Saving a stalled system

[niftyben]

Alrighty....

 

In preparing for my CFI I was talking to a non pilot friend about low rpm blade stall. I said it was unrecoverable. They said "Why not just flatten out the blade angle?" I said that the relative air, once the blade is stalled, creates an angle of attack that is too great to allow the stalled air to wash away, even at flat pitch. This creates WAY too much drag, causing the RRPM to slow, causing more drag, etc...

 

I then remembered a video I saw of a helo landing on some kind of military ship. The bird landed, came to flat pitch, then the rotor system bent down. ??? After a little research I found that the bird can push itself into the deck by about 1000 pounds. This must be done by creating a NEGATIVE angle of attack. Hm...

 

SO, would that negative pitch allow some kind of stalled blade recovery?

[Gomer Pylot]

The blades didn't stall because of too much pitch, they stalled because of too little speed. I think the only recovery would be through increasing RPM, and that takes time, thus altitude, of which there may be too little. If you let the blades get so slow they stall, you're SOL.

[Tenacious T]

I always thought the main thing that made it unrecoverable was that after the blades stalled and centrifugal force is lost the blades cone past the critical coning angle and basically just fold up. No recovering from that.

[PhotoFlyer]

The blades didn't stall because of too much pitch, they stalled because of too little speed. I think the only recovery would be through increasing RPM, and that takes time, thus altitude, of which there may be too little. If you let the blades get so slow they stall, you're SOL.

 

An airfoil ONLY stalls because it's angle of attack is too high. A decrease in speed means a higher AOA is needed to maintain altitude. That increases drag, which decreases speed, which means higher AOA. That will keep happening until the blades stall.

 

I always thought the main thing that made it unrecoverable was that after the blades stalled and centrifugal force is lost the blades cone past the critical coning angle and basically just fold up. No recovering from that.

 

I thought the same thing, but I am not 100% sure the blades would fold up. The blades are actually VERY strong, although they would not support the weight of the helicopter without centrifugal force. Recently we cut up some timed out blades. We placed the ends of the blade (300C) on two saw horses with a short section sticking off one horse to be cut off. Out of curiosity I sat on the blade in the middle, thinking it would flex down until it broke or bent. I was surprised when the blade only flexed down about 2 inches, but easily supported my weight (200lbs).

 

Once you exceed a certain coning angle, the amount of power that would be needed to accelerate the blades is greater than the engine can produce, which is why low rotor RPM is unrecoverable. If there was a way to eliminate drag from the relative wind and the large coning angle it might be possible to accelerate the blades again, but then you would have to move the blades back to a positive pitch, which would produce a large amount of drag and slow the rotor again. If it was possible I think we would already see a system for recovery on some helicopters today. Plus, Gomer makes a good point, increasing RPM takes time and at terminal velocity there wouldn't be much time to do it. Especially if you were only at 500 feet.

[Gunner]

Centrifugal force gives the blades rigidity and strength to support the helicopter... Thus with a stalled system you have upward bent blades that don't spin very well.

 

One of the first things NTSB investigators look for is upward bent blades... blade stall.

[Gomer Pylot]

Well, yeah, the AOA gets too high. But this is like the argument about whether attitude or power controls altitude or airspeed. Both control it, it's not one or the other. The AOA only gets too high because the RPM gets too low. Keep the RPM up and the blades will never stall, because the AOA never needs to be that high.

[Marc D]

In looking at low RPM stall accident reports it reveals that when the blades stall(in forward flight), the nose dives and when you counter with aft cyclic, you chop the tailboom off. The blades quickly "blow back" and chop the tailboom due to forward airspeed and lack of centrifigul force.

 

Marc D.

[helo08]

"In looking at low RPM stall accident reports it reveals that when the blades stall(in forward flight), the nose dives and when you counter with aft cyclic, you chop the tailboom off. The blades quickly "blow back" and chop the tailboom due to forward airspeed and lack of centrifigul force. "

 

Wouldn't this be retreating blade stall and not low rotor rpm blade stall? Somebody please correct me if i'm wrong or off topic.

[Bart]

Pretty sure the blades only fold upward if their hinges allow them to. The R-22/44 blades will fold up given a stalled system due to their coning hinges. Don't think any of the Bell or Eurocopter products rotor heads would allow this though (Not sure about Sikorsky, MD, Schweizer, etc..) just depends on the rotor head, and how they hit the ground.

 

I fly r/c helicopters that have a pitch range of +10/-10 degrees. I can enter an autorotation, add about +4 degrees of blade pitch and watch the blades almost stop completely, then slam in full negative pitch (10 degrees), and they will start turning again (provided its coming down at a level attidude and plenty of altitude). I don't think most helicopters have much negative pitch built in, maybe 2-4 degrees and thats probably only at the blade tips because of the washout built into them.

 

The real problem with a stalled rotor system, is that the pilot didn't react quick enough in the first place. If you have enough altitude and/or airspeed, theres no reason (aerodynamically) for the rotor system to stall given the collective is lowered immediately (assuming a correctly freewheeling rotor).

 

In looking at low RPM stall accident reports it reveals that when the blades stall(in forward flight), the nose dives and when you counter with aft cyclic, you chop the tailboom off. The blades quickly "blow back" and chop the tailboom due to forward airspeed and lack of centrifigul force.

 

This could happen any time you have a stalled or slowly rotating rotor and you apply aft cyclic abruptly...in which case you're going to crash anyway and not having a tailboom won't matter much. You might be confusing this with an improper Low G recovery!

 

Remember, in retreating blade stall, only one side of the rotor disk is stalled. Due to gyroscopic precession the disk blows back and instantly lowers the AoA of the retreating blade, thus "unstalling" it....you might actually overspeed it! I haven't been flying that long but I've never heard of a helicopter crashing because of RBS.

 

My2cents.

[RockyMountainPilot]

"In looking at low RPM stall accident reports it reveals that when the blades stall(in forward flight), the nose dives and when you counter with aft cyclic, you chop the tailboom off. The blades quickly "blow back" and chop the tailboom due to forward airspeed and lack of centrifigul force. "

 

Wouldn't this be retreating blade stall and not low rotor rpm blade stall? Somebody please correct me if i'm wrong or off topic.

 

 

When the rotor RPM is decreased, it increases the AOA of the blades. The retreating blade has a higher AOA, therefore it will stall first just like in retreating blade stall. It is retreating blade stall, just at a slower airspeed. Because the blades are not fully rigid from the inertial forces at normal RPM, they can flew much more and frequently will strike the tail boom during blowback.