Wuzak
26th July 2007, 23:13
In reading R-4360: Pratt & Whitney's Major Miracle I have discovered that using pusher props can be problematic.
The advantage is that the wing is free from prop wash, the disadvantage is, if the engine/prop is wing mounted, that the prop is effected by the turbulence from the wing trailing edge.
This was discussed in the section dealing with the XB-35. The Northrop flying wing had a myriad of problems, just one of which was the vibrations in the original prop layout - a contra-rotating pusher arrangement. The contra props were replaced with single rotation props, which lessened the vibrations but did not eliminate them. Needless to say that the engine/drive shaft/reduction gearbox combinations exhited no such tendencies when tested in the test cell by Pratt & Whitney.
The Convair XB-36 also had a similar problem, but to a lesser extent. It was solved in that case by adding strength and mass to the wing and engine mounting to attenuate the vibrations. Evidently with some success.
The author of the book opines that part of the reason the problems in the case of the XB-35 were so severe was that the trailing edge was angled with respect to the prop arc, meaning that different sides of the prop arc were affected differently by the trailing edge wake, and that the thrust line of the prop was very much above the line of the trailing edge. In the case of the XB-26 the thrust centreline was at the same level as the trailing edge of the wing, and the prop arcs were parallel with the trailing edge.
Greg may be able to tell us if there have been any problems with the N-9M, which is basically a small scale version of the XB-35.
Apart from that, has anybody heard of such problems in other pusher prop aircraft? Did the Horten brothers experience any problems with their experimental flying wings?
For the record...there were 15 XB-35 and YB-35s built. Only 5 flew. Pratt & Whitney developed 5 variants of the R-4360 specifically for the B-35 family!!
The advantage is that the wing is free from prop wash, the disadvantage is, if the engine/prop is wing mounted, that the prop is effected by the turbulence from the wing trailing edge.
This was discussed in the section dealing with the XB-35. The Northrop flying wing had a myriad of problems, just one of which was the vibrations in the original prop layout - a contra-rotating pusher arrangement. The contra props were replaced with single rotation props, which lessened the vibrations but did not eliminate them. Needless to say that the engine/drive shaft/reduction gearbox combinations exhited no such tendencies when tested in the test cell by Pratt & Whitney.
The Convair XB-36 also had a similar problem, but to a lesser extent. It was solved in that case by adding strength and mass to the wing and engine mounting to attenuate the vibrations. Evidently with some success.
The author of the book opines that part of the reason the problems in the case of the XB-35 were so severe was that the trailing edge was angled with respect to the prop arc, meaning that different sides of the prop arc were affected differently by the trailing edge wake, and that the thrust line of the prop was very much above the line of the trailing edge. In the case of the XB-26 the thrust centreline was at the same level as the trailing edge of the wing, and the prop arcs were parallel with the trailing edge.
Greg may be able to tell us if there have been any problems with the N-9M, which is basically a small scale version of the XB-35.
Apart from that, has anybody heard of such problems in other pusher prop aircraft? Did the Horten brothers experience any problems with their experimental flying wings?
For the record...there were 15 XB-35 and YB-35s built. Only 5 flew. Pratt & Whitney developed 5 variants of the R-4360 specifically for the B-35 family!!