Aces High Bulletin Board
General Forums => Aces High General Discussion => Topic started by: Yeager on May 31, 2000, 06:24:00 PM
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I been chewin on this one fer a spell so here go:
When the propellor is rotating faster than the air it encounters, it pulls itself forward sort of like biting into the air.
When the propellor is spinning slower than the air it encounters, it acts somewhat like an air brake (that is: it pushes, or fights against the rush of incoming air). This is *primarily* where the drag occurs and is missed so dearly i m o.
I know the physics involved are far more complicated and subserviant to a whole host of conditions within the flight envelope of any airplane, but suffice it to say that AcesHigh needs prop drag something fierce!
Hee Haw!
Yeager
[This message has been edited by Yeager (edited 05-31-2000).]
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Yeager, do we really have any data or anectdotes to estimate what kind of drag these planes should get from the prop? If we know better, I'll be the first to jump on the bandwagon, but this topic is beyond the scope of my theoretical knowledge and is beyond the scope of any flight test data I have seen.
[This message has been edited by funked (edited 05-31-2000).]
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It should be a hard mathematical formula Funked. Given that we know: the RPM of the blade, the width of the blade and the speed of the plane. It would be more complex in the case of real enviroments with wind, air density etc etc but those are solid values in AH. Once Pyro works out how much air the prop can push through at 100% efficiency everything over that is inverse porportionatly drag holding the plane back from moving faster.
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I once asked an aeronautical engineer about prop drag. He seemed a bit intrigued by the idea of coming up with a mathematical model for it. He preferred to refer to it as 'net thrust'. The idea is that at some speed the prop will create more drag than thrust. Some of the variables he thought would come into play were engine displacement, prop rpm setting, and whether the prop is attached to the engine via a reduction gear. The conversation had to end before I could pick his brain any more.
It all sounded way too complicated. There must be an easier way to guestimate a 'net thrust' factor.
ra
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I'll check some of my notes at work on `reducing RPMS and/or Throttle' and see how that effects Drag. However, I did find these equations (They are approximations for design) for a feathered or stopped prop:
Drag(Prop)=C1*q*sigma*A
where
C1=.1 for a feathered prop
C1=.8 for a stopped `non-feathered' prop
q=dynamic pressure=(.5)*rho*v^2 where rho is altitude air density and v is the free stream velocity
sigma=the solidity ratio (the ratio of the total blade area to the blade disk area)
A=blade disk area.
Hope that helps.
(http://devildogs.com/vmf111/sdsig2.gif)
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There's 2 types of prop drag that I can see being lumped into one.
1. Resistence to the rotation of the prop, which has no bearing on forward speed, only a rolling torque moment.
2. The propeller creates drag opposing the direction of motion of the aircraft when the propeller can no longer accelerate the incoming air mass (thrust). This usually means the propeller has reached it's maximum pitch angle and can go no further. It pretty much becomes a flat plate at that point and drag isn't too hard to guestimate.
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One of the biggest problems I see is determining what the pitch angle will be. That means you have to model the dynamics of the governor on each plane.
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We know that a propeller has twist from it's root to it's tip. We also know that the most thrust/drag comes out towards the tips, where the velocity is highest. The twist may be somewhere around 30 degrees from root to tip. In practice, there is very little difference between 60 and 90 degrees as far as drag is concerned. I would think limiting angles of 30/tip, 60/root would be typical (60/90 feather position). At full throttle, you'd never see any of the #2 drag until you hit well over 500 mph, but at idle, the drag would become prominent above about 200 mph. So basically, the RPM setting will determine the speed at which the propeller starts to act like a barn door!
[This message has been edited by wells (edited 05-31-2000).]