Convertable T/R

[Vfrpilotpb]

We all know the technology exists to swing an engine and rotor through an arc of some degrees ala the MV22 Osprey, so take that mechanical technology to the tail rotor and when in forward flight which does not need the stabilising effect of the counter torque produced by the tail rotor, then use that rotor as a pusher propeller by swinging it through 90Deg.
Has it been tried? Would it work? or would this sort of attachment be too risky or tricky to be of any use?

PeterR-B

[CRAN]

Pete,

I know of some work done by Dr. Simon Newman at Southampton university on the subject using a large ducted fan as a TR that then rotates through 90 degrees an acts as a pusher-prop at high speed. I think the work came to an end because he couldn't get investment interest in the technology. :(

In principle it will work over a limited increased speed range, but the problem of retreating blade dynamic stall still exists. You will therefore reach a speed at which it is impossible to sustain controlled flight. (That's if the rotor hasn't shook itself of by then. :D) Besides in the current airspeed range of traditional configuration helicopters it is not the capability of the rotor to produce thrust that is limited at high speed - it the ability of the rotor and indeed the helicopter to sustain the intense level of bone-shaking vibration that is induced by the highly unsteady flow.

When the Westlands set the speed record 249.09mph over Somerset in the 80's they weren't limited by rotor performance they were limited by engine power.

I guess what I’m saying is that yep, you could try it, and it would give some improvements in forward flight capability. But since the rotor would still be highly loaded to produce the lift required - with an ever diminishing control authority it would have very dubious control qualities in high speed forward flight. Vibration levels would be extremely intense and are likely to be prohibitive and efficiency would also be poor as you are forcing the rotor to operating in a very unpleasant environment. Not to mention the weight penalties of the mechanism and beefed up tail cone to take the axial loads.

Lift and Thrust compounding is believed by may to offer much more potential, allowing the rotor to be unloaded in forward flight and hence reducing vibration and offering improved efficiency in high speed forward flight over pure rotor powered flight. 300kts is claimed to be realistically possible and I know of at least one manufacturers attempting to source the finance to build a demonstrator which would be an upgrade to an existing helicopter. However this is for a military aircraft. Sorry folks!

Hope this helps
CRAN
:)

[Draco]

I believe the Americans (who else!) played with a huge range of different formats and propulsion systems in the 50s and 60s, including fitting jet engines to the sides of a Huey, with rapid results.

The effect of retreating blade stall was countered by the use of small wings on some machines (unloading the disc), which turned them into the compound category, but I believe they could top 300mph.

I'm sure that Nick Lappos will know more about these machines.

I'm not an engineer, but on the basis that the tail rotor is not really needed in forward flight, presumably the power that it drains from the engine is very little (as it will have only small loading). Any forward energy given by a rearward facing propellor would therefore have to be stolen from the main rotor, and I don't see why you would end up with any advantage. Perhaps someone with better understanding could enlighten me.

[Cyclic Hotline]

A number of variations of this idea have been attempted, with some success.

Sikorsky flew a variant of the S61A (N318Y) with the swing tail installed. The tail rotor swung through 90 degrees in forward flight. Flew very well by all accounts, although it would have appeared to have limited practical application. The canted tail rotor of the UH-60 and CH-53E maybe being the limit of the technological application.

Piasecki has a US Airforce contract to develop a thrusted H-60 in progress at this time.

[Nick Lappos]

Sikorsky flew several such machines three decades ago, including the ABC with two J-60's and an S-61F That flew with both a jet engine, and with a tail rotor that swung from anti-torque to thruster at speed.

Frank Piasecki is currently building the Pathfinder modification on an H-60 that will replace the tail rotor with a ring tailed thrust prop that uses rudders in the propwash to make anti-torque. He claims somewhat fantastic speeds.

These ideas are not new, the aerodynamics are quite well understood, and the problems all boil down to the compromise you are willing to make in hover payload vs the advantage in speed.

The Pathfinder was fully analyzed by our advanced design group, and we believe that the gain in speed is perhaps 30 knots (making it a 190 knot dash speed machine) but the weight of the device robs enough payload to actually reduce the range, and the loss of anti-torque capability compromises the Naval mission, where tail rotor authority is important.

The supreme speed rotorcraft, the Tilt Rotor, serves as another example, where for the same power and weight, and half the cost, a helicopter will carry twice as much and have the same range as a tilt rotor. Don't be fooled by the hype always published, where tilt rotors are compared to the next smaller class helicopter, V-22 vs Black Hawk or B-609 vs S-76 for example. The 609 has the empty weight and more engine power than a Black Hawk, all to carry 8 passengers like sardines. Put a 609 next to a Black Hawk, it will carry half the payload, and have less range, albeit at 240 knots vice 150.

There is no free lunch. Customers tell us what they want and we build it.

[sycamore]

As a new" kid" on this I`ll add my 10 penn`th.As already mentioned ,in 1964 two proposals were submitted to the US Army for the Advanced Aerial Fire Support System(AAFSS),which was to be a Chinook support a/c,all-weather,top speed of 220kts and a ferry range of 2400nm.Not asking much really,plus ability to hover at 6000ft at 95*f.Sikorsky and Lockheed both submitted proposals,the Sik. S-66,and the Lockheed CL-840. Both designs were similar with a narrow fuselage,tandem cockpits,sponson and stub wings for stores,The S-66 had a swivelling t/r to provide thrust as speed increased to 100kts or so,up to 200kts,and had a large"T" tail with rudders/elevators/trimmers.A modified S-61 (N-318Y) as a system test vehicle,but in practice the swivelling bits were not found to be very flexible(no pun!) in operation.Sikorsky later built the S-67 Blackhawk(original),which was very sleek,sadly it crashed at F`boro whilst doing aeros-very sad for all. So,Lockheed actually won the competition in 1965 ,their vehicle was to become the AH-56A Cheyenne,with rigid rotor,T-64 of (4200hp),conventional t/r and a fully reversible pusher propeller as well at the end of the fuselage.Eventually it could reach 215kts in level flight,245 in a dive,sustain +2g,demo`ed+2.6G and -0.2G.

[Nick Lappos]

sycamore,

The Cheyenne was cancelled after three of the 10 prototypes crashed, said to be for rotor stability issues associated with its unusual gyroscopic control system. The last crash involved a rotor-fuselage contact that killed the pilot. As a 22,000 pound helo with about 4000 horsepower, it could dash at 220 knots, and had a 10 foot prop on its back end.

The S-67 Blackhawk was developed by Sikorsky in 1970 after the Cheyenne production was cancelled, the two aircraft were not in direct competition. I have about 5 hours in the S-67, including several aerobatic maneuvers. It was a very fine helicopter.

[Dave Jackson]

Can't argue with the "no free lunch", but here's some cheap 'food for thought'. ;)

Theoretically, it should be possible to achieve 300+ mph in a rotorcraft that incorporates the following features: Very low tip speed.
Large chord blades.
Twin counter-rotating rotors.
Advancing blade concept.
Extremely rigid rotors.
Forward thrust device.
A light elaboration on this can be found at;
Large Chord and Low Tip Speed (http://www.unicopter.com/1090.html)

Arguments solicited.

PS. This posting will self-destruct if the CarterCopter does what it hopes it can do. :D

[Dave Jackson]

Piasecki Aircraft’s YSH-60/VTDP (http://www.continuum-dynamics.com/research/topics/piasecki_research/index.html)


Sikorsky Aircraft’s, Variable Diameter Tiltrotor (http://www.continuum-dynamics.com/research/topics/vdtr_handling/index.html)
` . . Are the rotor diameters reduced before they are rotated for forward flight :confused:

[Nick Lappos]

Dave,
The VDTR reducesbdiameter prior to or during early conversion, as the diameter won't clear the fuselage when in propellor mode. The power savings of the larger disk are the reason for the variable diameter, as the aircraft accelerates through translational lift, the diameter cn be reduced without performance penalty.

[Dave Jackson]

Thanks Nick,

From what you are saying, it appears that this design has the potential for a couple major advantages; over a certain unnamed craft.

You mention the power and performance advantages of the larger disk in hover and transition. Your comments also suggest the reduced likelihood of entry into the VRS, assuming that the rotor's diameters are changing in conjunction with their lateral rotation and the transitioning of flight.

[Lu Zuckerman]

As Nick had indicated the Cheyenne suffered from sever rotor instability. This was caused by the inherent stiffness of the blade and the fact that the blades had a different camber at different blade stations, which contributed to the instability. On past posts the subject of phase angle shift was discussed. The problem with the Cheyenne was that the shift was not predictable and because the blades would behave differently under different conditions resulting in the helicopter flying in an uncommanded direction. The shift and the aerodynamics overcoming the blade stiffness under certain aerodynamic situations caused the blade to dip down and hit the fuselage.

I believe there was only one crash caused by this inherent instability and it killed the pilot. This particular Cheyenne had a downward firing ejection seat but it is problematic if the pilot could have ejected being so close to the water surface. Besides, he was not sitting in the ejection seat. The second incident of blade instability took place inside the large wind tunnel at Ames Labs in San Francisco, California. The breakup was so sever the wind tunnel was out of commission for quite some time.

Lockheed worked on the problem by trying different positions of the swashplate in relation to control input and subsequent control output. It didn’t work and it was like a dog chasing its’ tail never quite succeeding. The problem was turned over to Parker Bertea the builders of the servo system and the rest of the hydraulics system. They worked the problem for over two years and finally came up with a solution. It worked something like a SCAS system in that cyclic input would be monitored and the subsequent movement of the blade was also monitored. If the blade did not move in the commanded direction a modifying signal was sent to the servo to correct for the unwanted movement. Pilots that flew the Cheyenne with this system installed stated that it was absolutely the smoothest helicopter they had ever flown. However the blade movement monitoring system was part electronic and part mechanical and it was full of potential single point failures as to make the helicopter extremely dangerous to fly. At this point the program which was on hold was cancelled.

I do not believe that the control gyro action contributed to the blade divergence. Lockheed developed the Cheyenne based on previous designs, which were highly successful. They believed that they could scale the design up and it almost worked. The Army however was constantly adding systems and mission capability to the point that the rotor system could not perform according to specs. Lockheed requested that they be allowed to increase the rotor diameter and the Army refused. Since they could not increase the rotor diameter they changed the design of the blades to increase lift and that is when the problems started.

[Dave Jackson]

Lu,

You mention that a wind tunnel was 'taken out' by the rigid rotor. ` Nick has previously noted another instance of a rigid rotor causing wind tunnel damage.

It's quite obvious that there's a need for a better design; ` of wind tunnel. :D :D

[sycamore]

A few minor corrections ref. the Cheyenne..A/C no.3(66-8828) crashed in March 1969 after experiencing a""half-p" hop in the collective channel leading to a"PIO" which went unstable,as the dampers had been selected off,either inadvertently,or deliberately, leading to the resonance,the blades struck the canopy,killing the pilot,Dave Beil.The a/c did not have a bang -seat ,they were only fitted as a result of this accident,only in the gunners position ,and it was an F-104 downwards firing one.A/c 66-8830 was damaged by an engineer whilst ground -running and the rotor oscillated,striking the canopy.The engineer was somewhat shaken!!A/c 66-8835(no.10) was put into the Ames large wind -tunnel by the Army against all advice,deliberately oscillated,and as it was tied down,it shook itself to pieces!This a/c was in Canada in Feb 1969 doing icing trials,when I was also there on icing trials on Westlands finest.We couldn`t persuade Don Segner that it was a fair "swop",a ride in a Wessex for a trip in the Cheyenne!!An awesome machine ,with todays systems,it would probably run rings around an Apache,which does look ugly,anyway!!! Nowadays you can get a trip in an Apache for a trip in a Wessex--how the world changes!

[Heliport]

Welcome to Rotorheads.
Look forward to reading more of your posts. http://www.stopstart.fsnet.co.uk/smilie/thumbs.gif

[Lu Zuckerman]

I was told that the Cheyenne that crashed off of Point Mugu NAS was equipped with a downward firing ejection seat and as I had indicated the pilot was not flying from the gunners position. I guess I was misinformed. I was also witness to the Cheyenne being loaded into a CL-400 Swing Tail for the trip to Canada.

It is my personal opinion that if the Army had permitted Lockheed to increase the rotor diameter the Apache would never had been built. There was one other problem with the Cheyenne that to my knowledge was never explored and that was it shared a design fault with some autogyros. On an autogyro when flying at low pitch with the weight supported on the wings if the pitch is allowed to go negative the blades will diverge from the tip path. In the case of the Cheyenne the cyclic was used to control the attitude (maneuvering) of the helicopter which at some point could involve the blades going negative.

There were also some severe problems relative to design for maintainability. In most companies the Engineering department had the final say-so relative to the design but when specific problems were brought to the attention of engineering they authorized the necessary design changes but they were rejected by production. They stated that if the changes were incorporated it would make it more difficult to install the equipment. So if certain problems arose in the flight control system the mechanic would have to remove some of the Plexiglas windows and cut a hole in the structure. There was a similar problem in the primary flight control system. All of the bell cranks that connected the pilots/co-pilots controls to the rear of the helicopter were mounted on a common shaft which required the removal of the entire assembly in order to remove a defective bell crank and, in order to gain access to the assembly the battery had to be removed.

[Vfrpilotpb]

Good morning Gentlemen,

I have read all your answers with much interest, in the case of speed becomming to much for the rotor system and the possibility of blades stalling, what would happen if the main rotor blades were then to be made Scimiter like, so that the tip was not going to be in the position of stalling, I realise I may be asking some fanciful questions but Scimiter shaped blades are currently used on Turbo -props and seem to cope well with the huge rev increase, whilst still acting as an efficiant way of turning burnt fuel into plenty of push. by the way thank you for your answers.:)