RFH airfoil question

[Helix]

Can someone explain the process behind this statement from the FAA's RFH handbook, page 2-1?

 

"When the center of pressure lifting force is behind the pivot point on a rotor blade, it tends to cause the rotor disc to pitch up. As the angle of attack increases, the center of pressure moves forward. If it moves ahead of the pivot point, the pitch of the rotor disc decreases."

 

I understand how the center of pressure moves forward as the AOA increases, but how does that cause the pitch of the disc to decrease?

[relyon]

"When the center of pressure lifting force is behind the pivot point on a rotor blade, it tends to cause the rotor disc to pitch up. As the angle of attack increases, the center of pressure moves forward. If it moves ahead of the pivot point, the pitch of the rotor disc decreases."

IMHO this passage is wrong. My understanding is that the RFM was written by Jeppesen for the FAA. The writing appears to have been done by persons with less than sufficent understanding of airfoil and/or rotor system [aero]dynamics and the error wasn't caught during editing or the edit wasn't made.

 

There are many reasons for choosing one airfoil as opposed to another. Symmetrical airfoils predominate because they have no pitching moment about the aerodynamic center. Asymmetrical airfoils, while producing lift at zero degrees pitch, have a negative pitching moment about the aerodynamic center. That moment is highest at the tips and produces a torsional stress that must be resisted by the blade span, blade root, and hub structure as well. Many helicopters use a symmetrical airfoil for the main rotor and an asymmetrical one for the tail rotor.

 

Bob

[Helix]

Okay, after MUCH browsing the web I think I found something. A Navy CNATRA training pub describes how the first helicopters to use asymmetrical blades had a habit of breaking blades due to torsional twisting, and excessive nose down attitude of the disc at high speeds because of that twist. Obviously we have overcome that engineering problem with todays materials. Leave it to the FAA to dangle that tidbit of trivia in front of us with no explanation.

[bqmassey]

Sorry for the revive.

 

I just came across this question in a quiz for one of my classes. It's clearly a poorly worded question, but I'm curious now.

 

Why would a shifting center of pressure on the blade, cause a pitch down of the rotor system in the direction of flight?

 

This is my best guess: the retreating blade is flying at a higher angle of attack (dissymmetry of lift->blad flap). Therefore, Higher AoA->forward center of pressure->pitch-up moment on the blade. If that retreating blade finally gives into the stress of that pitching moment, and futher increases it's angle of attack, gyroscopic procession would lift the disc at the back, causing a pitch down in the front of the disc, assuming this is all happening in forward flight.

 

Thoughts?

[Eric Hunt]

The question is totally backwards. If the CP is behind the feather axis of the blade, it will cause the blade to pitch nose down, i.e.decrease pitch angle and angle of attack. This causes big stresses on the rotor hub and grips, which are trying to hold the blade at a fixed angle.

 

That is also why, in the early days, blades were made symmetrical, because the CP stays fairly constantly at the feather axis - not much strength required to hold pitch. Strength = weight. But with the advent of composites, we can now have asymmetric blades, because they are vastly more efficient at producing lift, and we don't need massively heavy grips and hubs to hold the pitch angles.

[bqmassey]

Yea, this is terribly confusing.

 

So, the statement is backwards. Is the opposite still true then? At collective settings that put the CP ahead of the feathering axis, the aircraft will pitch up (assuming high forward speed)?

[iChris]

"When the center of pressure lifting force is behind the pivot point on a rotor blade, it tends to cause the rotor disc to pitch up. As the angle of attack increases, the center of pressure moves forward. If it moves ahead of the pivot point, the pitch of the rotor disc decreases."

 

I understand how the center of pressure moves forward as the AOA increases, but how does that cause the pitch of the disc to decrease?

 

 

As written in Eric Hunt’s post, the statement is totally backwards.

 

Pressure distribution over the blade surface is the source of the aerodynamic pitching moments and the aerodynamic forces. The lift developed by the blades is distributed over the chord, but not uniformly. The Center of Pressure (CP) is where a single force would act producing the same effect as the distributed lift. If the CP were ahead of the Aerodynamic Center (AC), which is normally set along the blades Feather Axis, the couple (pitch up) would tend to twist the blade and increase the angle of attack, making the lift greater and increasing the twist further, Figure [a].

 

The solution universally adopted is to construct helicopter blades so that the AC and Feather Axis of the blade remain ahead of the CP. As Figure shows, this results in a stable blade, since an increase in angle of attack producing more lift tends to generate a couple (pitch down) reducing the angle of attack.

 

The airfoil section selected for a helicopter blade will be a compromise to satisfy a number of conflicting requirements. One of these is a minimal migration of the CP over the normal range of operating conditions so that excessive feathering couples are not fed back into the controls.

 

In a cambered airfoil, the CP moves fore and aft with changes in angle of attack. The downward twist at high speed was enough to twist the blades against the pilot’s efforts on early machines and caused some crashes. For some time, helicopters used little or no camber in the blade section, but subsequently cambered sections were developed which reduced the CP movement. This along with the development of structures with greater torsional stiffness allowed the increase usage of cambered blades, although almost invariably in conjunction with powered controls.

 

The symmetrical airfoil at zero lift has no pitching moment about the AC. This is because upper and lower surface lifts act along the same vertical line. An increase in lift (increase angle of attack) on this airfoil produces no change in this situation; the CP remains fixed at the aerodynamic center, which is at or near the Feather Axis.

 

 

 

Edited October 30, 2012 by iChris