NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/rec/models/rc/2999

Text File | 1992-07-25 | 3.4 KB | 74 lines

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From: [e-mail] (Iskandar Taib)
Subject: Re: Taildraggers taking off (P-factor & Torque)
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Date: Sat, 25 Jul 1992 04:37:58 GMT
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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.


>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.


>> 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.

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. 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.

I think we're actually saying the same thing, you have simply
decided to partition thrust into "theoretical thrust" and "drag"
components.
--
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Iskandar Taib                          | The only thing worse than Peach ala
Internet: [e-mail] |    Frog is Frog ala Peach
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