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- Path: sparky!uunet!hayes!bcoleman
- From: bcoleman@hayes.com (Bill Coleman)
- Newsgroups: rec.models.rc
- Subject: Re: Taildraggers taking off (P-factor & Torque)
- Message-ID: <5873.2a8b98d6@hayes.com>
- Date: 14 Aug 92 11:28:22 EDT
- References: <5808.2a6da0b4@hayes.com> <BrxGvA.ML3@usenet.ucs.indiana.edu> <5828.2a758e95@hayes.com> <Bs6r6H.9u4@usenet.ucs.indiana.edu>
- Organization: Hayes Microcomputer Products, Norcross, GA
- Lines: 96
-
- 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
- Principal Software Engineer ! Packet Radio: AA4LR @ W4QO
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