Cry for technical help re foward swept wings

Hi, can anyone help me out with some calculations. As part of an A.TT concept project to explore super-short take off and landing (“S-STOL”) UCAVs I’ve come up with this configuration:

The drawing is roughly to scale. The dimensions are set within the guidelines (no bigger than equivalent manned carrierborne strike fighters).

The problem is whether the wings (which are restricted to 10m span in the spec) are big enough.

The only easy comparison to make is with the Grumman X-29. It’s vital stats are:
Span: 8.28m
Length: 14.6m
Height: 4.35m
Thrust: 16,000lbst
Weight (max): 17,303lb

My design’s vital stats:
Span: 10m
Length: 14m
Height: 5m
Thrust: EJ200; 20,250lbst with 23degree TV and AE-3007 lift jet 6,764lbst at 20degrees slant
Weight (max): 30,000lb

Any technical help on calculating wing area and predicting whether it would be STOL would be greatly appreciated. Also, design suggestions for making it perform better (S-STOL aspect).

Thanks in advance

can't do the calculations I'm afraid but my gut impression is that, with FSW and canards the lift jet is superfluous, but withouth it you could carry more fuel for the EJ-200.

Originally posted by waynos
my gut impression is that, with FSW and canards the lift jet is superfluous, but withouth it you could carry more fuel for the EJ-200.

Probably not if you want to land a Jaguar sized strike fighter on a ‘small’ carrier without the use of arrester gear whilst bringing home 10,000lb of expensive weaponry…
These links might make things clearer..
www.armbell.com...
www.armbell.com...

OK, see that now, is there a possibility of a 90 degree tiltable nozzle on the back of the EJ-200 for automated vertical recovery?

sorry bro i dont think it would b a very good STOL because of the forward swept design. the forward jet is asking for problem just as the JSF's simalar design has been nothing but problems we are just not to the point where we can put that much thrust with that relialabilaty. it a nice design though but the forward canards might mess with the lamanar flow over the main wing causing a bit of a problem. if it all worked out though it would be an amazing first responce dogfighter!

another thought, maybe the Vought designed tandem fan of the TF-120 would work well in this type od design and without the weight penaly of a lift jet?

OK, for rearward swept wings on commerical you effectively pick a lift coefficient curve, then depending on the wing loading add what 'extras' are needed for approach. Be it single slotted flaps, double, fowler or whatever.

I think leading edge slats are more for take-off, but honestly cannot remember, I'd need to check it.


In the low subsonic regime, for take-off, a FSW should be a little bit more efficent than a RSW due to the spanwise lift distribution - so you could maybe add a little to the basic wing CL - maybe 0.05 or 0.1 or something.


For working out the wing area, simplify the wing into as many 'blocks' as possible - form a series quadrilaterals/triangles and calc from that.



One other little thing, why the LERX and canard - the two don't go together - the canard wingtip will act as a kind LERX at high AoA anyway - thats why there is no LERXs on the EF or rafale.


[edit on 31-12-2005 by kilcoo316]

but there are LERX' on the X-29 and Berkut, so on a FSW type it seems that they do go together.

The X-29 has no real LERX to speak of - but the Berkut indeed does.


Why the Berkut does I have no idea - unless the russians are trying to combine the vortices from the two (LERX and canard) into one for improved performance. But they know alot more about it than me, so I'll not doubt there are reasons for it.




edit: Just remembered (from waynos post ) its Leading Edge Root eXtension -d'oh

[edit on 31-12-2005 by kilcoo316]

Thanks for the responses

I thought (correct me if I’m wrong) that FSW suits STOL as it has better slow speed control (aileron stall) and higher angles of attack (allowing slower landing speeds???). Also, the X-29 and S-37 are both cited as STOL designs… the canards and small leading edge extensions at the wing route can also be explained. (I’ll get back on that but at any rate I’m only regurgitating what I’ve read on the web, I’m not an aero-engineer.


Waynos, lift fans are on the cards as is augmented thrust engines but lift jets are the simplest and least complicated way –although the thirstiest and heaviest. With modern engines with better efficiency, thrust to weight ratios etc then the argument in favour of lift jets becomes stronger.

Originally posted by planeman
Thanks for the responses

I thought (correct me if I’m wrong) that FSW suits STOL as it has better slow speed control (aileron stall) and higher angles of attack (allowing slower landing speeds???).

For the 2 questions, yes, and dunno. In theory a FSW should be a more efficient wing (better L/D) but I don't think NASA found that to be the case with the X-29.

I'm trying to think about the high alpha for a FSW wing, but my hangover is killing me - sorry

S-STOL Concept –why the forward swept wing

(note that the outline A.TT requirement does not specify a forward swept wing nor lift jet use, those are merely characteristics of certain proposals being worked up).

The basic idea behind S-STOL (super-short take-off and landing) is to have many of the operating advantages of vertical take-off/landing designs without the weight and complexity issues. The S-STOL aircraft (a UCAV in this case) could thus take-off and land in such a short distance that it could operate from short stretches of road, small aircraft carriers or even barges –and bring home unused stores.

The key lift device is the lift-jet, with the main engine’s thrust vectoring and wings adding.

The S-STOL landing is largely dependant on incredibly slow landing speeds. This implies extreme aerodynamic breaking (high angles of attack) and would require exceptional stalling and slow speed stability. Of course, the thrust vectoring and lift jet ‘cushion’ thrust play a large part (particularly in making the landing ‘soft’) but the wing plan is crucial. The answer seems to be forward swept wings. Internet sources which back up this rationale:

NASA: www.dfrc.nasa.gov...

Research results showed that the configuration of forward-swept wings, coupled with movable canards, gave pilots excellent control response at angles of attack of up to 45 degrees. …. on the forward-swept wing, the ailerons remained unstalled at high angles of attack. This provided better airflow over the ailerons and prevented stalling (loss of lift) at high angles of attack.

Airforce Technology: www.airforce-technology.com...

The swept-forward wing, compared to a swept-back wing of the same area, provides a number of advantages: higher lift to drag ratio; … improved stall resistance and anti-spin characteristics; improved stability at high angles of attack; a lower minimum flight speed; and a shorter take-off and landing distance. …. … A substantial part of the lift generated by the forward-swept wing occurs at the inner portion of the wingspan. The lift is not restricted by wingtip stall. The ailerons - the wing's control surfaces - remain effective at the highest angles of attack, and controllability of the aircraft is retained even in the event of airflow separating from the remainder of the wings' surface.
The all-moving and small-area trapezoidal canards are connected to the leading-edge root extensions.

Russian Aeronautics: www.aeronautics.ru...

It is well known that a forward swept wing provides significantly increased manoeuvrability at low airspeeds and high angles of attack, less aerodynamic drag compared with a swept back wing, which in turn leads to an increase in range and improvements in take-off and landing characteristics. …..An aerodynamic layout which embodies an FSW guarantees a better blending of wing and fuselage and also optimises the pressure distribution over the entire wing and foreplanes. According to the estimates of US specialists, the use of an FSW on an aircraft like the F-16 should lead to an increase in turning ability by 14% and radius of action by 34% while take-off and landing distances would be reduced by 35%.

It is also interesting to note that both the notable FSW jets to fly have had canards and small LERX:
X-29:


S-37:

The canards are particularly relevant for trim at high angles of attack.

Right - finally got my head round it.

The reason why a FSW can operate at high AoA is a side-effect of the trailing wingtip vortices. On a RSW these are behind the majority of the wing so don't affect the wing itself.

However, on a FSW, the downward component of the vortex will act on the inner sections of the wing, thus helping to keep the flow attached a little longer. I'm not sure if this is also the reason for greater aileron authority at high AoA, but its very possible.



I'm also still of the opinion the 'LERX' on the X-29 isn't a real LERX as it will not induce the flow structures a real LERX will. The one on the S-37 is undoubtedly real though.

Ok, we’ve done the wing area calculations and comparison between my concept design and the X-29. Here’s how we estimated it:


and the X-29:


…crude, I know.

So my design (with the initial outline wing) has only 10% more wing area (21% larger main wing, 50% larger canard and 40% smaller rear aerofoil) but a proposed maximum weight so 73% greater.

My design (provisional specs) relative to X-29
Wing area = +10%
Wing span = +20%
Horizontal thrust (main engine) = + 26%
Intended max weight = +73%

So the two key questions are:
1) Is the Lift jet thrust sufficient to make it S-STOL
2) What impact is this going to have on wing loading and related airframe stresses?

My first thoughts are that I want to make the wing area greater –belts and braces approach. A lifting body design is of interest but given the depth of the lift jet not very easy to design.
Proposed part solution:


Any suggestions or technical help would be much appreciated.

Why is your MTOW so much greater - you don't need life-support for a pilot after all?

Even with the 2nd engine, it should not be this much greater??


What parameters did you use to come up with your initial MTOW?

Good question, the provisional specification parameters required the air vehicle to be no bigger than ‘light’ carrierborne fighters but carry 10,000lb of weapons (about the same as the Jaguar).

We also looked a fuel loads. The EJ200 engine is quite efficient and has high power without afterburner, but with the lift jet, the overall fuel consumption is going to be higher –so 10,000lb was allocated to internal fuel (slightly more than the Jaguar).

So far: 20,000 lb without the airframe.

That leaves airframe. We know the weights of the engines (2,286 lb and 1,617lb) so we made the guess that the rest of the airframe would be around 6,097 lb which brings us up to 10,000lb dry weight –by comparison to other types, 6,097lb looks about right since although we’d need to use a high proportion of composites (forward swept wings require it for strength), the wings all the same would be comparatively heavy to stand up to the stresses. And we’d need heavy landing gear for S-STOL landings.

So our MTOW is formed thus:
10,000lb max weapons
10,000lb fuel
10,000lb airframe.

It is worth noting that the X-29 had no provision for weapons load at all. We are taking that into consideration.

Here are some comparisons with relevant aircraft:

internal fuel
Jaguar: 9,240 lb
Skyhawk: 7,722 lb (est)
Super Etendard: 5,746 lb
Harrier II: 7,7432 lb
AMX: 5,993 lb
S-STOL UCAV: 10,000 lb

Weapons load
Jaguar: 10,000 lb
Skyhawk: 6,000 lb (est)
Super Etendard: 4,620 lb
Harrier II: 9,798 lb
AMX: 8,360 lb
S-STOL UCAV: 10,000 lb

Dry weight
Jaguar: 7,500 lb
Skyhawk: 10,778 lb
Super Etendard: 14,212 lb
Harrier II: 13,394 lb
AMX: 14,247 lb
S-STOL UCAV: 10,000lb

Maximum take-off weight
Jaguar: 26,740 lb
Skyhawk: 24,500 lb
Super Etendard: 24,578 lb
Harrier II: 30,034 lb
AMX: 28,600
S-STOL UCAV: 30,000 lb

These figures then need to be compared to engine thrust. It is particularly difficult to do with the S-STOL UCAV as the lift jet needs to be considered. But if we look at basic thrust to weight ratio in level flight at maximum load and maximum thrust:
Jaguar: 0.55:1
Skyhawk: 0.46:1
Super Etendard: 0.45:1
Harrier II: 0.72:1
AMX: 0.38:1
X-29: 0.92:1 (note, no weapons)
S-STOL UCAV: 0.68:1

Note that the S-STOL UCAV has a lift jet to assist take-off and landing.

Now if we consider the thrust to weight ratio with 50% internal fuel and 4,000lb of weapons:
Jaguar: 0.91:1
Skyhawk: 0.60:1
Super Etendard: 0.52:1
Harrier II: 1.04:1
AMX: 0.52:1
X-29: 0.91:1 (notional)
S-STOL UCAV: 1.06:1 (main engine only)



… so overall, 30,00lb MTOW looks about right.

OK, your 'weighting' may be acceptable - however, your internal airframe volume may not.

What is the comparison of physical dimensions between your machine and the jaguar. You won't be able to store 10,000 lbs of fuel if you dont have enough volume in the wings/fuselage free.

Also, I would expect the wing tank volume to be restricted on a FSW in comparison to a RSW as the structures have to be so much tougher (even allowing for composites).

There is an aircraft design book by Denis Howe which may be of use to you if you can get it from a library (but its about £100 to buy!!). It will help you get the sizing/weights right.

I agree re fuel in the FSWs and also had concerns about fitting that much fuel in. The scale image below shows the centre section of the fuselage where I envisage the fuel being.

When various structures like wing box, landing gear and fuel pumps are added the fuel volume will decrease compared to the representation –but it is clear that the fuselage is generally big enough to contain 10,000 lb of fuel (about 4500 litres)

We could also consider a fuel sleeve around the engine etc.

Uhm, 10,000 lbs of fuel does not equate to 4,500 litres??

Don't forget fuel does not weight the same as water - if I remember right its about 0.8 times the weight of water - 1000 litres of fuel = 800 kgs or so.

So for 10,000 lbs of fuel you need approx 5670 litres of room.


Have you compared your proposed fuselage internals to a cut-away drawing? (Just to see what else would be in there, and would there actually be the space proposed?)

Hydraulics and ancillaries take up alot more room than you'd imagine.


edit: Yup, Kerosene has a density of 815 kg/m^3


[edit on 3-1-2006 by kilcoo316]

Thanks Kilcoo, that’s great info and help. I went astray by comparing two sources, one of which said that the Jaguar had an internal fuel load of 9,240lb and the other saying 4,180 litres. I divided one by the other and came up with 2.2105lb per litre … about the same as water –yes, it doesn’t surprise me that it’s wrong –damn internet sources, lol.

Anyway, using your figures 10,000lb is around 5,560 litres.

Here’s the redrawn cross-sectional illustration:


Also, here’s something I should have posted before –the provisional layout of the D-0013-a design. The red sections are primarily fuel.


You are right, 10,000lb of fuel is going to be a struggle.