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
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-
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
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.
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
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
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
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.
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.
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.