Propeller questions

[caldera]

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?

[EagleDNY]

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...

[Tyrannis]

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?

[hitech]

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

[EagleDNY]

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). 

[EagleDNY]

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.

[Ardy123]

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.

[EagleDNY]

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. 

[kilo2]

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.

[BigKev03]

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

[Ex-jazz]

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

[caldera]

So the answer to question #2 is?  Anyone?  Anyone?  Bueler?  Bueler?

[Oldman731]

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

[MiloMorai]

IRL, the 109 has manual and automatic pitch control for the prop.

[Ardy123]

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.