Chuck Beaty--chord balance

[C. Beaty]

The author of that Army manual didn’t have a good understanding of his subject.

He didn’t appear to understand that with zero pitching moment coefficient, center of pressure and aerodynamic center coincide and stay coincident regardless of angle of attack and airspeed, within normal operating bounds.

Manuals of that sort are typically written by subcontractors. Some are better than others.

As transonic flow conditions are approached, pitching moment can become problematic.

[bryancobb]

How about the most recent Issue of The FAA Rotorcraft Flying Handbook and the questions on the FAA Knowledge Test all the way up to the ATP Knowledge Level? Was this text also written by people without a thorough understanding?

[bryancobb]

Here's what the FAA book says about why blades should be twisted. Same thing I have always been taught.

[Rotor Rooter]

Bryan

The bottom of the web page DESIGN: Electrotor-SloMo - Rotorhead - Blade - Root Twist - Variable Undercamber may, or may not , be of interest.

The NVFoil screen on the right is of an undercambered airfoil that was originally being considered for the root end of the blades on an Advancing Blade Concept helicopter, which are subjected to reverse velocity.


A modified version of it is now being considered for the root end of a rotor blade for the backpack helicopter. The undercamber is only good for low Reynold's numbers but the root of the blade is experiencing a low velocity at near-hover flight. It is intended that the profile will progressively reshape to a VR-7 at the blade tip. There are significant lift/drag advantages to cambered airfoils, but there are also other pros & cons to be considered.


The above is in agreement with Chuck's remarks, I think.


Dave

[C. Beaty]

I happen to have interfaced a bit with the preparation of the FAA rotorcraft manual.

Jeppesen was the contractor and the tech writer preparing the gyroplane section was very conscientious but if he'd ever been up close to a rotary wing aircraft was not obvious.

Most of the input came from Bill Clem but I spoke with the tech writer on a number of occasions.

A lot of the stuff is old boilerplate that keeps on getting recycled ad infinitum, ie., center of pressure travel.

Use all the junk you can when being paid by the page.

[bryancobb]

OK Chuck.

I will 100% accept that, being that you have intimate knowledge of how my highlighted errors made it into a several published reference texts. It is incorrect and reflects an ignorant view. It just SHOCKS me that incorrect boilerplate information would make it into the published version of US Government printed books, because it's more profitable if you are being paid by the page!!! If I were the person deciding what would be included or not, I would scrutinize every sentence for correctness before it got distributed to tens of thousands of helicopter and gyro pilots, and used to train Military aviators in numerous countries.

It should be flushed from the system and replaced with the correct information ASAP.

Would a corrected text say that untwisted blades are fine for helicopters, especially since they are a lot easier to manufacture?
Would it also say that reflexed semi-symmetrical blades are significantly superior to symmetrical ones for helicopters?

[C. Beaty]

Of course the “center of pressure” shifts vs. angle of attack with cambered airfoils. The point missed is that it does not with properly reflexed cambered rotorblades.

“Center of Pressure” shift is a red herring that does not belong in helicopter technical publications. It’s simply a space filler for publications written by contractors.

The first thorough textbook on helicopter theory was Aerodynamics of the Helicopter by Gessow and Myers. Not one word about center of pressure.

A more recent and comprehensive text is Helicopter Theory by Wayne Johnson. I’ve been unable to find a single word in all the 1089 pages about center of pressure.

Center of pressure is irrelevant simply because helicopter rotors must employ airfoils with nearly zero pitching moment coefficient, whether cambered or symmetrical.

[bryancobb]

Problem is, Neither of those texts you highlight are reference publications for the hundreds or thousands of flight schools around the world! The FAA test question writers probably have never heard of them. These books are probably written and used in academia to educate aerospace engineering students on theory. My guess is these authors have never even sat in a helicopter but that's purely a guess.

[bryancobb]

OK Back to the TOPIC OF THIS THREAD.

If Stan chordwise static balanced his untwisted Helicycle blade on his fulcrum blocks, discuss a little bit, the relationship of that distance from leading edge, to the feather axis from the leading edge, and to the aerodynamic center from the leading edge.

Please comment on the stability of the blades, the load or lack of load on the collective and pitch links, and generally the likelihood of divergent pitching "flutter" in flight.

[StanFoster]

I will repost the data from my post #1. The chord width of my blades is 7.25 inches. It balanced exactly 2.0 inches from the leading edge. That is 27.58% of the chord width. I have always read and even Chuck has mentioned that typically the balance point is 25% of chord. That is what initiated this thread directed to Chuck my concern over a slightly tail heavy blade. Had I painted my blades, the balance point would have been further aft................................Now, one point I never mentioned is that we adjust our drag links so as to sweep the blades forward till a 3 pound weight hung from the pitch horn balances the blade chordwise. This moves the CG forward enough to have the aerodynamic forces cause a slight nosedown tendency. This creates a down force on the collective. You do not want an upforce on the collective as some helicycle pilots have incorrectly had. The pitch rods should be in compression. Again, this how I understand the correct setup. I have had around 260 individual flights since my first fight September 1st, 2010. I have performed autos many times, and countless quick stops from plain to insane, and never any vibs, shakes or anything. I always ease off the collective and never ram it down. I also add back cyclic as I lower the collective so as to prevent aerodynamic precession from dropping the rotor at 12:00. This is mentioned as I am trying to minimize the time the pitch links are in between comression and tension. My understanding is that loose joints in combination with an improperly rigged roror could contribute to flutter. Stan

[bryancobb]

Stan,
I agree a down force on the collective is MUUUUUCH safer than a rising collective.
Doesn't the down force fatigue you on long flights? On the Brantly, the blades' trailing edge was bent at the factory to a point so the collective would almost stay anywhere you put it with NO FRICTION.

I flew a friend's Brantly once that you had to push down on the collective the whole time. It was awful. I told him he was going to get killed if he didn't fix that. He tabbed them IAW factory verbal guidance and got it right.

[StanFoster]

Bryan- I fly 121 miles to Mentone, and just tighten the friction on my collective where it "almost" holds it. I have flown the whole distance without the friction on, and its not that much down force. With the friction on, I can still perform an auto ok. Stan

[HobbyCAD]

Stan, tell me to move out of your thread if you wish me to, but I have a related question.

Bryan, why do we have the massive spring bias in the Mini-500 collective system, that springloads the collective to full up? What force counteracts the springs, to obtain the collective "feel" in flight? Is it the massive lead weight inside the end of the collective grip?

To the group, how would one need to statically set up your collective, should it be a perfectly balanced collective load, i.e. it does not drop by it's own weight, nor move up if no friction is applied?

[bryancobb]

The Rotax 582 had 3 springs that the collective must compress. The throttle piston on both carbs plus the oil injector lever. The weight on the collective was to cancel out those 3 springs.

Dennis engineered the Mini to have the correct "feel" on the collective and there was no way to "tab" the composite blades to change it. (Very latest S/N blades had a tab to make tiny tracking adjustments in cruise but that didn't significantly affect collective feel)

[StanFoster]

Francois- Please feel free to engage any related topic here! My collective has 2 springs to help reduce the download on the collective. Its not that much and the only time I put any collective friction on is on a long cross country or when wanting to free up my left hand to take a picture. Stan