Power and Propeller Optimization

[CJ]

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

[DB603]

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

[funked]

DB603, the Spits in AH all had automatic constant-speed props, as did every fighter in AH.

[janjan]

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.

[DB603]

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  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
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[juzz]

NACA report: Characteristics of the BMW 801D2 automatic engine control as determined from bench tests
(2.1Mb .pdf)

[This message has been edited by juzz (edited 05-10-2001).]

[niklas]

 

easy to understand, isn´t it ?  

niklas

[pugg666]

How much room did that thing take up?



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Pugg666 XO
Braunco Mustangs

[funked]

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).]

[funked]

Thanks Juzz, I saved that one.  

[DB603]

S!

 Thanx Funked..I might have mixed them up  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
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[CJ]

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

[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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Bodhi
-*Armageddon*-

[juzz]

He means all the planes in AH...

[CJ]

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..