Originally Posted by
scifi
Just some more observations on model airplane prop design that should carry over to the full size aircraft...
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Only to the extent that the blades are subsonic or not. Most model props have very high tip speeds, much greater than 12":1' scale. When the blades are sonic at the tips, the consequences are that positive camber results in less thrust than a negative camber would, from the effects of the shockwave formation. While the sonic tip speed results in oblique shock formation, the normal shock appears to be much more prevalent when tips are sonic. That oddity, of reversing camber at the outer radius of the blade has been on scale models by a researcher in NZL, (google it), and the only full scale evaluation was flown probably by me using rubber tabs on the TE of the blades to increase camber inboard so as to essentially reduce effective camber (well, really AOA) outboard. That was surprisingly effective from low speed up to the high speed WW2 high performance fighter we flew, but the blades physical profile remained as manufactured. In the latter cases, the tip velocities were subsonic, but there are normal shock formations on the blade in operation. tabbing inboard of the tip increased inboard loading of the blade, reducing the tip loading as AOA for a given thrust was lower, reducing the intensity of the normal shock outboard. The location of the tabs was chosen to give increased velocity profile inboard but trying to avoid excessive shock formation. The same tabbing was flown on helicopter rotor blades, only on a 2 blade system, and with a serious amount of safety precautions. Outcome was a very large change in the acoustic signature as was shown on propellers, and a very large change in required torque, and a much reduced blade stall rotor RPM. The limited flight test series includes some failure modes and the test helicopter, an R22B was in more ways than I care to recall less than an ideal test vehicle for extended testing. One set of comparisons comes to mind though; at test weight and conditions, normalised for w/delta, the rotor stalled out on normal blade at 82%Nr. At that time, directional control was in the process of being lost, full yaw pedal was attained.... (it is a test point with some risk attached, and is not done above a 1" hover height, stall a rotor at 10' and you are going to wake up at A&E, stall it at 50' and you are going to wake up in the after life. The risk of rolling up Franks infernal aerial contraption into an aluminium wad is non zero). The next test was with a series of conformal VGs on the outboard blade LE, and the torque required to hover reduced by 9%, and the blade speed for stall reduced to 77%Nr, 5% absolute, about 6% relative, and at the stall, residual left pedal authority remained, but only just, which is impressive). Adding a 1 meter tab mid span to the rotor with the VG on the outer blade for mass balance considerations, resulted in a reduction in power required by 17% from the base clean blade, and the rotor stall RPM dropped to 68% Nr. Approaching the stall RPM, 1/2 left pedal remained, and turns to the left could be accomplished without any difficulty, although anyone flying under speed rotors is nuts, and will have noted that the cyclic control becomes pretty wooly, while the aero affects are being amended the inertial effects become rather different to a normal rotor RPM case, so control is fun. limited transitional flight was conducted, and autorotation evaluated, with no adverse effects, the most notable difference being that transitioning through ETL was very soft, vibration was well down, and the rotor noise was considerably reduced. The concern on extended testing was that doing a failure mode in forward flight could result in tail rotor blade impact with a small piece of low density foam, and the RHC tail rotor is a swiss watch level of robustness... akin to tissue paper, and not overly robust. The failure tests in hover showed that there was low likelyhood of a shed piece of foam touching a blade, but it was not able to be ruled out, and an impact would lead almost certainly to a TR blade root failure, loss of 90 gear, and probable separation of the tail cone at the 5 point attachment, just the blade loss alone would result in a high probability of a mast bump, and being placed on aforementioned slab in 2 major pieces, head on one slab, remains of carcass on the other. The testing was discontinued for choppers awaiting access to a fensetron tailed beast or a UH60 which does a whole lot betterer with losing bits.
Back to subject, the model blades operate in a sonic tip velocity frequently, and more or less only the Thunder Screech did that on full scale. Almost all blades have some normal shock formation in flight, but oblique shocks are avoided where possible by limiting tip velocity. Sweep reduced the normal shock intensity...