Tiltrotor VS Tiltwing

Originally posted by ch1466

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.

A floppy wing on either a tilt wing or tilt rotor will play havoc with the design - however, I think a tilt rotor will have a stiffer wing per kg than a tilt wing anyday.

In flight there can only be 2 sources of lift, the rotors themselves, or the wing.

Now, in the case of a tilt wing, more of the weight is concentrated at the wing root (for wing twisting mechanism), and the engines themselves may be lighter (but the engines aren't a factor as they dont strain the wing root).

For a tilt rotor, the wing root is fixed, simple and lighter wing/fuselage structure. Ok, there is more mass out on the wingtip, however, this is not stressing the wing root in the air. The wing is spreading its lift more evenly in both directions, inboard and outboard. Same for the rotors, they are lifting at the wingtips, better to have the mass at the point of lift than have to transmit the lifting force through a longer load path to the wingroot. OK, so some of the rotor wake is ballsed up by the wing planform, but again, bigger engines dont really matter to the wing root in flight! (Strange as it may seem) and since the fuselage will be heavier than the engines, more loading will be on the wing root holding up the fuselage in flight than the engines on the ground.


For extremely low speed flight, the thrust is directly dependant on the swept area of the rotor, so decreasing the rotor blade size is not advantageous at all. The V-22 is wanted for marine work I assume (?? someone confirm this either way ??) so VTOL is an absolute necessity - needing the big rotor blades.

Originally posted by ch1466

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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.

The change in wing root loadings from a roll manoeuvre in hover mode would pale into insignificance compared to the gust loadings the wing has to be designed to withstand.

Besides, if the forces are coming from the wingtips, its better to have the mass located nearer to the wingtip to reduce the moment arm, not the case with a tiltwing.

Originally posted by ch1466
>>
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.

I'm not quite understanding you here. 'Stiff' is a very subjective word as all wings flex to varying degrees (for instance, the B-52's wingtips flex something like 30ft - just off on a wee tangent - I've heard there is work on dynamic gust loading compensation on the ailerons to reduce loadings and extend the airframe lifespan)


On your transfer loads to the point of suspension. If the tilt mechanism is at the wingtip it means less mass to hold at the fuselage (less upward force to transfer to the point of suspension as you put it).

Originally posted by ch1466

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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.

I don't understand your 'change the definition of what is wing and what is nacelle' - are you talking about having the wing twist mechanism at a location on the wing and not the wingroot?

Wing sweep?? On one of these kinds of aircraft?? Be less than 5 deg at the MAC I'd think.

Also, a tilt rotor can have a much longer wing chord allowing for greater wingbox thickness and what that infers for loading distribution through the wingspars.

If your working from boats, I'd say pure VTOL is still pretty important.

Killco,

>>
I don't understand your 'change the definition of what is wing and what is nacelle' - are you talking about having the wing twist mechanism at a location on the wing and not the wingroot?
>>

Yes, exactly.

>>
Wing sweep?? On one of these kinds of aircraft?? Be less than 5 deg at the MAC I'd think.
>>

Compound sweep in either direction and even the potential for fan-in-wing or other 'augment' designs all open up when you change the planing angle of the wing to the thrustline rather than the other way around.

>>
Also, a tilt rotor can have a much longer wing chord allowing for greater wingbox thickness and what that infers for loading distribution through the wingspars.
>>

Conventionally as with the X-18 and XC-142, yes.

>>
If your working from boats, I'd say pure VTOL is still pretty important.
>>

No. At least not for the air mech (air mobile/STOM) mission wherein heavy lift helos dominate for their ability to first-land a HUGE contingent.. Such is the class of machine I think they 'missed the boat' on when the Marines went with such a technologically adventurous route for replacing their Frogs. With such a requirement, the landing fields will be carefully chosen and the mission launch will be from larger ships or airbases with full runway capabilities.

Speaking of which, it should also be noted that both conventional and helicopter/assault carriers are very much an inefficient design.

The first because it both soaks and isolates a VAST area of flightdeck real estate with the island and angle decks separating the deck parks and 'interrupting the feng shui' flow of cyclical air ops from any kind of realistically efficient, let alone concurrent action.

The latter for the same reason that linear-axis flight decks were an accident waiting to happen in WWII: simultaneous deckops being impossibility and the dangers of having even an ESTOL platform (cockpit in front of engines) splatter it's aircrew in, thru or over any barrier also jeopardizing the remainder of the flightdeck and airwing.

BUT.

1. You put two runways side by hip on the same linear deck axis on two catamaran (SWATH) type hulls.
2. Recover forward to elevators on the centerline (effectively giving yourself a thru deck 'keel' to add rigidity to the wider above-water hull box while providing a margin in which to separate the flight decks).
3. Install a bilevel hangar with short-height spacing for UCAVs and a taller 'assault deck' for the troop transport/CSA and/or 'fighter' type assets.
4. Cellularize hangar /stowage/ for airframes not in use.
5. Add a ramp system or further elevators /aft/ so that the flow of aircraft is always forward, both to load the cats and get off the arrestors.

And it becomes fully possible to have simultaneous STOVL/CVTOL operations running side by hip with either conventional rotary wings. Or to employ ESTOL type miniairlifters while abandoning the ridiculous payload:range and _rate_ limitations of conventional helos altogether.

Furthermore, IT WORKS because the available landing area (full hull length) is still greater than any angle deck and the lack of crab angle, combined with a 40cm JPALS scatter error on a 60-90knot approach makes it possible to clear the spudlocker no matter the deck pitch or heel moment, come down in the first 1/3rd of a 500-600ft decklength. And still have the remaining 2/3rds to recover energy and make the bolter/failed reverse thrust+brakes etc. happen without worrying about how the airframe is going to roll off and lunge and how far the rotor fragments are going to go, when and if a conventional hover loses power.

Ditch or at least VASTLY downscale the island (MEMS Masted and virtually multi-cam'd) and, theoretically, 'special configuration options' precursors could even be in place to allow for rapid conversion via a C-130 type 'Super-COD' airframe acting to deploy combat battalions. Effectively /making/ an assault carrier from a conventional one in just a few transit loads.

CONCLUSION:
Non-compound/CRW type helicopters just don't have a place anymore, IMO. Even as ASW/SAR/SOF birds, the distances involved with modern weapons systems and the number of operating stations/hulls available to forward-preposition them just doesn't support using a 150knot platform to sanitize or recover or rescue groups of 10+ men (or missile firing submarines) over perhaps a 300nm radius of some 282,600 square miles.

Unfortunately, the U.S. Armed Farces are still too entrenched in platform specific (R&M turf preserve) glory-missioning to realize the extent of the basic paradigm change inherent to 4GW. And they have wasted five years of 'patriotic' funding surge to jury rig an ugly war while 'keeping hope alive' on those ego missions in their back pocket.

And now there is no money for a CV-X or even a CVN-79 in the Nimitz class. Nor is there the money to make the V-22 work in a way which highlights what it IS capable of. Versus what the rapid followon needs to do in turn to provide more generic air-mech capabilities for the light forces that are not merely what is needed today but all that can be afforded for a goodly number of tomorrows.


KPl.


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You're thinking too conventionally.

Imagine two CH-47 Chinook or better yet Mi-12 Homer powertrains, in tandem. Now (BWB) stick a pair of YB-49 centerbodies inbetween the each pair of rotors and graft the cargo compartment of a scaled C-130 inbetween.

Adjust the spanwise distribution of the thrustlines using the variation in the fuselage cavity widths (pilot/crew cabin and payload bay) to create assymetric (fore and aft) wingspan variation or employ staggerwing vertical spacing. So as to keep everybody's wake clear of everybodies airfoils on a large propfan buried, canberra like, at 'midspan'.

Now THINK what kind of an airfoil you want outoard of that thick center section and whether the engine nacelle should itself be articulated with or independent of it.

Provide /just enough/ variable incidence as thrustline change to both rapidly increase drag and _stabilize_ lift (dancing around the tip stall) using AMST (YC-14?) type active USB.

Alternately, consider making your engine-specific 'nacelle' into merely the partial cowling enclosure basis of a 'lift box' plenum in which, whatever side (LE or TE) you put your propfan, a takeoff shaft runs to a tandem fan or SDLF fan-in-wing type arrangement which in turn provides improved ejector performance that, along with wing tilt, allows you to both seek and modify max AOA lift curves all across the dual wings for any given payload:airspeed combinant.

Keep the gear short to ease ramp loading without kneel and provide rapid on-wheel reversal to the main cruise propulsion so that you can kill forward thrust /without/ destroying active-lift. Whether buried inboard or tilted out.

>>
I'm not quite understanding you here. 'Stiff' is a very subjective word as all wings flex to varying degrees (for instance, the B-52's wingtips flex something like 30ft - just off on a wee tangent - I've heard there is work on dynamic gust loading compensation on the ailerons to reduce loadings and extend the airframe lifespan)
>>

I'm well aware of how much the BUFF wings 'flap in the breeze'. But they don't have rotor disks which can mesh and thrustlines which can bunt to an angle independent of wing torsional (lateral) twisting, even /amplify/ the existing AE divergence.

>>
On your transfer loads to the point of suspension. If the tilt mechanism is at the wingtip it means less mass to hold at the fuselage (less upward force to transfer to the point of suspension as you put it).
>>

See above. Think bigger, blended, wings. And then find space for fuel and ESTOL constant, powered, lift. Without passing all that downwash over a body like that of a manta ray that has swallowed a mail tube.

It doesn't have to hover. But it needs _control_ on the descent with a big enough body to at least /consider/ a hummer class (10,000lbs X 14ft wide) internal bay carriage volumetric.


KPl.

I see what your thinking now - fixed inner wing sections where the cargo is held (on/in an integrated fuselage) and moveable sections outboard of the nacelle/rotors (well, essentially fixed to the nacelles which move).


It should work fine I think, but its kind of on a very different scale to what has been done before though

The Novel Innovative Competitive Effective Tilt Rotor Integrated Project (NICE TRIP) has choosen Tilt Wing over Tilt Rotor, as shown here:

www.aerosme.com...

Good find


They are just turning whats in the rotor blade wake to minimise downstream blockage - better hover.

I still think it is going to be pretty ugly in transition though - but its been done before, so should be workable.

Isn't that what I said when the thread started?

Originally posted by waynos
Isn't that what I said when the thread started?

Is EUROTILT the same project?

I dunno, just asking - true, I pretty much repeated exactly what you said in your first post on the thread.

But I wanted to reiterate the point I still see it being very aerodynamically ugly in transition.

No probs, according to the text in the link it does refer to 'the European tilt rotor project' without actually mentioning Eurofar. I don't think that acronym is used anymore, a bit like ACA/FEFA/EFA etc, they just like to keep changing.