb prop and a 160HP conversion on it, and I would have to pull throttle during decents to keep the RPM under redline. Not quite a dive, but once I was showing 150-160 mph IAS, the prop was wanting to turn around 2900 rpm. That being said, I really don't know the overspeed potential of the Jug, just seems like a prop optimized for 300-350 mph wouldn't tolerate that kind of speed. I could be totally wrong though. Bodhi works on them--perhaps he could tell us...
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Stoney: Ever take it to a prop shop and have it adjusted?
I had similar problem when I got my RV. It had a Areo/ pitts prop on it that never had it's min and max pitch stops adjusted. Min would over speed at cruise speeds and I would have to back off on power.
Ah has both min and max pitch stops. So they can over speed.
HiTech
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Okay. I wasn't sure of your basis. There seems to be a wide range of technical knowledge in this place. Of course, I know you're training staff so I figured you're technically clued in anyway but might like to see those EoMs.
I always appreciate your responses but my brain isn't wired for math. I get F=M*A but that's probably just based on experience.
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My Grumman had a climb prop and a 160HP conversion on it, and I would have to pull throttle during decents to keep the RPM under redline. Not quite a dive, but once I was showing 150-160 mph IAS, the prop was wanting to turn around 2900 rpm. That being said, I really don't know the overspeed potential of the Jug, just seems like a prop optimized for 300-350 mph wouldn't tolerate that kind of speed. I could be totally wrong though. Bodhi works on them--perhaps he could tell us...
Since you said "climb prop" I'm betting you had a fixed pitch prop in which case RPM will increase with an increase in airspeed.
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I've seen overspeed in AH but never damage from it.
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Since you said "climb prop" I'm betting you had a fixed pitch prop in which case RPM will increase with an increase in airspeed.
Yes, but its the same principle. Once the blades on a constant speed prop hit the stop, any increase in speed beyond that will create an overspeed condition.
And I didn't get it re-pitched. After I had two cylinder base nuts shear from the high-compression pistons (long story), I overhauled it and put low-compression pistons back in. That dropped the power enough that I couldn't get it past 2700 RPM much at all.