From: bcoleman@hayes.com (Bill Coleman)
Newsgroups: rec.models.rc
Subject: Re: Taildraggers taking off (P-factor & Torque)
Date: 14 Aug 92 10:33:28 EDT
Organization: Hayes Microcomputer Products, Norcross, GA

In article <1992Jul29.192503.9464@bmerh85.bnr.ca>, mkfeil@bcrki9.bnr.ca () writes:
> In article <5828.2a758e95@hayes.com> bcoleman@hayes.com (Bill Coleman) writes:
>>Read the following again, Iskander. If the thrust exceeds the total
>>of weight and drag, it will not slacken off in speed.
> 
> All Iskandar is saying is that a propeller is not a thrust machine. It
> creates LIFT, and this lift decreases as the plane's airspeed increases since
> angle of attack decreases. 

You aren't paying attention.

In the section quoted above, the airplane is in a vertical position. The
three applicable forces are:

  THRUST - with a vector pointing upwards
  WEIGHT - with a vector pointing downwards 
  DRAG   - with a vector point downwards

Those are are the forces we consider here. Read the quote again. If
Thrust is GREATER than the sum of Weight and Drag, the airplane will NOT
slow down. The airspeed will not slow down. The AOA will not change because
the plane will not slow down, and the lift will not decrease.

This is basic physics. Indeed, if the sum of the three vectors points upward,
the aircraft will ACCELERATE upwards, not slow down.

Note again, we are talking about vertical climbs here, not normal level
flight.

Note also that the thrust must execeed the SUM of BOTH Weight and Drag. If
Thrust merely equals Weight, then the Drag vector remains, meaning the
aircraft will decellerate and must eventually return.

> It follows that a greater than 1:1 static thrust
> may not translate into unlimited vertical ...

*I* *NEVER* *SAID* *THAT*

I said, as I said again above that Thrust must be equal (or greater) than
the sum of both Weight and Drag in order to affect an unlimited vertical
climb.

> As an example, consider a VERY large diameter, VERY low
> pitch prop. This prop will have high static thrust since it moves a lot of
> air, but long before form drag on the plane becomes significant, the maximum
> speed of air moving through the prop will be reached. If this is too slow to
> keep the plane flying properly, then it will fall off to one side.

Guys, this "slowing down" bit occurs not because of the Prop or the engine,
but because there's insufficient thrust to overcome both the weight and
drag of the aircraft.

Consider a helicopter. It has a very large, very low pitch prop. Yet 
helicopters can climb vertically (practically, you can't see that high)
without limits. Thrust exceeds weight and drag.

>>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 don't think this is right. AOA and lift are defined relative to the air
> stream. If there is less lift than weight, then the airplane will accelerate
> downward. 

Right, this is what I meant. Sorry I wasn't clear

> If lift is equal to weight, the airplane will continue doing what it
> was doing, i.e. either climbing steadily, flying level, or descending in a
> steady glide. 

Yup. Basic physics.

> At zero AOA the plane is in free fall.  

Not at all. Zero AOA merely means the relative wind is coming straight on
to the chord line of the wing. Depending on the shape of the wing, it
could mean that the wing is producing considerable lift, or even producing
a negative lift component. (Consider the case of a flat-bottom airfoil
flying inverted at a zero AOA)

By "free fall" you mean that the wing produces no net lift. That might
be true of symmetric airfoils. But, we were discussing props.

Getting back to props, my original posting was prompted by Iskanders 
assertion that prop blades can stall. I think I've adequately demonstrated
that props don't stall in normal use. Unless you intentionally over-pitch
a prop, the AOA of a prop from stopped to flying decreases, not increases.
Since you can't increase the prop AOA from the static condition without
moving the aircraft backwards, a prop that is unstalled in a static test
won't stall in flight. (Tailslides excluded)

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