WaspAir
Supreme Allied Gyro CFI
- Joined
- Oct 21, 2006
- Messages
- 6,796
- Location
- Colorado front range
- Aircraft
- Bell 47G-3B-1, A&S 18A, Phoebus C, SGS 1-26A, etc.
- Total Flight Time
- rather a lot
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The only fix I could see that could be done to existing blades would be possibly a trim tab. The fix that Chuck suggested would be a complete change in construction to achieve quarter cord balance.Sorry guys, I arrived late to the party. Referencing back to posts 1, 6, & 7, I fly with the aforementioned set of “red blades” except mine are white and 29 ft. diameter. Having experienced this uncommanded pitch up as described I am interested in better understanding of the phenomenon and future avoidance. My understanding thus far is that this is not caused by retreating blade stall and instead the lack of torsional rigidty allowing the advancing blade to twist to a greater AOA when speed increases. The rotor rpm is kinda low (290-310) and I theorize that maybe more revs might make things more stable. I also have a set of 28’s that have not yet flown so will be trying those this Spring. Being on an open frame gyro it gets uncomfortable over about 65 knots anyway so Having divergence at 75 is not too bad, Mostly I guess it would be nice to have a bit more stable ride when experiencing convection. Mike mentioned that Chuck had suggested a possible fix. Do you recall what that was and would it be retrofittable.
Is it quarter cord balance or torsional (lack of ) stiffness? How would a trim tab therefor affect stiffness? If the blades are seeing, guessing,300 mph at the tips, how can an increase of 10 mph created such an issue? I can, however see an angle of attack being exceeded and a sudden stall occur. We all agree that input affects are not felt until 90° of rotation. How then is the force being applied to pitch up? Is this sudden lift occurring at 90° of the advancing blade suddenly causing an extreme amount of lift at this point? To have more lift at that point then it is inferred an increase in angle of attack caused the sudden lift. Would it not be logical that an increase of AOA at this speed felt by the rotor at this point being the driven section, that an accelerated stall would occur? If the blade then failed by twisting downward, then loss of lift would occur.The only fix I could see that could be done to existing blades would be possibly a trim tab. The fix that Chuck suggested would be a complete change in construction to achieve quarter cord balance.
From my understanding it is both. The blade is not quarter cord balanced. In smaller disk sizes it doesn’t seem to be a factor due to the stiffness of the blade in torsion. Up to 27’ foot blades it was not an issue it was the disk sizes above that where the problem manifests itself. I am no expert and am going by the limited knowledge imparted on me by Chuck Beaty and My Father.Is it quarter cord balance or torsional (lack of ) stiffness? How would a trim tab therefor affect stiffness? If the blades are seeing, guessing,300 mph at the tips, how can an increase of 10 mph created such an issue? I can, however see an angle of attack being exceeded and a sudden stall occur. We all agree that input affects are not felt until 90° of rotation. How then is the force being applied to pitch up? Is this sudden lift occurring at 90° of the advancing blade suddenly causing an extreme amount of lift at this point? To have more lift at that point then it is inferred an increase in angle of attack caused the sudden lift. Would it not be logical that an increase of AOA at this speed felt by the rotor at this point being the driven section, that an accelerated stall would occur? If the blade then failed by twisting downward, then loss of lift would occur.
The solution of a trim tab would in theory keep the AOA constant. But, it seems this is also ties to lower RRPM. Why would this pitch up not manifest itself on departure if the machine is lightly loaded? The faster the RRPM the greater airspeed retreating blade stall occurs. For me the issue of why is far from settled.
Jean Claude would you elaborate on that statement, please?And also, due to the low Rrpm the divergence of the longitudinal flapping angle will occur at a slower forward speed
If you remember Doug the first several years dad produced blades they have very little reflex. Once he figured out how to change his oven and tooling to put more in he did. This enabled him to eliminate the trailing edge rivets. It made the blades a little less efficient but a much more docile flying set of blades. I flew one set of non reflexed blades. I didn’t like them at all told Dad I wouldn’t fly them again and he should sell them.To clarify just a bit: Bensen blades were not true 8H12 (McCutchens are more accurate). The '12 has a convex bottom surface, while Bensen blades, both woodies and metal, had dead-flat bottoms. Made them easier to build, and the flat bottom was an easy reference point for measuring pitch.
Reflex adds trim drag to a blade (much as a down-loaded H-stab adds trim drag to a FW plane). Therefore, any designer who is chasing efficiency would like to minimize trim drag by using less reflex than needed to produce a zero pitching moment.
Some gyro blade designs have employed virtually ZERO reflex. No doubt the designer hoped that the torsional stiffness of the blade would prevent the twisting of the blade at higher blade airspeeds. And sometimes that worked, and other times it did not.
Anecdote: My best gyro-flyin' buddy died many years ago in the crash of a Cloud Dancer motorglider. The plane went straight in after a typical departure stall-wing drop.
It turned out that the plane was designed without download on the H-stab. Instead, the wing's pitching moment was offset (at one airspeed only!) by aft CG. The normal CG was at 40% or more of wing chord. To make the pitching moment worse, the wing had a drooped trailing edge (the opposite of reflex). These design tactics are used in some sailplanes for the sake of drag reduction. The consequence in this case, though, was that the plane was statically divergent with respect to airspeed. Once it slowed up, it tended, on its own, to slow up even more. This was not entirely clear in the flight manual; it probably should have been written in bold capitals. Moreover, the real stall speed, per another friend with many hours in this model, was considerably higher than disclosed in the manual.
For me, this crash was a bitter lesson in pitching moment and CG location. I still miss my man Bill.
Thank you for the treasure of information found in the website your provided. The paper presented by WIeńczySłaW StaleWSkIA small collection of reports for all those interested in autogyro blade profiles is available here:
https://magentacloud.de/s/pZRTxbt9mE89wGK
The password is naca8h12 (all lower case). The stanislawski report is a more recent one which gives values for variations on the 9H12M profile and proposes a variable plan form. Another interesting one might be the PractialIssues....., which gives a few values of several profiles with and without trim tabs.
Have fun and may there always be an inch of clearance between your aircraft and the nearest obstacle...;-)
Cheers,
Jürgen