NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/rec/models/rc/3080

Text File | 1992-07-29 | 6.6 KB | 139 lines

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From: [e-mail] (Iskandar Taib)
Subject: Re: Taildraggers taking off (P-factor & Torque)
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Date: Thu, 30 Jul 1992 04:59:05 GMT
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In article <[e-mail]> [e-mail] (Bill Coleman) writes:
>In article <[e-mail]>, [e-mail] (Iskandar Taib) writes:
>> In article <[e-mail]> [e-mail] (Bill Coleman) writes:
>>>In article <[e-mail]>, [e-mail] (Iskandar Taib) writes:
||
|||| Which is why one often can't have
|||| unlimited vertical climb even if static thrust exceeds weight.
||
|||Huh? I don't see how this is related to propeller performance.
|||You can't change the laws of physics. In a vertical orientation,
|||the primary forces acting on a plane are thrust, weight and DRAG.
||
|| This happens because dynamic thrust, especially at higher speeds,
|| is often less than static thrust. Thus the airplane slackens off
|| in speed, often to the point where it is directionally unstable.
|
|Read the following again, Iskander. If the thrust exceeds the total
|of weight and drag, it will not slacken off in speed.

I say it "slackens off in speed" because I am assuming that the air-
plane enters the vertical climb with some horizontal speed greater
than the stable vertical speed. You'd be right in pointing out this
is irrelevant. I suppose what I should have said was that if you use
a prop whose pitch is on the low side, your vertical velocity might
be less than that attained if static thrust were used in the calcu-
lations.

|||Even if thrust exceeds weight, you still have to have extra thrust to
|||compensate drag. If thrust exceeds the sum of weight and drag, the
|||aircraft will accellerate upward. If thrust is equal to that sum,
|||the aircraft will continue upward at the same speed. (For the limited
|||heights used by model aircraft, this would result in an "unlimited"
|||vertical climb)
||
|| You forget to mention that the airplane should also have to fly
|| up fast enough to be directionally stable. If your thrust decrea-
|| ses with speed, then the speed attained is less than the theo-
|| retical speed you'd obtain if you did your calculations with static
|| thrust.
|
|If the thrust exceeds the sum of weight and drag, the aircraft
|will not slow down. It will speed up. If the thrust is equal to that
|same sum, the aircraft will not slow down. It will stay at the same speed.
|
|The ONLY condition underwhich the aircraft slows down is when thrust
|is less than the sum of weight and drag. It doesn't matter at that point
|if the aircraft will maintain directional stability or if the dynamic
|thrust is different at different speeds. What matters is the thrust
|available at the point when the aircraft is vertical, versus the weight
|and drag. Everything else is secondary.

Yes you are right. I was simply pointing out that if one wants un-
limited vertical climb the airplane must be directionally stable at
vertical climbing speed. I agree that it is irrelevant to this issue.


|||| Which is why props can act as brakes in a dive.
|||
|||Props act as brakes in a dive because you are changing the angle of
|||attack. As you move the plane faster than the prop, the AOA decreases,
|||hence the induced drag decreases and the torque required decreases,
|||so the prop spins faster. At some point, the engine can't spin any
|||faster since the torque output decreases faster than the requirements.
||
|||At this point, the engine resists further rotation. Even if the props
|||are at a highly negative AOA (which results in a forward (spinward)
|||pointing induced drag component), the engine can't turn any faster.
||
|| Right! And thrust decreases because you are changing the angle of
|| attack.
|
|I believe that is what I said.
|
|| If the prop is actually providing drag (since, as you say, the
|| airspeed is so high that the blade is at negative AOA) then the
|| prop can't be providing thrust, can it.
|
|Sure it can. Just because a wing is at a slightly negative AOA doesn't
|mean it produces no lift. What makes airplanes descend is having less
|lift available than you have wieght. The same principle applies here to
|props. It is the forward-pointing component of lift that cause the prop
|to speed up (think of it as negative induced drag)
|
|| I'd say that before this
|| happens the prop blades operate at ever decreasing angles of at-
|| tack as the airplane speeds up (and, as you mention, the engine
|| cannot produce more RPM). At some point the engine has unloaded
|| to its maximum RPM and beyond this the thrust produced by the
|| prop begins to decrease.
|
|Actually, the total thrust probably decreases long before we get to this
|equalibrium point (where the engine isn't driving the prop, and the prop
|isn't driving the engine).
|
|| I think we're actually saying the same thing, you have simply
|| decided to partition thrust into "theoretical thrust" and "drag"
|| components.
|
|I disagree. You seem to have a fundamental problem with the contribution
|of drag to the flying equation. The drag caused by the production of lift is
|induced drag. The drag endemic to the friction of air is parasitic drag.
|Both contribute greatly to the performance of wings and props.

Actually, I'm very aware of the difference between parasitic and
induced drag, and the fact that drag increases with airspeed. What
I thought we were discussing here was my statement that thrust
produced by a propellor can decrease with speed, which I think
we've more or less proved between the two of us. I'd say that
given a shallow enough pitch the point at which the thrust will
decrease to very low levels will occur at low airspeeds, where
drag of the airframe, while still low (since the speed is low),
will prevent the airplane from speeding up any further.

How would you classify the "drag" of a windmilling prop? I see it
as "negative lift" rather than induced _or_ parasitic drag.






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Iskandar Taib                          | The only thing worse than Peach ala
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