Autorotation question

[Vanquish]

So recently I've been wondering about auto's.

If, for example, you were to drop a trex 700 out of a real aircraft at lets say 10000 feet, flick throttle hold on and straight away enter a full negative pitch dive all the way to the ground what would happen?

Would the blades reach a certain speed and not go any faster or would the head speed continue to increase until the heli exploded and rained 700 all over the place? either way how fast could the head speed potentially be upon impact with the ground/mid air explosion? Is there a maximum "safe" headspeed heli manufacturers recommend and has anyone knowingly exceeded this?

Not suggesting i'm going to try this just interested...

[StreetDragster]

I believe that whilst the heli is in negative pitch descent the drag of this actually acts as a brake, so you'll reach a certain headspeed where the drag matches the heli's weight and it'll just continue descending at that terminal velocity/headspeed.

Matt

[pchristy]

The headspeed will stabilise at a figure determined by the pitch.

Actually, the rpm *reduces* with more negative, and increases as you move towards positive pitch, when at some point it will start to slow down again.

However, large negative values *accelerate* the headspeed quicker. You use the negative to get the blades spinning, but then reducing the amount of negative will increase the headspeed until you reach a point where you start generating real lift again, and taking energy out of the disc.

Umm - hope that's clear! I can picture it quite clearly - not sure I've explained it that clearly!

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Pete

[Sharpy]

So doing a normal auto with say -6 at bottom stick would it be best to hit the switch, go straight to bottom stick then feed it out.
I tend to hit the switch and feed in the negative.

[Steve]

I've also been lead to believe that -6 deg is about the optimum negative pitch for autos. So, I fly -10 to +10 deg in idel up, and have -6 to +12 in throttle hold. The extra 2 degrees at the top helps you to really slow the blades right down just before you set it down on the ground.

However, if you're intending doing inverted autos, then maybe you should consider -12 to +12 (well, 2 deg more than your flying mode) and then you need to find the + or - 6 deg position within the pitch range rather than at full bottom stick.

[Sharpy]

I was thinking more for speeding the blades up. I have always been told to feed the negative in slowly to control the decent but if I'm reading pete's post right would it not be better to whack in all the negative at the start then feet it out.

[pchristy]

It depends what you are trying to do. Think about negative as the gears in you car: You don't start off in top gear, you start in first, to get the thing moving, but the top speed in 1st is low. Once you've got it moving, you can change up towards top gear (less negative) where the top speed (head speed) is higher.

Reducing the amount of negative will increase the headspeed and reduce the rate of descent.

If you go to negative BEFORE you hit the hold, the motor will actually drive the heli down, and you will be throwing away useful height.

Watch the good F3C guys entering an auto - they will kill the motor whilst still at flying pitch, the smoothly reduce it until they get the rate of descent they want. That way the heli doesn't suddenly lose altitude when entering the auto. Remember, there is nothing so useless as safe height *above* you!

The rate of descent is controlled by varying the pitch. If its not coming down quickly enough (overshooting), you can always reduce it a bit, but few experienced pilots just use full negative all the way down.

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Pete

[Sharpy]

Thanks Pete