Comments on: Speed Limits – Part 1

By: George Carey

George Carey — Thu, 02 Jul 2009 20:36:53 +0000

Thanks for the note Ehud. What you say makes sense but what if the main rotor RPM was increased? For a given forward airspeed would this not increase the speed of the retreating blade further above it’s stall speed? You could then increase the forward airspeed by that amount. Of course then the advancing blade speed would also be higher and limited by approaching the speed of sound. This is why it seems that main rotor RPM is set to give you a happy medium between the retreating blade stall speed and the advancing blade reaching the speed of sound.

I remember Airwolf but it has been a long time!

George

By: Ehud Gavron

Ehud Gavron — Thu, 02 Jul 2009 20:10:21 +0000

Yes, but that isn’t as big a factor as it might seem. For example, on the R22′s high-speed rotor the tip-speed of 672FPS works out to (x3600/5280) 458MPH. That’s 300MPH slower than the speed of sound at sea level (760MPH).

Thus if this were a primary factor, it wouldn’t come into play until a forward airspeed of 300MPH (260 knots). Instead the forward airspeed is truly limited more by retreating blade stall.

Of course Airwolf had both bases covered… “Give me both turbos, Dom!”

Ehud

By: George Carey

George Carey — Thu, 02 Jul 2009 15:44:48 +0000

I understand the limitation of forward flight being due to the stall speed of the retreating blade. Is there not also a limitation of the advancing blade approaching the speed of sound? Is the limitation a tradeoff between these two speeds? If it were just the stall speed of the retreating blade you could increase the Nr rotor speed. If you do that though then you increase the advancing blade speed toward the speed of sound.

By: Tim McAdams

Tim McAdams — Mon, 29 Jun 2009 15:00:34 +0000

Thanks Kurt and everyone else who have contributed their thoughts and ideas. I like to put a topic out there and have readers who really have a good understanding of the subject provide comments. I think we all learn a lot that way.

By: Kurt McKibben

Kurt McKibben — Mon, 29 Jun 2009 02:56:04 +0000

Thank you Tim McAdams!! Exelent discussion!!! I learned the CL: coefficient of lift is a combination of angle of attack AND blade shape.

I wish I could write as well as you do!! Thanks again

Kurt McKibben
CFI, Rotorcraft, Helicopters

By: Kurt McKibben

Kurt McKibben — Mon, 29 Jun 2009 02:47:45 +0000

Oh, by the way Dan, now that you have my attention, the CG moves constantly through out all flights as fuel burns out of the tanks. We figure CG with full fuel and with empty fuel before takoff to assure we can be within CG limits the entire flight for a possible autorotation anytime if an emergency arises.

For CFI’s there is an FOI ( Fundamendals Of Instruction) FAA test question that states how important it is to learn the correct information in the first place. This is refered to as “Primacy.” It means the first thing a person learns are remembered best. To learn something wrong first, means you have to go back, do unlearning, and break bad habits before you learn the correct information. It is difficult to break bad habits and relearn information that was false.

No wonder flight instruction is so expensive!!!!!

Kurt McKibben
CFI Rotorcraft, Helicopters

By: Kurt McKibben

Kurt McKibben — Mon, 29 Jun 2009 02:27:48 +0000

Hello folks,

Maybe we should write something about cyclic feathering to differentiate flapping and feathering. The collective controls pitch angle of all blades equally, via pitch horns. The cyclic controls pitch angles of each blade individually, via the swash plate. Flapping is an aerodynamically forced movement on the blades, via the physics of the lift equation. You build a rotor system to accommodate flapping no matter what because it’s physics you can’t deny. For anyone to say they can see blade flapping with the naked eye is nuts. The blade is flapping up and down so fast you can’t see it happen. It’s very small, and is just enough to equalize lift across the disc. You don’t see this with the naked eye, and the plane of rotation still resembles the horizon. THIS IS ALL CHALK BOARD TALK TO EXPLAIN THE PHYSICS for the correction of dissymmetry of lift.

Flying behind in a line of choppers and viewing different pitch angles from one side of the rotor disc to the other side is just viewing a function of cyclic input to maintain a given direction of flight. IE. forward, backward, sideways. If those same helicopters made a right turn those blades would change again side to side from a new cyclic input to turn right, via the swash plate.

You were looking at a cyclic control setting to create the blade configuration necessary to maintain air speed in forward flight.

Zen,,,,,,,,,,,,,,,,,,

Kurt McKibben

By: Dan Olson

Dan Olson — Sat, 20 Jun 2009 04:04:17 +0000

Check You Tube for videos of helicopters. One in particular is “NH-90 NATO Frigate Helicopter”. This shows a helicopter in forward flight approaching a ship to land while the ship is under way at 25 knots. The rotor is horizontal, not higher on the advancing side.
Dan

By: Dan Olson

Dan Olson — Sat, 20 Jun 2009 03:42:21 +0000

Ehud, I suggest you watch the rotor blades from behind during forward flight. You will see that both the advancing blade and the retreating blade are at the same height. I have been the tail end charlie of a flight of Hueys in trail. All the rotors are horizontal except for coning of about 3 or 4 degrees. You can also see that the advancing blade is at a reduced angle of attack while the retreating blade is at an increased angle of attack. The limit of this is when the retreating blade reaches its critical angle of attack and stalls. This is “retreating blade stall” and is one of the factors that sets Vne (never exceed speed) for a helicopter.
And the CG does not move. The CG is the center of gravity. This will remain fixed unless some internal load is moved about. If the center of LIFT were to move as a result of a pitch change from the swashplate, then the rotor would tilt. When the rotor tilts, the overall lift vector of the rotor tilts and results in translation or banking in that direction.
In steady flight the lift vector is ALWAYS perpendicular to the rotor. In steady forward flight the rotor is tilted forward to provide a forward component to equal the drag of the fuselage. In steady turning flight the rotor is tilted in the direction of the turn and the fuselage swings out directly under the rotor from the centrifugal force of the turn.
Thanks again,
Dan

By: Ehud Gavron

Ehud Gavron — Sun, 14 Jun 2009 01:19:56 +0000

Dan, the swashplate increases the angle of the advancing blade and equally decreases the angle of the retreating blade, but this in and of itself is not sufficient to change the relative lift of the two blades and the dyssymetry of lift.

Flapping means the advancing blade flaps up, moving the CG inward and compensating for the blade not being able to “lag” as if it had a lead-lag hinge on a fully-articulated rotor-system. The retreating blade flaps down, moving the CG outward, and therefore producing more lift without needing to “lead”.

The flapping hinges do not cause the “helicopter to be freely suspended from the rotor system”. The underslung rotor system does that without regard for a flapping hinge.

The swashplate does not equalize lift. It provides some of the input necessary to this — the flapping of the rotor blades does the rest.

I am not a Viet Nam era UH-1 crew chief and have respect for all of you who risked your lives for our country flying over dangerous enemy territory.

Respectfully,

Ehud