How much trim control do LEVCONs provide?

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collimatrix

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Unread post10 Aug 2018, 22:12

One of several design decisions about the HAL Tejas that does not make sense to me is the selection of a tailless delta planform.

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This aircraft is supposed to have a carrier-based variant, and a tailless delta is a horrible choice of wing planform for a carrier-based fighter.

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Remember how much these sucked?

The most demanding and dangerous part of carrier operations is landing. Landing is considerably easier if the aircraft in question has a lower stall speed, since it can perform this delicate operation at a lower airspeed.

Lowering the landing speed of a tailless delta is harder because tailless deltas can't use flaps. Because tailless deltas lack a second set of independent control surfaces that is not in line with the trailing edge of the wings, they can't trim out the pitch changes of deploying flaps. A tailed design or a design with all-moving canards (or even canards with an adjustable flap like the SAAB Viggen) can keep even trim even when the flaps are deployed.

So I had written off the use of a pure, tailless delta as just another one of the many nonsensical decisions made in Indian defense procurement.

But looking at it again, the Tejas has those big honking LEVCONs. Could LEVCONs possibly be used for trim adjustment?
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vilters

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Unread post10 Aug 2018, 23:39

Delta's makes the strongest wings platforms and have room for a lot of fuel internally and strong for stores and landing gear..
They are also "naturally stable" platforms enhancing overall flight performance.
Ok, they need a high AOA on landing, but with proper configuration, even that should not be an issue.

PS : The Cutlass was dangerous, not because of it's wing (at the time that was a very GOOD wing) but they where massively underpowered.


A Delta with LEVCONS comes very close to some other aircraft like the Rafale and the Tiffy.
Only that the canard is attached to the wing, thus reducing RCS while enhancing lift and performance if the controlling software is up for the task.

In a good configuration it will also make a delta less prone to energy loss during manouvering.

Adding a LEVCON to a delta makes a lot of sense IMHO.
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element1loop

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Unread post11 Aug 2018, 04:24

It is primarily higher wing sweep angle that pushes up approach AOA, not delta shape.
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collimatrix

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Unread post15 Aug 2018, 03:41

All of this is interesting and none of it answers my question; can LEVCONs provide independent trim control and trim out the pitch change from the use of flaps the same way canards or conventional horizontal stabs can?
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Unread post15 Aug 2018, 04:08

I’m guessing that the levcons mostly just change the center of lift at high angle of attack and tweak the formation of vortices. They probably don’t have much ability to do much if the aircraft is at a low angle of attack, because any lift they generate is at the expense of the wing they are attached to.
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element1loop

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Unread post15 Aug 2018, 09:19

collimatrix wrote:All of this is interesting and none of it answers my question; can LEVCONs provide independent trim control and trim out the pitch change from the use of flaps the same way canards or conventional horizontal stabs can?


That sure is an interesting question. I'll opine 'no', as they're not designed for trim stability during any phase of flight. They're only designed to modify and manage the stall onset flow detatching at low speeds, rather than to adjust trim for sustained low speed flight in negative lift and high drag.

I can't see how they could provide enough 'leverage' as a trim control surface in a slow speed air flow, to counteract the strong flap or flaperon pitch trim change needs in multi-position flap extensions, when hot 'n heavy with highly variable CG.

Elevator trims are much better for low speed because they're often bigger, and much further from the CG and the pitch rotation axis, and out further from the wing root's center of lift, so provide much more leverage and adjustment on a slow air flow. So provide for much better fine-tuning of attitude with changing CG. The LEVCONs would fail those requirements, as they will be much weaker in those areas.

Plus CG balance limit envelope is contolled by pitch trim leverage range, on the airflow. The more pitch trim range (leverage) you have, the further you can fly into an out of weight and balance condition before stalling (due to a lack of available thrust to over come the induced parasitic drag, from high AOA, as TOW, poor balance, or OAT rises, or all three).

But the heavier and slower the aircraft, and the more away from CG balance it is, the more leverage you need. So if LEVCONs could trim at all, they would have to be rather large and/or rather long, or else you're going to quickly run out of leverage on a slow air flow, that is already close to detatching from the control surfaces.

Seems like a dangerous mechanism, with not much leverage, or enough surface area to delay air detatching from them suddenly.

Consider how weapons, internal fuel and EFT loads would tax their available range limits, with balance and pitch changes on approach, and as wheels come down, then flaps deploy, and pitch it again. Too dodgy I think, especially when low and slow in a sink. The last thing you want is less than solid pitch trim leverage towards the stall as speed drops closer.

And what about if you have engine failures, or must glide, or the wing or control surfaces lose area due to battle damage. You want a large excess of trim leverage for those situations.

Canards are again big, with a wide range of movement, and their leverage comes from being well forward of the axis of pitch rotation. LEVCONs are just too small and too close to the dynamic axis of pitching, and not going to have enough spare margin.

So IMO, no, there's not enough bite on the air flow at flap deployment V-speeds for a fighter. That said, it could work on some light or experimental category GA aircraft, etc., but adds complexity and weight. So why use that approach at all?

[I'm guessing you're asking with regard to LO impact of removing canards or elevators, and using LEVCONs to trim? In which case, why not use TVC (which T50/Su57 has already]?
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vilters

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Unread post15 Aug 2018, 13:30

LEVCONS are the inboard extension of LEF.

The F-16 moves its LEF 2°UP for supersonic flight.


A delta wing is a horizontal S for automatic inherent stability .

LEVCON and LEF and trailing edge flaps all help to control the curving of that horizontal S optimising it for maximum lift and minimal drag during the flight envelope.

Can LEVCONS trim => Sure they can, just like all other control surfaces can.

With canted rudders, even the rudders can trim. (and do so on some aircraft)
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element1loop

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Unread post15 Aug 2018, 15:24

Most trim range is used up at highest Temp, highest landing altitude, MTOW, MLW, highest certified out of balance condition, dirty at lowest speed with max degree extended flap. So will LEVCONs adequately trim at wide enough flight envelope conditions at slow enough airspeed, for an 80,000lb fighter?

That demands a large dedicated pitch-trim control surface located away from the pitch rotation axis.

But yes, at higher speeds any surface that works with the force and buffet could act as a trim surface. i.e. above certified flap deployment speed.

But at low speed and configured on finals, pitch trim requires a larger deflection surface located further from the pitch rotation axis. Using LEVCONs to attempt to trim with full flaps will shrink the available certified envelope so much that it may become non-viable, or so constricted in performance envelope as to be impractical.

i.e. no one will buy it.
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element1loop

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Unread post15 Aug 2018, 15:44

But if the jet has a conventional dedicated pitch trim, and the question is simply, can a large enough auto LEVCON movement (alone) net counteract the flap deployment trim needs (alone), then that could (if they are big enough) be sufficient to maintain stable pitch trim as flaps deploy, near to finals speed range.

Not really sure why you'd want to, but it could improve the available total trim authority near to the full dirty stall speed. But unless you can keep the airflow attached (i.e. use vortex generators, etc.) why bother?
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vilters

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Unread post15 Aug 2018, 17:27

Modern software can do many things.

On the PAK-FA, those LEVCONS also work as "ailerons".

And supersonic trim needs very little leading edge movement to get large results.
Remember that you are working on the Leading edge curve here, and a small movement has a huge impact further down the wing at any speed and AOA combination.
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element1loop

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Unread post15 Aug 2018, 17:50

I'm ignoring that aspect only because it has nothing to do with the question, which is:

collimatrix wrote: ... can LEVCONs provide independent trim control and trim out the pitch change from the use of flaps ...
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vilters

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Unread post15 Aug 2018, 22:23

element1loop wrote:I'm ignoring that aspect only because it has nothing to do with the question, which is:

collimatrix wrote: ... can LEVCONs provide independent trim control and trim out the pitch change from the use of flaps ...


You can ignore whatever you want but you are missing the point.


In a previous post you write : "can large Levcon movements . . . . . "

You don't need large LEVCON movements. => You are working on the leading edge of the wing.
For trim you only need very small LEVCON movements for trim because even a tiny leading edge movement will have a huge impact on the airflow over the entire wing.
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element1loop

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Unread post16 Aug 2018, 05:38

vilters wrote:
element1loop wrote:I'm ignoring that aspect only because it has nothing to do with the question, which is:

collimatrix wrote: ... can LEVCONs provide independent trim control and trim out the pitch change from the use of flaps ...


You can ignore whatever you want but you are missing the point.
In a previous post you write : "can large Levcon movements . . . . . " You don't need large LEVCON movements. => You are working on the leading edge of the wing. For trim you only need very small LEVCON movements for trim because even a tiny leading edge movement will have a huge impact on the airflow over the entire wing.


"Vilters" didn't you even read the question yet? What don't you comprehend? You're going on about speeds well above flap deploy V-speed range. More ancilliary 'info' about supersonic and transonic leading edge effects has zero to do with it as flaps rip away if deployed at those speeds. :doh: :mrgreen:

This is only about LEVCONs retrimming for the pitch change induced by flap deployment--AT LOW SPEED.

You haven't addressed the question at all, and are trying to argue with someone who did! :doh:

Interesting tactic. :mrgreen:
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vilters

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Unread post16 Aug 2018, 14:40

For Christ sake.

A Delta Wing HAS no flaps.
Why do you keep coming with flap deployment when there are no conventional flaps on the thing?

And a LEF or LEVCON ripping? Only after a technical defect. => Ask Gums. => He landed an F-16 with a ripped LEF after a technical defect..

PS, as I already wrote, the F-16 LEF go UP 2° supersonic. They don't rip off at all. (if all bolts, nuts and cotter pins are installed.)

Again, a pure delta wing HAS no conventional flaps.

And if you think a bit more then the length of your nose?
Trimming the leading edge is far more economical AND safer then trimming the trailing edge.
=> You ADD lift instead of removing it as one does with conventional trim systems.
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element1loop

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Unread post17 Aug 2018, 01:32

Rubbish, all deltas I'm aware of have trailing edge surfaces that are designed to act like flaps, to droop the main wing trailing edges lower, in order to drop the flyable pre-stall speed lower.

Why do you think the question is about can LEVCONs be used as trims to counter "flap" deployment on a delta wing?

I mentioned FLAPERONS in my second comment, for that reason, i.e. some delta wing designs treat ailerons as a flap, via drooping both equally, while maintaining their normal aileron up-down deflection operation as well. Hence the trailing edge control surfaces can operate as ailerons, elevons and flaperons. Do you want to just pretend flaperon type implementations are not present or used as true flap-devices, on delta wings? Do you want to pretend these produce no induced pitching on deployment, such as during approach? :roll:

Delta wings do have flap devices, they just don't use high-deflection flap devices, but they aim to do the very same thing, namely, to lower the stall speed.

[And btw, the reason why delta wings use less flap deflection is because, as I already pointed out for normal elevators, the further from the pitch rotation axis of the aircraft the flap-type devices are, the more leverage the control surface has on the slow airflow. The delta wing may use less flap-type surface-deflection ANGLE, but that does NOT mean they have less FLAP EFFECT. They have the same effect as a regular high deflection flap design, simply because of where they are located, further out from the pitch rotation axis, which creates more leverage on the airflow, so the flaperon config still induces a strong pitching when deployed, which still requires substancial trimming to counteract ... "no flaps" ... my butthole.]

Hence the question is, can LEVCONS counter the pitching induced by "flap" deployment on delta wing aircraft.

The flap devices are deployed at LOW SPEED (not supersonic speed), so the question is asking can LEVCONs deflect enough airflow to trim at low speed, to counteract the induced pitching of deploying flap devices on a delta wing?

All the rest of your stuff about trimming supersonic flight with LEVCONS, or bringing in other leading edge control surfaces, or denying that delta wings even have flap control surfaces, is irrelevant. :doh:
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