PPRuNe Forums - Maximum speed

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Hi,

I'd like to ask what happens as a helicopter approaches its maximum speed at level flight. What observations would we make if we increase the speed gradually towards maximum?

Specifically:
- Engine RPM
- Lift and drag forces acting on the main rotor blades
- Helicopter's attitude to maintain level flight ( How should pitch and roll change )
- Flight qualities (i.e does it tend to roll, pitch etc. )
- Vibrations and sounds

Nickel

suggest sticking "retreating blade stall" into a browser

In level flight I would say nothing else but an increase in ‘windscreen noise’.

As you approach VNE at maximum power it is my experience that the significant give-away is the nose attitude, which is well below the horizon.

German Version of Wikipedia has a good description for afore mentioned "retreating blade stall" (my investigative eye tell´s me you can read that too... :hmm::suspect:):

Quote:

Hubschrauber erreichen prinzipiell nicht die Flugleistungen von Starrflügelflugzeugen:
Die Höchstgeschwindigkeit liegt meist zwischen 200 und 300 km/h, einige Kampfhubschrauber erreichen über 360 km/h. Der Geschwindigkeits-Rekord liegt bei 400,87 km/h und wurde am 11. August 1986 mit einem Westland Lynx erzielt.
Die Höchstgeschwindigkeit wird dabei durch die Aerodynamik der Rotorblätter begrenzt: Das jeweils nach vorne laufende Blatt hat gegenüber der von vorn anströmenden Luft eine höhere Geschwindigkeit als das nach hinten laufende. Nähert sich nun das vorlaufende Blatt im Außenbereich der Schallgeschwindigkeit, kommt es dort zu Effekten wie Abfall des Auftriebs, starke Erhöhung des Widerstands und große Blattbeanspruchung durch Torsionsmomente. Für den Piloten äußert sich dies zum Beispiel in starken Schwingungen, die die Kontrolle des Hubschraubers erschweren.
Meist wird die Geschwindigkeit eines Hubschraubers jedoch durch das rücklaufende Rotorblatt begrenzt: Hier führt die Kombination aus hohem Anstellwinkel (zyklische Verstellung, s. o.) und geringerer Strömungsgeschwindigkeit beim Rücklauf zum Strömungsabriss und damit Auftriebsverlust. Viele Hubschrauber kippen daher zuerst in Richtung zum rücklaufenden Rotorblatt, bevor das vorlaufende Blatt in den Überschallbereich gelangt.

Or - free to download from the faa website - out of the "Rotorcraft Flying Handbook":

Quote:

Retreating Blade Stall

In forward flight, the relative airflow through the main rotor disc is different on the advancing and retreating side. The relative airflow over the advancing side is higher due to the forward speed of the helicopter, while the relative airflow on the retreating side is lower. This dissymmetry of lift increases as forward speed increases.

To generate the same amount of lift across the rotor disc, the advancing blade flaps up while the retreating blade flaps down. This causes the angle of attack to decrease on the advancing blade, which reduces lift, and increase on the retreating blade, which increases lift. As the forward speed increases, at some point the low blade speed on the retreating blade, together with its high angle of attack, causes a loss of lift (stall).

Retreating blade stall is a major factor in limiting helicopter’s top forward speed (VNE) and can be felt developing by a low frequency vibration, pitching up of the nose, and a roll in the direction of the retreating blade. High weight, low rotor r.p.m., high density altitude, turbulence and/or steep, abrupt turns are all conducive

to retreating blade stall at high forward airspeeds. As altitude is increased, higher blade angles are required to maintain lift at a given airspeed. Thus, retreating blade stall is encountered at a lower forward airspeed at altitude. Most manufacturers publish charts and graphs showing a VNE decrease with altitude.

You might want to read up on "dissymentrie of lift", that makes it a bit more clear!

Quote:

In level flight I would say nothing else but an increase in ‘windscreen noise’.
As you approach VNE at maximum power it is my experience that the significant give-away is the nose attitude, which is well below the horizon.

The nose on the type I fly stays nicely on the horizon even at Vne, 168 kts, which can be reached in level flight at about 85% or less matched torques. The first indication the pilot gets that he has reached a speed 6 kts below Vne (or hit a gust at normal level cruise speed) is is a woman's loud voice announcing "Airspeed...AIRSPEEEEED" and the knowledge that she has made the DAU log an exceedance "against" the pilot, even if he never reached Vne. Very annoying, a lying female spy in the sky!

ShyTorque:
Thanks this was what I wanted to know. From what you say I deduce that you do not see any reduction in rotor RPM also.

Are you flying an especially powerful machine or can we say that in helicopters this is generally so?

And, if you know, what would happen if you exceed Vne? Do you start to lose performance slowly ( loss in RPM and/or lift etc...) or would the helicopter get uncontrollable before you notice any performance loss?

Retreating blade stall

Nickel

Phill77's reference to Wiki gives a very precise account.

Low rpm only comes from not enough engine power.
Retreating blade stall comes from both lots of rotor power (and so engine power) and lots of speed.
So the only correlation between low RPM and retreating blade stall is that both are influenced by the power factor.

Constructors VNE kind of takes worst case of all parameters (density, TOW, speed)
The R44 states 130 VNE, but in many practical flight conditions retreating blade stall will not be significant until 150-160 knts. In all my experience MR rpm is the most significant factor : a few % can make a big difference (for instance using 95% instead of 102% will create retreating blade stall in many normal R44 flight conditions)

Retreating blade stall is by the way in fact "retreating blade tip stall", because close to the hub the blade always stalls at some point in forward flight.

d3

On turbine powered helicopters the rotor RPM is automatically taken care of by the engine computer; if there was an increase in rotor drag this would result in the torque increasing at a constant collective pitch setting, rather than the rotor RPM decreasing.

From my own experience, it doesn't happen when operating below Vne, as I always do.

Retreating blade stall isn't the only issue beyond Vne. The phenomena is usually preceded by suddenly increasing vibration levels, which do the aircraft harm over a period of time. Vne is set to take this into account. A helicopter designer calculates the strength of his airframe and components to cater for vibrations caused by time spent in the hover, climb, cruise, descent, autorotation and high and low speed flight. He assumes percentages of total flight time that the average aircraft will spend at each stage of flight.

To operate outside the design envelope, or spend long periods close to the edges of it is therefore unwise in the long term as well as in the short term.

Retreating blade stall and blade design

Sorry to move the thread away slightly from where it started but I have a question.

Main rotor blade design seems not to have changed much for many years - ie blade section is similar throughout its length.

Some Fixed wing modern props have very different profiles to those seen in years gone by - we are told for improved efficiency

Why have heli main rotor blades not seen this kind of change ? and what would a blade whose width is wide at the hub and narrow at the tip do to performance ?

thanks for any intelligent answers ! (plus funny ones)

richard

Richard,
The main reason is that a rotor blade spends much of its time flying sideways, when compared to a prop. Rotor design engineers study rotor performance in the flight regimes that ShyTorque has already mentioned (in that case it was fatigue life evaluation). Each design point (eg speed) has an optimum design, with hover looking very much like the prop you mention. Cruise requires a combination of good tip stall performance for the retreating side and good flutter stability in general. The latest machines now also have anhedral tips.

This is a complicated area, so i would start by reading Prouty "Helicopter Aerodynamics" from HeloBooks. Delta 3, has done some rotor simulation work, so is also a knowledgable chap.

Quote "Why have heli main rotor blades not seen this kind of change"

Lost again

Look at some recent published changes in rotor upgrades and you will find improvements of up to 15% (load/efficiency/speed)

By compairison : Some winglet addons speak of 3-5% improvements (cruise efficiency, MTOW etc).

So in my opinion it is not all that bad, my R44 many times cruises higher and faster than a PA-28 that I am overtaking.

It is fair to state that due to the complexity of rotor-wing design, optimisation is quite difficult because all the different constraints that have to be met, even fair that we may be close to max keeping the same architectural choices. As you will see on different threads on this forum major break through will probably come from different designs.

Skytorque

True but the published enveloppe is mostly simplified-conservative.
My R44 sim predicts quite well retreating blade stall, and there are many regions outside the published envelop that will not create problems, altough I am not advocating here to ignore the published envelop for the reasons you quote.

Conversely the R44 rotor blade problems in the Andes (high/heavy/fast) probalably were within the published enveloppe, but the systematic hitting of one of the boundaries can create fatique problems.

A very practical limit (irrespective of envelops) : vibrations are a very good health sensor I think, even if one theoretically would be within the enveloppe.

d3

Delta 3, I agree. So, you think the R-44 designer didn't get it 100% correct?

But it's SHytorque, btw :)

Max Speed

ShyTorque (sorry about the sky..)

I am not saying they got it wrong. I am just saying that very many parameters come into play. Hitting once in a while 140, while flying most of the time 115 will be better than flying 130 all the time.. But that would be difficult and dangerous to put in the manual wouldn't it. After all fatigue comes from a overall mix in usage and pre-scribing a mix is difficult.

From practical experience I can see 10-15 knts difference between well balanced/paired and super clean blades as opposed to not so well balanced/paired/clean blades. So that would enter my personal limitations : if the thing is not well balanced I would not fly 130, let alone faster and by the way you would certainly feel it.

Now some results (be it on the edge of the precision, because I think in this regime aero-elasticity starts playing a big role (and would push stall further than in my calculations.
The most significant curve is the so-called 8-curve where for one full rotation the angle of attack is plotted versus mach and compaired to the stall curve (with prandle like correction)

1. Average weight R44-I at VNE :

http://www.portmyfolio.com/prive/heli/Level%20130.JPG

2. Heavier : less margin:

http://www.portmyfolio.com/prive/hel...30%20heavy.JPG

3. Going to fast, in descent : still OK according to the sim (NOT ADVOCATING TO DO THIS!):

http://www.portmyfolio.com/prive/heli/Descend%20140.JPG

4. 5% lower MR rpm : BIG difference, so if by over powering the engine, RPM would drop off then you are in bad shape. This for me the big difference between R44-I and II in fast/high : The R44-I runs out of steam and well with the envelop limits the RPM can drop off what is dangerous):

http://www.portmyfolio.com/prive/hel...30%2095pct.JPG

d3

I'm sure you're right. ;)

Shytorque

let me try it this way :

High speed -> close to Retreating blade stall but OK,

Trying to go faster -> progressive increasing danger for stalling

But if by doing so we overpower the engine and let RPM drop -> DANGER ZONE

d3

Yes, I don't fly the R-44 but I can certainly follow that.

But why would anyone in his right mind do it? Surely, compliance with the RFM limitations would ensure this didn't happen?

Well the answer is NO in mho !.

I fly very often high and fast (over the alps FL 85 -105)

A R44-I starts running out of steam, the manual allows you to pull full trottle, but raising the collective at some point doesn't help because the engine can't follow. As long as you would keep RPM you are OK, but the limitations don't take the RPM drop into account (an omission I think)

The R44-II doesn't suffer from this power hole.

d3

Other Factors;

Limit of Cyclic Movement - Fairly self explanatory you will literally reach the stops.

Compressability of the advanciing side - I am not clever enough to explain that

Structural Integrity - The designers say it will break up at "x" speed, this you will notice:)

The most recent improvement I am aware of, prepared for correction, is that of the British Experimental Rotor Program. The paddle at the end of the blade seems to delay the tip reaching the speed of sound due to the increase in surface area, therefore allowing the advancing blade/helicopter to go faster.

Due to the cr@p that the mill and police strap onto their aircraft I have never been able to achieve VNE straight and level. We live in hope.:cool:

D3, that simulation is truly excellent! I hope it will be wielded on a regular basis to provide that extra level of technical expertise. :ok:
(I can't remember if you posted a link)

If your Prandtl-Glauert adjusted AOA max is based on static Cl plots, then retreating stall will in practice be much higher. This is because when the blade/wing stalls there will be a seperation vortex as the rotation is lost (opposite of the starting vortex on a fixed wing takeoff). This seperation vortex produces high speeds over the blade while being shed, so that transient lift is produced for a while. I can't describe this in more detail, but i gather that the vortex shedding can provoke flutter if the blade natural frequencies aren't tuned. As you say, vibration is bad.

But either way torque goes up, so power required goes up. The machine cannot achieve level flight. I can imagine Nr droop as you comment.

As FayeDeck indirectly comments, in a teetering rotor retreating stall would cause massive flapback to limit speed. I imagine this happens beyond Vne, so should remain a theoretical discussion. If Nr droop caused the retreating stall you suggest, wouldn't that just limit speed?