rotor rpm's vs airspeed?

[nacamijdijd]

hello,
i need some information regarding relation of rotor rpm's and airspeed in autorotation. nothing specific, just as a general rule how does rpm's behave in different airspeeds in autorotations.
or, if someone has actual diagram, it would be nice to put it here.

thanx

[Vertical T/O]

Increase IAS and RPM increases. Decrease IAS and RPM Decreases, but by loading the disc in the aft cyclic movement to decrese IAS the RPM increases before it decreases and visa versa. So when you pull aft cyclic to slow down, RPM will increase initially and then as the loading comes off the disc RPM will decrease to lower than it was before the cyclic movement. Presuming no collective input to conteract is used. Conversely by moving cyclic forward you unload the disc leading to decreased coning angle which causes RPM to reduce initially but as the IAS increases so will the RPM.

[Shawn Coyle]

A relatively open ended question - do you mean the effect of airspeed on rotor RPM at constant collective position? Or, the effect of rotor RPM on rate of descent at constant airspeed?
The Bell 206 has a maximum airspeed in autorotation of 100 KIAS because above that, the rotor RPM can decay pretty quickly, assuming the collective is all the way down.

[nacamijdijd]

thanx for replies

this is what i thought:
for certain weight, temperature, density,..., colective is ALL the way down, and CONSTANT attitude is held (constant airspeed), no cyclic movement to play with RPM's.
- airspeed 0 (vertical autorotation), RPM's = X
- airspeed around say 30 Kts, RPM's = Y
- airspeed around the value for best endurance, RPM's = Z
- airspeed around the value for best glide ratio, RPM's = ...
- airspeed rising towards max speed, RPM's = ...

Now, what's the conection between RPM's and constant values of airspeed. and, is it possible to maintain RPM's through the whole range of usable airspeed?

[Aesir]

and, is it possible to maintain RPM's through the whole range of usable airspeed?

No. Like Shawn said some helicopters have airspeed limit in auto due to Nr decay above those speeds. the 206 & 222 do have Vne auto.

[NickLappos]

Having been the guy who chose the Vneauto speed for the S-76 family, I have a vote here. I hate to disagree with Shawn, who is right 99.99% of the time, but....I do not believe that the maximum speed for autorotation has anything to do with rotor rpm decay on entry. I base this on the fact that they cannot have a higher power on Vne unless they show safe auto entry from the higher Vne, as well as the ability to safely and easily transition to the lower Vneauto.

I think the max speed for auto is much more a product of the rule that requires positive longitudinal static stick stability, (a nearly useless property of helicopters, not at all related to their stability). The method of demonstrating LSS in autorotation requires that you must trim at .9 Vneauto and push to 1.1 Vneauto and show that the stick settles forward at the 1.1 Vneauto. For high speed helicopters, this is a big problem. In autorotation, the vast speed increase (almost 30 knots if Vneauto is 150 kts) creates a big increase in rate of descent, which swamps the rotor's natural LSS with a very large fuselage negative contribution. Most helos simply cannot pass the LSS at Vneauto above perhaps 100 to 120 knots.

This makes all manufacturers eager to reduce auto Vne to a much lower and more practical value, or rely on some artificial means to cure the negative LSS at high speed (or seek an equivilent safety ruling.)

Here is my specific answer to the question on the table:

-A. airspeed 0 (vertical autorotation), RPM's = low green, perhaps 90%
-B. airspeed around say 30 Kts, RPM's = low green, slightly above A
-C. airspeed around the value for best endurance, RPM's = highest rpm, about 105%
-D. airspeed around the value for best glide ratio, RPM's = slightly below C, call it 100%
-E1 airspeed rising towards Vneauto RPM's = ever decreasing rpm, down to the low the green at Vneauto call it 90%
-E2 airspeed rising towards Vne power on RPM's = ever decreasing rpm, down to below the green at power on Vne call it 85 to 90%

[nacamijdijd]

nick, thanx for reply, that's what i needed.
while we're there, is it then OK to say that rotor RPM's are safe in speed range from 0 to Vneauto? that is, without too much jibbing cyclic?
also, can it be drawn as a general rule, the value of Vneauto? In terms of high/low rotor RPM speed helicopters, high/low disc loading haelicopters, teetering/articulated/rigid rotor system helicopters,...
nothing precise, just something like in relation to speed of nominal RPM's (V best glide ratio, endurance)for characteristic types. like, its round 1.? (or 2.?) of recomended Vauto.

[Shawn Coyle]

Nick:
206 Vne in auto - It wasn't about decay of rotor RPM in the entry - it was if you went faster than that in auto, it could decay very quickly.

nacamijdijd
'Fraid I don't understand the term 'jibbing cyclic'. Please explain. And the auto RPM range is safe for everything from zero (and backward, if you really want) to Vne auto.

Some helicopters have a Vne auto that is set for reasons the manufacturer will understand, but is not required to give out. I can think of a couple that had a Vne in autorotation of 80 KIAS, for reasons which were never explained....

[nacamijdijd]

Shawn Coyle:'Fraid I don't understand the term 'jibbing cyclic'. Please explain.

sorry, i was refering to cyclic movement for and aft and it's influence to rotor RPM's.

and for those helicopters that had a Vne in autorotation of 80 KIAS, what was their recomended speed for autorotative flight?