Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: CJ on May 09, 2001, 12:40:00 PM
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I hear a lot of people saying things like the following;
-this aircraft has a higher power to weight ratio than that aircraft.. why can that aircraft outperform this one?
Optimization of propellers has a great impact in the thrust available at an airspeed for a plane. Some props for a given engine are optimized for high speed, and some for low speed. The propeller twist, diameter, pitch range, number of baldes and airfoil sections are all optimized for a certian range of flying. High speed props tend to be smaller diameter, and in order to absorb the power may need more blades. This keeps tip speeds below Mach 1, and also has lower "prop drag" than a large diameter prop. Low speed propellers have thicker airfoils, different twist distribution, and plan form.
Since all planes use constant speed props, if a plane is flying below the optimum speed for the diameter, airfoil section, number of blades, etc, it will be "slipping" a lot more than a blade that is optimized for that regime, and therefor wasting more power.
So a plane with a lower power to weight ratio, but is operating in a much more efficient regime for it's propeller design, it could outperform that other plane.
It all just boils down to understanding every detail about your aircraft's flight envelope, and optimizing your flight profile to fit the plane's strengths.
I'm sure that HTC can't model every detail about propeller propulsion efficiency... that program would probably cut FPS by 20% alone, but I'm sure they modeled quite a bit. Actually, it would be interesting to know how the propeller modeling is done, so that we could fly accordingly, but I'm sure they don't want to give everything away to the competition..
S!
CJ No. 272 Squadron
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S!
At least in 109/190 the propeller RPM adjusting should cause no sweat,since they are automated to give best efficiency over a wide range of engine settings.The thingy was called Kommandgerät(command device)...But no exact info how AH has modeled this.Spits should for example have manual prop pitch trimming,but in AH it is fully automated.
It is understandable that not everything can be modeled,but is there any links or info what we actuallt have now modeled?
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DB603
3.Lentue
Lentolaivue 34 (http://www.muodos.fi/LLv34/)
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DB603, the Spits in AH all had automatic constant-speed props, as did every fighter in AH.
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Think CJ is saying there is more about propeller optimization for speed than blade angle.
DB: Kommandoberät only in 190, not in 109. It controls MAN/RPM ratio + fuel richness.
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S!
JanJan..the system 109 uses is pretty close to what 190 uses.The aneroid(and much other things) controlled system kept the engine/prop RPM in a predefinet ratio..It was very close to 1:1.6 or something.Gotta check from book.Also 109 had fuel injection so it has pretty automated mixture adjustment (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)And Funked...I read that Spit pilots wished they had the same system as in 109/190 too,so it is prolly they had manual adjust of prop settings?Maybe later models had automated ones...Thanx for comments gents!
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DB603
3.Lentue
Lentolaivue 34 (http://www.muodos.fi/LLv34/)
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NACA report: Characteristics of the BMW 801D2 automatic engine control as determined from bench tests (http://naca.larc.nasa.gov/reports/1945/naca-wr-e-192/naca-wr-e-192.pdf)
(2.1Mb .pdf)
[This message has been edited by juzz (edited 05-10-2001).]
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(http://www.vfa.tu-cottbus.de/fw190/images/bmwkommandogeraet.jpg)
easy to understand, isn´t it ? (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)
niklas
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How much room did that thing take up?
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Pugg666 XO
Braunco Mustangs
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DB603 you seem to be confusing the Kommandogerät with the prop. speed regulator. Two different systems.
The regulator (present in real life on every plane that we have in AH) controls prop pitch to maintain the desired RPM. The only Spitfires which lacked this system were of pre-war vintage.
The Kommandogerät (present on Fw 190) allows the pilot to control boost setting, supercharger speed, mixture, spark advance, and RPM with a single lever. There was still a prop. speed regulator which took orders from the Kommandogerät.
The difference is that the Kommandogerät chose the RPM setting while in the other planes it was set by the pilot. Pilots could also override the regulator and manually set prop. pitch.
[This message has been edited by funked (edited 05-10-2001).]
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Thanks Juzz, I saved that one. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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S!
Thanx Funked..I might have mixed them up (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)But as said...In 109 You didn't need to adjust anything but the throttle lever.Prop RPM was on Auto.Darn..gotta read the 109 data again..LOL!
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DB603
3.Lentue
Lentolaivue 34 (http://www.muodos.fi/LLv34/)
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I realize that all the aircraft modeled in this sim have adjustable/constant speed props, but what i was getting at is that constant speed props are not all the same. Different blade numbers, pitch ranges, twist distribution, airfoils, diameters, can all make the thrust curve of the powerplant, with velocity, vary considerably for identical power levels. The primary reason for this is the same reason that different types of turbo fan jet engines have varying bypass ratios. For high speed flight, you have lower bypass than for lower speed flight. This means that you're accelerating a smaller mass of air to a higher velocity, which yields lower thrust overall for a given power level (fuel burn) but the higher velocity gives a higher top speed, with a reduced low end acceleration and climb performance.
Propellers are the same way... Helicopters are basically lifted with large diameter constant speed props that also have cyclic thrown in. They generate tremendous thrust, but the flow velocity is not very high. Using the same engine to turn a small constant speed propeller would not generate nearly the same thrust, but would allow a higher top speed.
These are extreme cases... the variance in propeller type between similar powerplants in ww2 is a less extreme case of propeller optimization...
CJ
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CJ,
you need to check your information. All planes do not use constant speed propellors. Cripes, that is arrogant presumption. If you do not know all about a subject I suggest you do not make a blanket statement like that.
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Bodhi
-*Armageddon*-
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He means all the planes in AH...
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Bodhi,
You're right. I meant to say that all aircraft in Aces High use adjustable propellers, or constant speed propellers. Simply a slip.. I don't always proofread everything i write in here 3 or 4 times. I guess that makes me an arrogant American, and that slip completely negates everything else that i wrote in the message..
Have a nice day..
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Quote "I'm sure that HTC can't model every detail about propeller propulsion efficiency... that program would probably cut FPS by 20% alone, but I'm sure they modeled quite a bit. Actually, it would be interesting to know how the propeller modeling is done, so that we could fly accordingly, but I'm sure they don't want to give everything away to the competition"
Ever heard of a program called X-plane? (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)
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Oops, Bodhi is having one of his "spells" again, he actually isn't such a bad guy.
Here Bodhi (hands Bodhi a paper bag) breate into this, buddy.
StuB
Originally posted by Bodhi:
CJ,
you need to check your information. All planes do not use constant speed propellors. Cripes, that is arrogant presumption. If you do not know all about a subject I suggest you do not make a blanket statement like that.
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I did try X-plane. It had a great jet flight model, but when i tried with prop planes, above a certian speed, the prop would "run away" and suddenly the aircraft would accelerate, go out of control, and crash really hard (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif) Not sure what the problem was, but overall a very good feel on that sim.
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Since you guys are talking about the prop, I'll add one more question. Dunno if it fits in this thread... but hope so (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif) I have imagined that it has something to do with prop drag or so.
I cannot remember how many times my landings have turned to ditches when I have not been able to stop my plane on the runway in time. This never happens if my engine is running, but if I have lost my engine or ran out of fuel, the plane just glides endlessly. It is almost impossible to even push it to the runway. It seems to have additional lift as well even though I try to approach with slower speed than when engine is running.
Just yesterday I lost my engine, one aileron and rudder. Normally I could have landed straight from my position and slowed down with rudder, but now I glided all around a large airbase and could not push it to the runway (http://bbs.hitechcreations.com/smf/Smileys/default/frown.gif)
Is this how it should be or is there something missing in the prop modelling?
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BlauKreuz
Lentolaivue 34 (http://www.muodos.fi/LLv34/)
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I'm not sure about that one. I had problems with engine out landings a lot initially in this sim, but have gotten used to it. I think most of the reason why is that when i lose an engine i get kind of excited and tend to try to "get it down" on the runway, and probably just come in with too much speed. I've been trying to manage my energy and airspeed from a ways out lately, and with slipping, flaps, and gear, can usually get it slowed down now. I know how in the heat of battle, or after damage I sometimes forget to manage E as well as i should and end up blasting over the threshold with too much speed, just happy to have made it, and end up rolling off the end.
As for prop drag in a engineless aircraft in AH, im not exactly sure how it's modeled. Here's how it is in the planes i've flown though, for those who are interested...
It depends on a lot of things. In a real single engined plane with a constant speed prop, like the Cessna 182, when you loose the engine, the prop goes to fine pitch because of several factors.
First of all, the prop is windmilling, which forces the oil pump (which powers the propeller pitch changine mechanism) to keep working, which causes the constant speed prop to keep functioning. Since the command rpm( as set by the prop lever and governor) is invaribly higher than the engine is allowing the prop to turn, the pitch tries to flatten out, and allow the engine to run faster. This simply causes more drag, and since the governor does not know that the engine is off, functions as if the engine was running, but the prop was at too high of a pitch angle to allow the rpm to reach the set level. To reduce drag on a single engine aircraft with a failed engine, you can do two things.
1. Slow the plane to the point that the prop stops, and hence the blades stall, which actually reduces to total drag component on the propeller. The pitch also flattens beacuse there is a spring that reduces the pitch at this point since there is no hydraulic pressure to counteract it. Once it stops, the pitch stays flat, and the prop will generally stay flattened until you dive to a much higher speed to get it spinning again.
2. Pull the prop back to minimum RPM which will increase the pitch on the blades, and reduce the rpm and drag. The prop will keep spinning, and the engine driven oil pump will therefore keep supplying pressure to the pitch changing mechanism, which will keep it in a course pitch. the prop is still unstalled, but the high pitch and low rpm mean that it's absorbing much less energy out of the surrounding airstream than a flat, unstalled, fast spinning propeller.
In my commercial training in a Mooney M20E, we did a lot of engine out practice, and once the instructor even pulled the mixture and cut the engine while we were flying at about 5000 feet AGL. The prop kept turning, and he told me to notice the glide angle, and get a feel for it. He then pulled the prop knob all the way out, and the prop slowed, and i noticed quite a reduction in drag and the airspeed climbed substantially at the same flight attitude. We did not try stopping the prop and getting the blades to "stall" but i have seen a video about this with a Cessna 182, and the person int he video demonstrated that the lift/drag went from around 9 with the prop windmilling to about 12 with the prop stopped. That is quite a reduction of drag by stopping the prop, and I'm guessing that the reduction by going to course pitch with a windmilling prop would be a similar amount. It made about as much difference as dropping the gear would. (again just guessing, but i'll bet it's within 25%)
Now, in multi engine aircraft, the props are usually designed so that they they can be made to "feather" after a failure. This is simply because the assymetric thrust makes it so that the aircraft already handles badly without thrust on both sides, adding drag to the failed engine side just makes things worse, and increases the Vmc (airspeed for minimum single engine controllability) for the aircraft. When flying a Cessna 310 of a friends a couple of years ago, we had already reduced mainfold pressure, and we were pushing the props forward for the "GUMP-Gas Undercarriage Mixture Prop) short final check. i mean the glide ratio went through the floor! In fact, we sort of used the prop levers as brakes during final since the props created so much drag in the fine pitch mode. I'd guess that without power and with the props feathered, the drag would be a lot less, but I don't have any numbers to back it up.
For what it's worth