From: bcoleman@hayes.com (Bill Coleman)
Newsgroups: rec.models.rc
Subject: Re: Taildraggers taking off (P-factor & Torque)
Date: 14 Aug 92 11:28:22 EDT
Organization: Hayes Microcomputer Products, Norcross, GA

In article <Bs6r6H.9u4@usenet.ucs.indiana.edu>, ntaib@silver.ucs.indiana.edu (Iskandar Taib) writes:
> In article <5828.2a758e95@hayes.com> bcoleman@hayes.com (Bill Coleman) writes:
>>In article <BrxGvA.ML3@usenet.ucs.indiana.edu>, ntaib@silver.ucs.indiana.edu (Iskandar Taib) writes:
>>> In article <5808.2a6da0b4@hayes.com> bcoleman@hayes.com (Bill Coleman) writes:
>>>>In article <Brr5K9.MID@usenet.ucs.indiana.edu>, ntaib@silver.ucs.indiana.edu (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. 

Speed is irrelevant. Thrust is the important component. Even if the
aircraft were falling in a tailslide -- if you introduce enough thrust,
you will get an unlimited vertical climb.

> 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.

Low or high-pitch makes no difference. We're talking Thrust here. And not
just static thrust, but the thrust that exists in the conditions of vertical
flight.

Thrust is a combination of engine and prop. And pitch isn't the only 
significant factor in a prop. The diameter, shape of the airfoil and
twist are all significant.

But when speaking of Thrust, we ignore the details of how exactly you 
got that much force for the moment. That's the third part of the problem.
The first problem was, "is an unlimited vertical climb possible?" I believe
I've proven that it is. The second problem would be, "How do I create an
aircraft with an unlimited vertical climb?" The answer is to generate 
enough thrust to overcome weight and drag. How you generate enough
thrust -- that's the third problem. 

Come on, the Space Shuttle demonstrates unlimited vertical climbs. An F15
can climb 30K feet vertically with no problem. Why is it so hard to believe
the same is possible for models? 

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

Good question.

Let's assume there's no torque from the engine at all, so were discussing
aerodymanic drag. 

As the air blows over a still prop, its AOA with the wind causes it to
produce lift. The lift causes it to rotate. As the prop rotates, it is 
moving faster (more lift) but also lowering the angle of attack (less
lift). Further, there's some drag caused by airflow over the prop
(the parisitic drag, which may not be sigificant in this case) and more
importantly, the drag caused by the production of lift (induced drag).

In a windmilling prop, all of these factors have equalised to a steady
state. The prop is still producing lift, since it continues to rotate. If
it were to stop producing lift (perhaps by the application of some magical
lift-destroying laquer) it would eventually stop rotating, since the
effects of parasitic drag would slow it down.

So it isn't negative lift at all, but normal airfoil lift. The difference
here is that the energy to rotate the prop is coming from the wind, not
the engine.

If we add engine drag into the equation, everything is as before, except
that we reach a steady state at a much lower prop speed. (Since more
lift is needed to overcome the drag of the engine) If the engine has too
much drag, the prop will slow down and stop.

Going backwards, if we start the engine, then the engine will introduce a
negative drag to the system, and the prop will speed up to a higher
steady-state speed. And that is where we started.

-- 
Bill Coleman, AA4LR                ! CIS: 76067,2327    AppleLink: D1958
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