RRPM changes with airspeed?
 

Hi Guys,

Can someone explain why rotor rpm increases with forward airspeed and decreases with slower airspeed?

Thanks :confused:

On which aircraft? On many it doesn't increase with airspeed. On some types rpm is increased to delay retreating blade stall.

The usual is that the RRPM remains constant in all phases of flight. In autorotation it is the way around. With increasing TAS, your RRPM will decrease (when collective pitch remains constant).
Maybe you mean the tip speed of the advancing blade. This will increase with increasing airspeed.

cheers eivissa.

Rrpm Up As Ias Up!
 

What RJMP states is absolutley correct - RRPM does increase as TAS increases and vice versa on all helicopters if you do not control it in any other way. It is a simple statement that deserves a simple answer.

As TAS increases the efficiency of the rotor disc increases as translational lift increases which for a fixed power setting requires less pitch and therefore less rotor drag so the RRPM increases.

As TAS decreases the efficiency of the rotor disc decreases as translational lift decreases which for a fixed power setting requires more pitch and therefore more rotor drag so the RRPM decreases.

Simple eh?

Autoranger:D:D

Rotor efficiency vs airspeed
 

For a given collective setting an increase in airspeed increases the rotor efficiency.
If you maintain a constant pitch angle with collective and accelerate to a higher airspeed, the rotor efficiency will increase.
The power required to maintain the collective setting reduces. The RPM will then want to increase due to more power being in use than required.
On a governed machine (R22/44 or turbines) the governer senses the RPM increasing and reduces the power (MAP or TQ) to maintain RPM.
On a non governed machine (H300) the RPM will increase and the throttle will need to be manually reduced.
The opposite occurs if airspeed is reduced due to the rotor efficiency reducing.

If you fix the collective and have manual throttle (i.e. no governing of rotor RPM), and change airspeed the rotor RPM will increase for the following reason.
The power being put into the rotor system is constant, and the increased airspeed will increase the mass flow of air through the rotor disk. The engine is providing power to overcome the drag of the rotor blades, and the increased mass flow of air through the disk will decrease the drag on the blades.
Viewed the other way around, if you have a governor that maintains rotor RPM constant, and hold the collective fixed and increase airspeed, the power to keep the rotor RPM at a constant value will decrease, and you'll see a reduction in manifold pressure or torque.

A further cause for RRPM increase on most types is the offset on the vertical stabiliser which unloads the tail rotor as airspeed increases - assuming the helicopter is flown in balanced (trimmed) flight with a fixed throttle position.

JJ

Thank you for the information.
But why does rotor efficiency increase with forward speed. The rotor disc is tilted forward, so surely some airflow is coming from above, increasing induced flow, effectively pushing lift and total reaction aft?
At slow speeds upto say 50 knots I can see the reduction in Induced drag, being true, but at higher speeds with the disc tilted is the rotor efficiency better?

R22,R44 and B206.

Way back in 1981 with the S76A in the North Sea we experimented with Rrpm and torque. At 100% NR/87% TQ at 5000ft it would achieve 135knots/ 630Lbs/hr with moderate vibration. By beeping up the rotor to 107% and lowering the lever to return to 87% TQ it would cruise at 142 knots/600 lbs/hr with insignificant vibration. Still got the notes from my kneeboard.

FED,

You are not comparing apples with apples there. 87% Q at 100% RRPM (N2) and 107% RRPM (N2) ain't the same. The torquemeter only reads correctly at 100%. By adjusting the RPM and considering that the M in RPM = Minutes and Power (not torque) also involves time you are in fact using more power.

http://upload.wikimedia.org/math/f/a...c5cb056b36.png

The reduction in the fuel flow would be from an increase in efficiency in the power turbine and an increase in efficiency of the gas generator up there where the curve is pretty steep considering the size of the engines. i.e SFC. (Which is why they are small in the first place) :8

As to the original post, never really thought about it as most helicopters have a pretty small RRPM range power on, thats where it stays. An increase in RRPM will occur in increasing forward flight at a fixed power input but as RRPM is usually constant it would actually be a decrease in power. Otherwise we are just talking about the "bucket curve" here aren't we? i.e. in forward flight until drag becomes a factor the rotor is "less inefficient".

I am assuming of course we are not referring to some aircraft where it is adjusted automatically - EC145, EC135 (not all) ...............................

Sure about this? My understanding is that induced drag decreases throughout the speed range of the helicopter, so other things being equal and ungoverned, the RRPM will increase with airspeed. Type shouldn't have any bearing.

Good explanations given by Autoranger, Rotortorque and Shawn Coyle already.

TT

I'm not sure where the term 'rotor efficiency' came from, but it's probably pretty misleading.
Nothing changes about the characteristics of the blades - just the airflow acting on the blades.
If you understand that the engine exists to overcome the drag of the blades, then things become much more clear.
A similar effect happens when the helicopter is accelerated out of the hover- the change in airflow into the disk causes the drag to decrease. In non-governed helicopters, this is seen as an increase in RPM (at a fixed power setting). In governed helicopters, it's seen as a decrease in torque.
But 'efficiency' changes?? I don't think it's the right words.

And then Eurocopter confuses the whole issue further by changing the rotor rpm according to the yaw pedal positions in flight and even ground for noise and "efficiency" as per the EC130. Step on that fenestron pedal in flight and watch the rotor rpm change.

Efficiency................
 

"ell-dee" ring a bell.

Lift/Drag ratio. Total Lift to Total Drag. The curve on a helicopter isn't much different to a F/W, just the scale is different. And no a helicopter cannot fly backwards, its just possible to point the fuselage in the wrong direction.

Lift to Drag is very seldom used in helicopters as the source of lift and drag operates in a totally different manner.
I've never found the ratio to be any use in helicopter terms, but am more than willing to be corrected if someone can show me how it's used!

Helicopter Lift & Drag Values
 

L=1; D=0
L=0; D=9995435234459454354354395.23434

or

If the lift is broken and you have to push the helicopter out of the hanger, the drag factor is large. :8

TT