Glide angle in autorotation

[Stuck_in_an_ATR]

A plank driver here (though just starting helo training)...

I've been reading helicopter aerodynamics books for a while now and came accross one, which is more for engineers than pilots (quite a lot of maths there etc.) In the chapter on autorotation, the author states that for a typical helicopter, the glide angle in an auto for min sink rate is about 16.6 degrees IRRESPECTIVE OF MASS AND HELICOPTER SIZE/TYPE

The math behind this are the two following formulae:

Vy=0.14*omega*R
wmin=0.04*omega*R,

where:
Vy - autorotation speed for minimum sink rate
wmin - minimum sink rate
omega - rotor angular velocity
R - rotor radius

The author claims that they hold true for ALL conventional helicopters... Simple conversion of these formulae allows to obtain the aforementioned glide angle of 16.6 degrees.

Is that really like that? I buy the "mass" part, as the planks have the same best glide ratio for any mass (though at different speeds), but can't quite understand why it would be the same for different helicopter types :ugh: (have flown fixed-wings with best glide ratio ranging from 4 to 50)

I'm sorry if this question is too "nerdy", but I'm trying hard to understand why and how the helos fly... :}

[RaymondKHessel]

Just looking at the formula, since rotor radius is a variable the glide angle can't be the same for all helicopters. So the author's generalization doesn't seem to be quite correct. That's my interpretation anyway (and keep in mind that I rode the curve through calculus 2).

I'm a little surprised that drag doesn't factor into the equation, and that there isn't some correction for solidity of the rotor disc. What I take from this is that your sink rate increases with an increase in angular velocity (=rRPM, right?), and conversely it decreases with a decrease in rRPM. If you let your rRPM increase, you will sink faster; if you want to make it to a distant emergency landing spot, you want a lower rRPM!

Welcome to helicopters. They be addictive.

[Shawn Coyle]

stuck:
Absolutely not correct that there is one glide angle. Can you tell me what book this is in?
The rate of descent in autorotation depends on a whole list of variables. send me a pm and I'll send you a chapter from Cyclic and Collective that discusses this in much more detail.

[Ascend Charlie]

Watch a B47 in auto, it is almost vertical.

And even at best min descent speed,you can change the glide angle with RRPM, so it would seem that like all generalisations, it is wrong, including this one.:8

[Stuck_in_an_ATR]

Thanks for replies - this theory seemed a bit too general to me too... Out of curiosity - can anyone provide the values of min autorotation sink rate and corresponding airspeed for their helicopter type? I'd like to have a general idea...

The book I quoted was a Polish book "Budowa i pilotaz smiglowcow" by Mr. R. Witkowski - I don't think many PPruners are familiar with this one :) Can anyone recommend other aerodynamics handbooks? (preferrably a bit more nerdy than the "FAA Rotorcraft Handbook" :8)

Cheers,

Stuck

[jayteeto]

Read the question carefully..... I don't know the answer myself, but it says that min sink rate if you fly a certain angle. The ROD and speed will be DIFFERENT at various masses, however it states that at that angle it will be the best you will get?? Might be true, a similar thing is true that all aerofoils stall at the same angle of attack. Boffins work it out for me please.......

[Jim Tierney]

Hi Stuck,

Try either Shawn Coyle or Wal Wagentdonk books!

Both extremely good.

Cheers

JT :ok:

[RaymondKHessel]

The Wagtendonk book fills in all the holes left by the FAA book, plus some. Should be required reading for every student pilot. Coyle's book I haven't looked at cover to cover, but it looks like it goes a step further than Wagtendonk's. You can't go wrong with either of those books.

[topendtorque]

Next time that you drive past a wind mill that either pumps water or generates power, stop, get out and observe.

As the wind increases speed does the mill spin faster or alternatively does its tail swing the mill out of wind to govern the speed? and do you think that there would be more or less reactionary force against the tower with more or less wind?

When you fly a kite does it fly straight up in a light or strong wind or sag limply on the end of the line, either way?

Knowing the wind speed and direction at all times, even when you are sound asleep is a really good idea if you want to survive in a helicopter for long.

Minimum sink rate is GOD. angle of glide may even be backwards in a very strong wind, who bloodycares.

I always believe that the lighter the wind, and thus the further one goes forward, that it is just like a cow walking out to pasture. It takes more power to walk further, think about it.

There is only a certain amount or Kilojoules in a kilogram of grass. A cow loses weight when it walks too far without enough energy to sustain the habit - oh, and the body fat. think about that. A very simple computation.

What is it that powers engine off autorotation? I bet you it aint variable.

So, as you drive, or ride in a train to your school tomorrow, concentrate on the specifics. Do you have enough lunch money or will you eat less today?
cheers tet

ps, well you asked the bloody question, not me.

[n5296s]

I'm just a bit beyond the same stage you are (late in PPL training hoping to do the checkride in the next month or two), but like you I've read a bunch of stuff beyond the basics about how these things actually work.

Clearly you can vary the landing point, but that involves NOT flying at Vy. Glide ratio is (speed)/(sink rate) so anything that affects both equally (like R and omega in the formulae you quote) cancels out - insofar as those formulae go at least.

I suspect that the formulae oversimplify - if they WERE true, so would the result be (for the BEST glide slope). But they don't seem to take into account parasitic drag, which presumably varies quite a bit from one type to another (think Pitts or Stearman to high-performance glider).

I've watched my instructor fly two autos in succession, one at 20 KIAS (with a strong headwind and we were going backwards) and 100% rotor RPM, and one at 90 KIAS and 90.001% rotor RPM (R44) and the results are VERY different, that much I know. 90 knots is way above Vy but clearly gave much better results (assuming you WANT to go a long way of course).

Personally I'm at the point where I can just about nail the numbers for a "textbook" auto but I'm not encouraged to mess with things. That comes later assuming I can afford to keep flying the thing! Though I did do one "maximum range", I was somewhat terrified with the rotor horn blaring and the needle hovering (of course - this was a heli after all) on the lower redline, but it did work.

n5296s / n9888s

[Gomer Pylot]

All this theory is fine, but in the end, you have to make the helicopter do what you need it to do in order to make a successful landing. The airspeed, Nr, theoretical glide angle, etc don't matter at all (within limits, of course), you just use the speed and collective you need to get it to a safe area. Doing textbook autos to a runway do not even begin to make you a proficient or safe helicopter pilot. Get a proficient instructor with a few thousand hours to show you how things work. You can enter an auto from exactly the same point, same altitude, same airspeed, and put the helicopter on almost either end of a runway by varying the airspeed and Nr.