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Author Topic: Weathercocking into wind  (Read 607 times)
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Indoorflyer
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« Reply #25 on: June 04, 2019, 08:55:46 PM »

I had an engineering prof who said a wing in controlled flight depended on a delicate balance of "downwash" and "hogwash." Roll Eyes
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Make the same mistake on both sides; nobody will notice...
Prosper
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« Reply #26 on: June 05, 2019, 05:39:10 AM »

Well said Urs. When I was making pendulum aileron setups - in which inertia is just about everything - I thought that if there were no inertia then an aircraft with a given thrust would accelerate or decelerate instantaneously to match any change in the wind. This would mean for example that it wouldn't climb or dive into gusts or lulls. . .to a ground observer it would appear to speed up, slow down, yaw (it wouldn't roll I imagine) all without any vertical disturbance. I know this is ludicrous but it helped me visualise how inertia affects things.

I was thinking it's significant that this into-wind thing occurs when the prop stops and the model glides - a drop in airspeed (makes the model more gust-sensitive until trim speed is regained?), a change in stability (does the 'forward fin' of the prop change its notional area when freewheeling as opposed to being driven, giving the tailfin more authority?). Propwash disappears or nearly so, removing an insulating blanket of air from the tail surfaces thus making them more gust-sensitive. . .

Perhaps when Bill gets back indoors from his 100 test flights he can tell us how the rubber Jungmann is trimmed to fly in still air; left circles relaxing as torque decreases to a right-tending glide?

Stephen.
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Yak 52
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« Reply #27 on: June 05, 2019, 05:47:32 AM »

After launch/takeoff the ground doesn't exist as far as the model is concerned.

That might be true in the textbook world.

Well it depends on level of the textbook! My comment was an over-simplification to make the distinction between an object in physical contact with the ground versus one that is free to rotate about it's centre of mass in flight. It's true that the air is never stable to that degree in reality but it's a valid thought experiment.

It is important to understand this simplified model before moving on to the details of how the ground still influences flight - in much less direct ways. It's the ground's friction that causes wind gradient, it's heat that causes thermals and in fact the ground actually supports the aircraft in flight by means of a large footprint of slight over pressure at ground level Smiley
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