Lockheed XH-51A

[Spaced]

Hey guys, off memory there are a few guys here who worked on the Chyenne progam, so Im hoping they may have had some involvement with this aircraft as well although any input is welcome.
The '51A was a compound variant of the 51. heres a picci
http://www.russian.ee/~star/vertigo/foto/lok_compaud_2.jpg

I recently bought a book on the Chyenne, and was surprised to find that the 51a only had an engine on one side. What furthere surprised me was it was on the left side.
So a couple of questions;
Why the engine on the left? Its pushing the same direction as the torque reaction of the fuse.
Was there a consistent reaction when power was applied? ie, a right yaw tendancy.
Were there any other unique characteristics involved with this layout.
Ive seen some fottage of it flying, it looked like a ball.

[B Sousa]

Wow Big Aerodynamic Subject. Your gonna have to get opinions from at least three folks who visit the forum. Lu Zuckerman (with one N) Nick Lappos or Shawn Coyle. They eat this up. You can PM them from other posts within the Rotorheads Forum.
All I know is the XH-51A was a Test Bed for the Engine combination to drive the Off-Loaded Rigid Rotor. Never to be produced like that.
Good Luck

[sycamore]

Spaced, the XH-51A (3 a/c) were built by Lockheed in 1962 to investigate rigid rotors prior to the Cheyenne programme,and were powered by a conventional PT6 driving a 3 or 4 bladed rigid rotor system. It was capable of up to 200 kts in a dive(4-blader),. and aerobatics, loops, rolls up to 3g. No151263 was modified as part of an Army contract , with Bell,Kaman and Sik. to develop a compound, and a P&W J60, of 2600lb thrust, off a scrap Sabreliner, was fitted, to look at off-loading the rotor in flight. The cockpit transparencies were also strengthened as they were bending at high speed. The fin and stabiliser were also enlarged, and the batteries were carried in a pod on the stbd. wingtip. The gear was also retractable on all a/c.

The engine was mounted on the left, as the fuselage would turn left with the rotors offloaded (I think !)
It performance allowed it to offload the rotors in fwd. flight, totally, achieve 263 kts in level flight, sustain 3g at 200kts, and accelerate from the hover to 200 kts in 45 secs.Unfortunately it only had fuel for 20 mins..

Then along came the Cheyenne ....!

Have we progressed much in the last 35 years....?:ok:


PS,BERT,hope your G50 is in racing trim...

[B Sousa]

Sycamore Writes:"PS,BERT,hope your G50 is in racing trim..."

Unfortunately its one of those I Wish I still owned. It was one of 50 brought over in 1964 to make it official so that Dick Mann could legally race Flat Track. He took the AMA #1 slot that year....
History.or Hindsight is 20/20.

[Lu Zuckerman]

To: sycamore

Then along came the Cheyenne ....!


In designing the Cheyenne they tried to scale up the XH-51 both in the design concept and the blade design. During the development phase the Army increased the weapons specifications which impacted the weight of the helicopter. The aerodynamics department determined that the blade design would be marginal and requested the Army to allow Lockheed to increase the rotor diameter. The Army refused. Lockheed determined that in order to generate the necessary lift and maneuvering capability they would have to redesign the blades to incorporate a non symmetrical airfoil using a cambered cross section and to vary the camber from one blade station to the next. A cambered blade has a very strong pitching moment and this pitching moment was different for each blade station, which made the blade highly unstable. The new blade design generated the necessary control power but it set in motion the downfall of the Cheyenne.

The blade instability led to a variance in the phase angle so that if the pilot inputted forward or lateral cyclic he did not know which way the helicopter would move. I have a personal opinion as to why the Cheyenne suffered two blade incursions one in flight and another in a wind tunnel.

With the rotor unloaded the Cheyenne is in essence an autogyro. In an autogyro if the blades go into negative pitch they could flap down. During unloaded flight the rotor system is used for control and the wings support the weight of the helicopter. During maneuvering the blades can go into negative pitch and the instability of the blade and its’ inherent pitch instability can cause rotor divergence.

Lockheed tried for a year or so to correct the phase angle shift and eventually turned the problem over to Bertea the makers of the Cheyenne hydraulic system to include the flight control servo package. They worked the problem for almost two years and finally found a solution. It is my understanding that the Lynx had a similar problem in that the blades would dip down to the left when the pilot input forward cyclic (15-degree offset). The problem was solved with an electronic device that sensed the blade movement and caused the servo input to compensate for the blade movement so that with forward cyclic the helicopter would fly forward. The Cheyenne system devised by Bertea did the same thing but instead of electronic it was mechanical.

It was installed in one of the ten Cheyenne’s’ and the pilots reported that it was the smoothest flying helicopter ever and it was totally free of vibration. The new control system although it did the job was composed of many mechanical feedback linkages, each of, which was a single point failure that could cause loss of control. Soon thereafter the program was cancelled.

I saw the two experimental X H-51s fly but I never got near them. The original proof of concept helicopter was in the next room from our offices.


:E :E

[Ascend Charlie]

Well, that was the technical reason.

Any mug can look at the photo and work out why there was only one engine, and on the left side.

If there were one on each side, you couldn't open any of the doors to get in...:8