tilt rotor vs rotor tilt

[maxtork]

Good morning Gents!
Well at least it's morning for me. I had a thought last night and wanted to run it by you all to get your input.

I understand that larger diameter rotors like we have on our conventional style helicopters are not as efficient when turned face on into the airstream like the tilt rotor does. I also hear that the tilt rotor is not quite as efficient at a hover due to the high disc loading of the prop rotors. The relatively small size of these prop rotors was required to keep them from being too large once tilted forward. So here is my question...Could we use a similar type tilting mechanism on a conventional helicopter to tilt the rotor to gain speed without going all the way to 90 degrees where the rotor becomes too large to act as a prop?

What if we could tilt our rotor disc to about 30-45 degrees (yes, numbers pulled directly from rectum) in forward flight. We could delay compressability effects on the advancing blade and retreating blade stall on the other side. We would probably be less efficient than having the rotor flat but not as bad as having it at 90 degrees ala tilt rotor.

This is all assuming that the curve between bad efficiency with rotor at 90 vs good efficiency while flat is linear. I don't know this to be true which is why I am posting here. I also understand there would be control issues as well since our helicopter style antitorque system would have to change to work more around the roll axis as the rotor is tilted forward. The main rotor controls would also become more complex just as the tilt rotor controls have but I believe with todays technology it is acheivable.

The big question here could be "how much return on our investment do we get?" How much foward speed would we get by tilting our rotor disc forward like this without adding more power? I know the power required to hover is higher than the power required to fly foward after translational lift but the curve increases again in order to fly faster towards vne. Would this tilting of the rotor actually add speed for the same power or simply increase the vne limit such that we could fly faster than we can today by simply adding more power?

The tilt rotor is an interesting idea as it has supposedly the best of both worlds, high speed of a fixed wing and VTOL capability of a helicopter. I'm willing to bet it would be easier to sell an aircraft that was a helicopter that could cruise at an honest 200kts as opposed to an airplane that can take off and land vertically within a certain set of limitations.

What do you think?

MaxTork

[Haggis Hunter]

"how much return on our investment do we get?"

I'll give you a £1 to get going...

Interesting thought, need to get some of the P of F boffin's into this one.

[Lu Zuckerman]

To: maxtork

If you tilt the rotor in relation to the mast you would encounter excessive lead lag which could possibly tear the rotor to pieces. This assumes a fully articulated system which also has design limits relative to flapping.. There is a possibility that excessive tilt on a Bell rotor system would exceed the design limits of the underslinging and you may encounter excessive lead lag, which will exceed the structural limits of the rotor system. This does not include mast contact.

But then again, I’m not a graduate engineer.

As far as the efficiency or the V-22 Prop rotor ® is concerned in an auto rotation the ROD can reach 6000 feet per minute. I believe this is the reason for certifying any prop rotor aircraft as a powered lift aircraft and not as a helicopter. That way they don’t have to demonstrate auto rotation.

The length of the Prop rotor ® blades are such that if the aircraft makes a run on landing the blades extend beneath the fuselage and are designed to fracture in small pieces keeping the possibility of shrapnel intrusion into the fuselage to a minimum.

I’ll leave it up to NickLappos to explain the relative merits of the two designs.

:E :E

[maxtork]

Thanks Lu,

Actually I was talking about tilting the rotor AND the mast together which hopefully would eliminate the lead lag problems you speak of. The downside to this of course is designing the gearbox such that it could be tilted without misaligning the engine input or twisting up the control system inputs. I've thought about these aspects as well and I think I can get past them without too much trouble.

Is high disc loading the reason for the tilt rotors high ROD in autorotation? It makes sense to me that it is but I don't want to make any foolish assumptions

Cheers
MaxTork

[Dave_Jackson]

maxtork,

Sikorsky has a number of patents related to a 'variable diameter rotor blade actuation system'.

Theoretically, this could be a fairly significant improvement to tilt rotors.


Lu,

"If you tilt the rotor in relation to the mast you would encounter excessive lead lag which could possibly tear the rotor to pieces."

The V-22 claims to have a Constant Velocity Joint in its hub. If it is a 'true' CVJ, it will eliminate THIS cause of lead/lag. Regrettably, it does not look like a true CVJ and it has been impossible to find a conclusive answer.


Dave

[Lu Zuckerman]

To: maxtork

Regarding the interrelationship between the tilting transmission and the stationary engine this could be accomplished with a second transmission driven by the engine and this transmission would be analogous to a sidewinder transmission which would be stationary and in turn allow the main transmission to pivot around the sidewinders input to the main transmission. It would be required to design the main transmission oil system so that the transmission could be lubricated in the horizontal position. The sidewinder transmission would not only make the system more complex and less reliable it will also increase the weight of the helicopter.

Regarding getting the control system to function in both rotor positions this can be easily solved using fly-by-wire just like the V-22.

I believe the high disc loading and the inability of the prop rotor ® to back drive in autorotation and provide the necessary lift. There is an excessive amount of machinery that must be driven by the Prop rotor ®.


To: Dave Jackson

The V-22 claims to have a Constant Velocity Joint in its hub. If it is a 'true' CVJ, it will eliminate THIS cause of lead/lag. Regrettably, it does not look like a true CVJ and it has been impossible to find a conclusive answer.

The CV joint is similar to an elastomeric pitch change bearing used on some helicopters. It is not truly a spherical bearing but it has similar characteristics in that it can be rotated about its' theoretical axis to permit rotor tilt and flapping while transmitting engine torque to the Prop rotor ®. Lead lag is mechanically restrained. It also serves as a hub spring providing restraint. Electronic sensors sense individual blade flapping and this signal is transmitted to the flight control system resulting in servo displacement that moves the swashplate to counter the flapping.

These same sensors detect movement of the Prop rotor ® due to gyroscopic turning moment during maneuvering while in the aircraft mode.

The hub spring is a CV joint as all movement about the shaft pivots about a theoretical axis, which intersects the driven axis.


:E :E

[Ian Corrigible]

I remember reading about Sikorsky's work on scimitar-shaped variable diameter main rotors a decade ago. Did this ever proceed beyond paper studies ?

In addition to SAC, Bell and Boeing have also undertaken research into variable diameter tiltrotors (VDTRs) and folding-rotor tiltrotors.

I/C

[maxtork]

Lu,

Funny you mention the sidewinder transmission as that is what I was thinking about calling the whole machine! I had envisioned something like an astar main gearbox and engine assembly turned sideways to the airframe and made to pivot around the input. Now since you have the engine off center in the airframe you move the cockpit/cabin area slightly off center the other way to balance the machine. this also opens up the area just in front of the mast such that when tilted forward we aren't chopping up the cockpit! :ooh: I was already expect the use of either fly by wire flight controls or even a hydraulic or teleflex cable type arrangement to overcome that situation.

As for the work being done on variable diameter rotor systems, is it a benefit to a normal helicopter layout or only to a tiltrotor type set up? It seems like a very complex solution to an already complex machine. At least my idea just moves things that aren't spinning at high speed!;)

Thanks for the discussions so far, it has been most interesting. Keep the replys coming!

MaxTork

[slgrossman]

Maxtork,

When you tilt the rotor disc forward how do you propose to account for the decrease in the vertical component of the lift vector - add a wing - add more power? Sounds like you'd have to approach the tilt-rotor / tilt-wing configuration, which I think is where we started.

-Stan-

[maxtork]

Well that kind of depends on the situation. It could really go either way. We could add more power and leave it at that or we could add the wing assembly to help out with the loss of lift. This is probably the way to go for one not so small reason. As we tilt the rotor disc we would also be tilting the torque axis. This means that our trusty tail rotor is going to be pushing the wrong direction to counteract the torque. If we were to mount short wings we could either use fixed wing type control surfaces to balance the torque that would be at this point acting about the roll axis. I even thought about trying to mount the tail rotor on the end of one of those wings much like the Fairy aircraft I have read about(sorry can't remember the name of the particular machine at the moment but it wasn't the rotordyne). By doing this we would also put the tail rotor drivetrain inline with the engine and thus the pivot point for tilting the main gearbox.

I see your point though....if we would go through all this trouble would we not end up with a machine very similar to the tilt rotor itself. Yes and no, it may be just as complex but I believe it would perform better in a hover due to the larger diameter rotor and more helicopter like areodynamics. It may not however be as fast as the tilt rotor and this is the question that would make or break the whole deal. If it is every bit as complex and therefore expensive as the tilt rotor but only marginally faster than a traditional helicopter then why bother? This is what I was getting at earlier when I mentioned "return on investment". I'm really not sure of the answer, hence this thread.

MaxTork

[Dave_Jackson]

Thanks Lu,

Your comments are in agreement with the information that I have dug up. It appears to not be a 'true' CVJ.


maxtork,

IMHO, Stepniewski's Low Tip Speed Design Philosophy (http://www.unicopter.com/1093.html) will be the future direction of rotorcraft. Basically, it is a compound helicopter except that the rotors and wings have been functionally combined.

It may never quite match the top speed of a tilt-rotor but it will offer other major advantages; such as greater payload and safe transitioning between hover and forward flight etc.


Dave