Tiltrotor VS Tiltwing

Can someone explain me why Bell (and US military) chose tiltrotor concept for Osprey over tiltwing? For me it looks like tilt wing has many advantages over tilt rotor and no disadvantages. What's the difference - by TR solution the rotors are moving/tilting while by TW the rotors are fixed and the whole wing is moving. In theory TW should have numerous advantages :
1. when the wings rotate they don't block the downward air (as we know such thing can result in very nasty accidents).
2. you don't need to send the shaft through the whole wing like by Osprey - that means the wing is lighter and much less comlex (Osprey is hydraulical nightmare).
3. Because the rotation point (and the complex machinery) is on the body instead on wingtips the whole system is less vulnerable (body of aircraft is usually better armored than wings).

Hiller X-18 and Canadair CL-84 were the tiltwing prototypes tested. I don't understand why tiltwing research was stopped and tiltrotor gained popularity.

I share your view on this, the tilt wing does seem to offer many advantages over tilt rotor as you say.

Interestingly, according to the 2004 Jane's All The Worlds Aircraft, EUROTILT, formerly known as EuroFAR, switched from a tilt rotor to a tilt wing design because it obviated the blocking effect of the V-22 style arrangement (as you said)improving lift by 12%, allowing a smaller diameter rotor to be used and increasing cruising speed, it would also allow near-conventional landings with the engines tilted only slightly (about 5 degrees).

I don't know if EUROTILT is proceeding or not but it does seem to be the best solution.

If anything Tiltwing should have more disadvantages than tiltrotor and less advantages, when you tilt the wings, what tends to happen is your plane stalls if the wing tilts upwards to above the critical Alpha point. When this happens, your plane stalls, now in this configuration, the only way to pull out of a stall would be to tilt the wings downwards and put full power.

I believe that the TiltRotor is better, despite having more moving parts, I would take a tiltrotor over a tiltwing.

Shattered OUT...

I found this:

...
Disadvantages, however, are that an additional method of control such as a tail jet or rotor is required for control in hover, and ailerons change from roll control in horizontal flight to yaw control in hover. Control is especially difficult in hover during gusts due to the "barn door effect" of the wings in a vertical position.

www.aiaa.org...

I think its easier an cheaper to just tilt a rotor, instead of an entire wing. I guess it would also make the Osprey heavier, with a mechanism to move two wings with engines.

But surely it doesn't matter about the wing stallling as you are hanging off the rotors by the time this happens?

Anyhow, in the EUROTILT drawing pictured in Jane's it is only the outer section of the wing (the part that would otherwis be 'masked' by the rotor) that tilts, maybe the best of both worlds?

I was thinking the EUROTILT design sounded familiar, and it was. Here is the 'Weserflug' design for an identical arrangement circa 1944;

Trust the Germans to have already thought of something like that :-) Is there anything in the aviation world they hadnt already thought of or planned out ?

Originally posted by bmdefiant
Trust the Germans to have already thought of something like that :-) Is there anything in the aviation world they hadnt already thought of or planned out ?

Well, everyone has thought of everything, actually getting it through into the concept and design phase is a whole different story.

But yeah, I'm sure they haven't had Intergalactic Warships capable of making the transition from a planet's atmosphere into space without aditional rocket power.

Shattered OUT...

Originally posted by Zion Mainframe

I think its easier an cheaper to just tilt a rotor, instead of an entire wing. I guess it would also make the Osprey heavier, ith a mechanism to move two wings with engines.

Well the rotating mechanism would be smaller compact - that means with less weight. It was proven that tiltwing is lighter than TR, and there were already TW projects in early 60ties.

Originally posted by waynos
But surely it doesn't matter about the wing stallling as you are hanging off the rotors by the time this happens?

Your not though - the wing would stall at an AoA of say... 20 degrees (and thats a bit optimistic!) so your rotor would still be 70 degrees from providing pure vertical thrust.



Also, blowing the stream over the wings would produce a pitching moment, unless the centre of pressure line of the wing is mounted through the centre of mass of the aircraft - which would bring in other complications. This would require a 3rd vertical thrust source with a lever arm to counter-act.



[edit on 23-12-2005 by kilcoo316]

I don't know enough about pure aerodynamic theory to say whether you are right or wrong on this (but I am sure your points are basically correct) but how come it worked on the CL-84, X-18 and XC-142 then? They all successfully demonstrated the tilt wing concept in the late 50's and early 60's, so why not now.

I've been kinda mis-representing myself here.

It will work, but there are disadvantages, I've outlined them.

A tilt rotor should have more control in hover/transition than a tilt wing. So the designers probably went with what they thought was the safer option.


As usual, there is no right or wrong, just a series of compromises that can be worked different directions depending on what the designer wants.

Originally posted by kilcoo316
Your not though - the wing would stall at an AoA of say... 20 degrees (and thats a bit optimistic!) so your rotor would still be 70 degrees from providing pure vertical thrust.
[edit on 23-12-2005 by kilcoo316]

I believe that in straight and level unaccelerated flight that the AoA before stalling is 65 degrees, but only if your plane is in the green, if you have flaps and stuff, it's different.

Shattered OUT...

Longbow,

>>
Can someone explain me why Bell (and US military) chose tiltrotor concept for Osprey over tiltwing?
>>

The problem is the engine count. To be really useful as a _heavy_ (CH-53, not 46) replacement (which is what we need), the airframe really should have four engines and once you do that, you 'might as well' go for TW, if only because you are going to be fore-and-aft mounting them to keep CG and CL (thrust) margins equitable within a wide payload variance. How much of the wing panel should actually rotate is another story...

>>
For me it looks like tilt wing has many advantages over tilt rotor and no disadvantages. What's the difference - by TR solution the rotors are moving/tilting while by TW the rotors are fixed and the whole wing is moving.

In theory TW should have numerous advantages :
1. when the wings rotate they don't block the downward air (as we know such thing can result in very nasty accidents).
>>

Vortex Ring State is a problem for all rotary wing aircraft. That said, what destroyed the V-22 practicing assault landings at that Arizona or New Mexico airport was inherent to a case of a bad pilot applying bad habits for a new type of aircraft. The Tiltrotor is a plane that has ESTOL capabilities. If you confuse it with a helicopter, you're in for trouble.

>>
2. you don't need to send the shaft through the whole wing like by Osprey - that means the wing is lighter and much less comlex (Osprey is hydraulical nightmare).
>>

Not true. If you don't cross link all the active propellors, losing one side is as good as losing both. About the best you can say here is that the linkages are less complex (fixed) than they are with a translating cowl and frankly, I don't believe that a sudden loss of power in one nacelle can be compensated for, in time, during the most dangerous segments of transition in a combat area.

>>
3. Because the rotation point (and the complex machinery) is on the body instead on wingtips the whole system is less vulnerable (body of aircraft is usually better armored than wings).
>>

Ainh. Vulnerability studies are quite complex in their factored determinancies of 'total area' (wherein a critical hit will down the aircraft) for any given flight mode. While it might be true that the nacelles are not particularly well armored (and are /terribly/ densely populated) the fact remains that a STOVL assault bird of any type likely to be in a ballistic threat area at less than 200 knots is one of the few types which can be fired _down_ upon, by ground forces. And this exposes the entire upper deck and wingroot. It should also be noted that when something buzzes over you at 300 knots making a /helluva/ passage+blade racket noise, if you have the presence of mind to shoot at all, it will most likely be at the 'center mass' of a plane-shaped hole in the sky. Not at the wiggly bits on the end.

A better critique might be the adverse weight and stress factors on the wingroots due to having that much weight outboard in rolling moment away from the (lateral) CG.

Again, my answer to this is that soldiers don't play Huey games in a modern war. They pick a point well away from the threat and then DRIVE to the sound of gunfire. In a Gator, Shadow/RSTV-

www.armyrecognition.com...
www.evworld.com...
www.army-technology.com...

Or similar vehicle of sufficiently compactible footprint as to be stuffed in the back rather than slung underneath (size limitations on internal payload is one of the chief drawbacks of the V-22).

>>
Hiller X-18 and Canadair CL-84 were the tiltwing prototypes tested. I don't understand why tiltwing research was stopped and tiltrotor gained popularity.
>>

You try lifting the mass of a fully fueled wing 7-8 times higher than the variable incidence pop-top on an F-8 Crusader. Then keeping it there with sufficient residual hydraulic power to retract again at the worst possible moment: as the aircraft is accelerating forwards once more.

As other's have noted, the proprotors on the V-22 have cyclic functions as well as those associated with conventional pitch controls on a propellor driven airframe. As such, within an admittedly narrow speed band, they can play either/or on fixed/rotary wing performance values and thus pay less of a penalty in nacelle transition phases than the TW does.


ARGUMENT:
The area where helos /always/ fall down (at least non compounds) is that they pay their own lift-at-drag penalty in shifting the disk off-axis to push the helicopter foward. In worst case scenarios, this can 'bow' the whole airframe with a MASSIVE increase in presented area. But even in cruise, the amount of forward thrust generated vs. lift cushion variation from norm is pathetic. The TW is much the same way, only in a narrower critical speed band (of much shorter duration).

OTOH, the reality of life is that any airframe which can land in 100-150ft of flat area with no more than a 10ft obstacle at either end is going to have access to 90% of most nation's landmass within a reasonable distance of any target you might want to hit. You simply execute a rolling VL or 'ESTOL' (Extremely Short Takeoff/Landing) mode approach which is actually safer because you retain more options (directional control among others) at 60-90 knots forward speed than trying to come to a hover and blasting everyone underneath you with downwash in the multi-hundred decibel equivalency.

However, in wars of maneuver defined by air mobility, often the definition of your 'sphere of influence' around any given base point is that of 'reach vs. risk'. Which is to say the amount of time it takes to bring a force in and out of a given area, if that is indeed possible at all. Most helicopters, despite advertised ranges in the low hundreds of miles actually only operate on a 50-150nm radius. Whereas any airframe which can achieve true wingborne flight sufficient to convert all lift to propulsive thrust can acheive 250nm or more with _no payload penalty_. Furthermore, at a typical cruise speed it can define this radius as a 2hr servicing period (out and back) from whatever amphib or land base it is flying from. So that if you have 10 such aircraft in the air and each is dropping off one team on the outbound leg to pick up (or resupply) another on the return, you actually have a time between potential availability of mere minutes (assuming your force is small enough to be extracted by one or a few such aircraft).

This is something that NO penny-farthing RW aircraft can match because it's fuel reserves and best cruise for power setting X are always much more tightly constrained such that it is unlikely that it can even /reach/ that far, let alone recover anything (which is why we have FARPs). This mode of operation, along with the native payload:airspeed constraints of slingloading any vehicles is where you get the 'air assault' stereotype with wave upon wave of Huey's setting down into the elephant grass because they cannot bring vehicles and whatever boot force they land has to be able to stay for the extended period that a typical UH (90-100 knots on the Huey, 120-140 on the Crashhawk) must take to RTB, refuel and bring another wave or prep to extract.

The key differentiator then becomes cost vs. risk. Present replacement price for a UH-1 is around 4.7 million dollars (I remember a day when we were getting them for considerably less than 2). A V-22 (or likely a tiltwing equivalent) runs you about 50-60 million dollars. We lost 3,305 out of 10,005 hueys doing the dumb (put enough targets in front of an amateur with an assault rifle and he WILL hit a few) of 'wave' attacks in Vietnam.

www.vhpa.org...

EVEN ASSUMING that a tiltrotor or tiltwing can completely leave the trashfire envelope (or at least speed through it too fast for accurate optical tracking on a short TFR horizon) if we kissed off a similar 1/3rd of the 6-8 Tiltrotors you can (max) expect to be spotted on an LHA/LHD class vessel, we would be back to roughly the same number of (transit time) hours between reinsertion/transfer of troops away from imminent threat and to new areas of interest. As we would with an equivalent force (again, assuming the could reach as far inland) of UH-1s

.33 X 5 = 1.65 tiltrotors or 99 million dollars.

99 million / 4.7 million = 21 UH-1s (which an LHA could actually carry as their spotting factor is much smaller than a V-22s).

Which is where it becomes wiser to go with 'RAP' or Recon Attack Platoon teams of 3-5 vehicles with imbedded weapons (Netfire CLU trailers behind Shadow RSTV's for a preference) and simply put the vehicles down some X (10-20) miles from ANY threat.

Because this lets the grunts have a way to carry heavy weapons at constant speeds of 25-30mph. While attacking over the horizon (using UAV lookdown targeting and ROVER remote terminals) to stay or at least /git/ out of trouble. Even as it, again, lets you use any stretch of road or farmer's field you think appropriate for your airframe weight and rotor diameter. With ZERO, risk of 'rotorborne vs. wingborne' flight mode control problems due to adverse landing or hover requirements in a hot LZ.

CONCLUSION:
As soon as you start treating the tilt-anything more like an airplane with very special landing modes and less like a really fast helo, you bypass 99.999% of the bad assumptions inherent to the 'but isn't it bettter to...' variable geometry options over shifting the powerplant as a function of the airfoil or as a nacelle imbedded thrustline in it's own right.


KPl.

That Hiller was a prototype, and never went into production.

A helo is a helo and a plane is a plane and mixing the two results in an aircraft that neither rotorheads nor wingjockeys can save from the laws of aerodynamics.

Tilt-anythings will go the way of the dirigible. After enough funerals.

Originally posted by ShatteredSkies

I believe that in straight and level unaccelerated flight that the AoA before stalling is 65 degrees, but only if your plane is in the green, if you have flaps and stuff, it's different.

Shattered OUT...

Eh??

Uhm, the exact stalling point would depend on the aerofoil section and the wing planform and integration of 'tricks' (LE Slats, LERXs etc).

But, most aerofoils stall at around 15-18 degrees, maybe another couple of degrees can be squeezed out with smart design (don't forget this is not going to be a fighter wing!), but 65 degrees? Sorry, I cannot for the life of me see it getting to half this without stalling!

woops, wrong post.

Shattered OUT...

[edit on 24-12-2005 by ShatteredSkies]

You have voted ch1466 for the Way Above Top Secret award. You have two more votes this month.

That was a most excellent post ch1466.


Um, not really anything else to add, great thread in general, and I am awed by the sheer amount of aerodynamic and aerospace knowledge shown in the posts so far.

Originally posted by ch1466

A better critique might be the adverse weight and stress factors on the wingroots due to having that much weight outboard in rolling moment away from the (lateral) CG.

KPl.

Uhm, don't forget the wing is holding up both the engines and fuselage, having the engine on the wing itself will reduce the wing root loadings (from a static point of view).

Dynamically? Well, in 'hover mode', the rolling moments will be coming from the engines themselves, again, less stress on wing root.

In 'flight mode', hmmm, ailerons are inboard of the engines, but still away from the wingroot. I think in that case there would be greater tension in the wing spar, but still less bending moment at the wing root.

So a tilt rotor would actually reduce the wing root loadings in flight (in comparison to a tilt wing).

Could well be wrong though

Kilcoo316,

>>
Uhm, don't forget the wing is holding up both the engines and fuselage, having the engine on the wing itself will reduce the wing root loadings
(from a static point of view).
>>

No becasue the downwash is killing as much lift as you get back in the hover and the accelerative force, whether applied by the ailerons or by the hub controls is going to put added load factor against the natural P/torque induced one and thus increase stress to the wings regardless. Too accomodate this, you must beef the wing spar thickness' to compensate for the weight outboard. This in turn plays right back into the effective lift (and power) that the wing itself must be able to accept as the airfoil becomes stiffer and heavier while the structure /around/ the wingroot/roofmount must accept and dampen whatever transferred vibration and flex modes are generated without sheering said airfoil off the spine of the aircraft. A floppy wing on a tiltrotor means either drastic efficieny losses under forward flight OR hover loads as aeroelastics either make it hard to hold the engines level with the effective airfoil AOA plane under load. Or bow the wings (and thus rotor disks) dangerously in the hover.

Meaning yet more weight on BOTH sides of the root to keep those outboard mounts planed exactly where they should be.

The TW decreases effective hover downwash area and /may/ perform like some kind of exotic blown flap depending on what the various leading/trailing edge surfaces look like and how fast they can act during transition. Against this is the notion that there is no multiple thru-spar transfer, side to side and what does rotate must accept a lot of 'slosh' factor on fuel as well as presented area.

But if you accept ESTOL rather than VTOL as your principle landing mode, you can also increase the number of engines while decreasing prop diameter and flat out /pulling/ the proprotor and nacelle feed articulation crap in favor of an RCS system. All of which should drastically reduce weight ON MOUNT with the engine itself.

At which point, all the idiocy inherent to a design point 'centered but never meshed' on a pair of 38 foot rotors effecting every element of span loading and transitional stress (how much of said wing must rotate and whether you want more area/sectional depth on the inboard sections to offset a shorter over all wingspan and more root area to bear the loads outside the direct wash diameter) must also be reassed.

>>
Dynamically? Well, in 'hover mode', the rolling moments will be coming from the engines themselves, again, less stress on wing root.
>>

And it is the separation of the thrust lines NOT the thrusline:fuselage of BOTH engines which will drive the load bearing as you must accound for the accelerative and cushion effects on the loaded wing going down and the unloaded wing _bearing the engine weight_ coming up. As summative stresses requiring equal reinforcement of the roots.

>>
In 'flight mode', hmmm, ailerons are inboard of the engines, but still away from the wingroot. I think in that case there would be greater tension in the wing spar, but still less bending moment at the wing root.
>>

The wings are stiff. They absolutely -have to be so- in order to accept torsional loads and longitudinal sheer and compression factors. Because you cannot afford to have one side flying out of the plane of opposed thrust OR lift (AOA) curve when the wing is flying or hovering with that bloody fuselage suspended like a pig between two jealous eagles.

Stiff wings are heavy wings and they transfer loads to the point of suspension which only adds yet more to the loads and vibrations that it too must accept.

>>
So a tilt rotor would actually reduce the wing root loadings in flight (in comparison to a tilt wing).
>>

See above, you can do things with a TW that change the definition of what is wing and what is nacelle and how the loads are distributed by wing sweep and total area AT THE ROOT. As well as how many such are blowing vs going in what particular mode.

The only thing you MUST do is accept is the notion that pure VTOL is tactically useless compared to a true 'rough field and road strip' optimized airframe can do with 2-4 times the internal weight and 10 times the cubic volume.

Something we never seem to understand in our fixation with STOL as a function of a 2,500ft + 30ft obstacle and 'how to shortland a C-17'. 60-100ft can be almost found anywhere you've got satellite driven topomapping. Just ask an Air America Porter pilot.

But a half a mile is not nearly so simple.

And landing ontop the heads of some yutz /shooting at you/ (leaving mines in the trees, spears in the grass, mortars preregisterd) in a hot LZ is the casepoint definition of military moronicism.


KPl.