Musings on naval S(V)TO(V)L

[hagaricus]

I've been grinding these ideas around in my head for ages, so I thought I'd put them out there, even if just to be shot down in flames.

VTOL: the main thing about this, and the reason it has largely evolved into STOVL is the penalty imposed on the aircraft for the hover. Has anyone attempted to take the experience of all the VTOL research and apply it to STOL? after all, if you can combine artificial stability, blown surfaces/ejectors, vectored thrust and variable geometry to create a mach 2 aircraft with a landing speed of, say 40 knots you've achieved a large percentage of what VTOL actually does, possibly for less of a penalty? I'm thinking this might be a better way to allow smaller STOBAR carriers..

Another way to minimise the penalties of VTOL is the tailsitter. As I understand it the main problem of these is the pilot's orientation in transition & hovering mode. UAVs don't have this problem, and I'm pretty sure spaceflight has taught computers how to operate on different axes, so the step from there to tube-launch and the "submarine aircraft carrier" seems obvious? Tube-launched & deck-recovered, UAVs could give the sub it's own airborne capability.. I expect something like this is already on the forum, but I tried searching..

Anyhow, these are the mumblings of the bloke down that end of the bar, and mine's a bloody mary. cheers!

 

[cluttonfred]

I agree that tailsitters might still still be worth a closer look. Even without artificial stability and the like, something as simple as a a couple of video cameras, both regular and infrared, would go a long way to correcting the visibility issues.

 

[hole in the ground]

I think it was the other references which made vertical control difficult. I think an automated landing system is probably a requirement for tail sitters. One that isn't beyond the realms of possibility.

 

[Broncazonk]

There is a lot of great information in this thread: http://www.secretprojects.co.uk/forum/index.php/topic,14200.0.html

Here is what I was thinking--very similar to your thinking:

“I'm trying to invent a new class of aircraft: the STOSL. Everyone knows what a STOVL is, but STOL has various meanings that nobody agrees on. See dicussion here: http://en.wikipedia.org/wiki/STOL

Anyway, I propose that a STOSL (Sto-sul) aircraft should be defined as: Short Takeoff and Short Landing (STOSL) - The ability of an aircraft in a fully loaded condition to clear a 50-foot (15 meters) obstacle within 150 feet (50 meters) after commencing takeoff, or in landing in a fully loaded condition to stop within 150 feet (50 meters) after passing over a 50-foot (15 meters) obstacle.

Although an aircraft like this may have LHD/LHA application, I'm more interested in learning/reading about all the different STOL concepts that I know nothing about.

Think about an airplane about the size of a PC-6 (something with the space to carry 8-10 people) that can get up and down better than a Storch. An airplane like that would allow rural hospitals everywhere to have air-medivac capability. Medivac helicopters are great, but they are so expensive to operate that in rural areas only the big regional hospitals have them. A STOSL airplane would also be useful to special forces operators and military medivac operations, etc.

Thinking bigger, 30-40 and 70-90 place STOSL aircraft would make great regional commuters. Nasa thinks something like that will be important soon.

Getting up and down vertically creates a lot of troube vis-a-vis weight and design and noise and fuel consumption, but if you could come up with something that 'darn near' or 'comes real close' to having vertical capability, I think there would be value in that.

Bronc”

 

[Abraham Gubler]

VTOL: the main thing about this, and the reason it has largely evolved into STOVL is the penalty imposed on the aircraft for the hover. Has anyone attempted to take the experience of all the VTOL research and apply it to STOL? after all, if you can combine artificial stability, blown surfaces/ejectors, vectored thrust and variable geometry to create a mach 2 aircraft with a landing speed of, say 40 knots you've achieved a large percentage of what VTOL actually does, possibly for less of a penalty? I'm thinking this might be a better way to allow smaller STOBAR carriers..

You may want to look into some of the proposals for the British OR. 346 requirement from 1960-62ish. These aircraft designs were to land at low speeds (<80-100 knots) and used some interesting approaches to achieve this.

http://www.secretprojects.co.uk/forum/index.php/topic,1049.msg165744.html#msg165744

As to a Mach 2 aircraft being able to land at 40 knots without equivalent penalties as being able to hover I think is a bit overly ambitious. But even an aircraft able to land/takeoff at 80 knots is a 55% reduction in the arresting and catapult energy compared to a typical 120 knot carrier fighter of today (of equal weight).

Another way to minimise the penalties of VTOL is the tailsitter. As I understand it the main problem of these is the pilot's orientation in transition & hovering mode. UAVs don't have this problem, and I'm pretty sure spaceflight has taught computers how to operate on different axes, so the step from there to tube-launch and the "submarine aircraft carrier" seems obvious? Tube-launched & deck-recovered, UAVs could give the sub it's own airborne capability.. I expect something like this is already on the forum, but I tried searching..

The control issues were solved in conception in the Convair tailsitter program but they never got to build a block 2 aircraft to show it. For the XFY they proposed placing the pilot in a pod above one of the tails where he would be standing upright for takeoff and landing and so be able to orientate to ground. This design was also carried over to a Convair jet powered tailsitter.

http://www.secretprojects.co.uk/forum/index.php/topic,12730.msg125583.html#msg125583

Later in the Model 49 for the AAFS program Convair proposed a simple tilting nose design ahead of the propulstion that would keep the pilot in the right orientation for the flight configuration.

http://www.secretprojects.co.uk/forum/index.php/topic,6188.msg50962.html#msg50962

But the big problem with tailsitters is like any VTO aircraft it needs to generate 1.2 times the thrust of the takeoff weight. Which when loaded with ordnance and fuel is something no conventional strike fighter can do.

 

[robunos]

But the big problem with tailsitters is like any VTO aircraft it needs to generate 1.2 times the thrust of the takeoff weight. Which when loaded with ordnance and fuel is something no conventional strike fighter can do.

Agreed, plus a 'flat rising' VTOL type, (Harrier, F-35) can perform a rolling VTO or STO and thereby use wing lift to supplement it's engine thrust in order to increase it's warload, which the tailsitter can't...

cheers,
Robin.

 

[Tony Williams]

I had similar ideas a while back, prompted by reading that the RN plans to use rolling landings for the F-35B as well as rolling take-offs. The reason for this is that if STOVL fighters return from patrol with a full warload, their gross weight may be higher than the maximum for a vertical landing. In those circumstances, they have to dump their weapons before landing. Not too important for dumb bombs but you really don't want to be dumping your very costly precision-guided munitions at the end of every flight. So the RN was experimenting with rolling landings in a Harrier, as the addition of wing lift raises the maximum bring-back weight, and wants to do the same with the F-35B.

So I thought: if they're not going to use vertical take-offs or landings, why bother to have the capability - why not just design a super-STOL plane?

What I had in mind was a development of the Harrier which replaced the rear nozzles with a single, straight-through nozzle at the back end which could be deflected downwards by (say) 45 degrees. The front nozzles would remain the same. So when landing, the front nozzles would still be pointing down 90 degrees, the rear one by c.45 degrees to provide some forward as well as downward thrust. For take-offs, the rear nozzle would be pointing straight back, the front ones downwards (but not by as much as 90 degrees).

The advantages of this concept would be that a single rear nozzle would avoid the friction losses involved in the convoluted exhaust path to the twin nozzles thereby releasing more thrust, plus would make it possible to add an afterburner to the rear nozzle to boost take-off and sprint performance. In combination, the extra thrust from these changes should allow a significant increase in the permitted gross take-off weight to the benefit of payload/range performance. It might even be possible to delete the complication of the puffer jets used for VTOL control, although I'm not sure about the exact landing speed which would permit that, and whether such a speed would be OK without arrestor gubbins.

Thoughts?

 

[Abraham Gubler]

What I had in mind was a development of the Harrier which replaced the rear nozzles with a single, straight-through nozzle at the back end which could be deflected downwards by (say) 45 degrees.
Thoughts?

There are a truck load of problems from such a design such as how would you keep the nose up when you move a lot of the downward thrust to the rear. The aircraft will pitch forward on its centre of gravity. In effect you will just have a Harrier that can’t hover and without any avionics because they were displaced via the engine exhaust duct.

If you want a STOL fighter the best point is to start with a conventional fighter. The advantage of such is you can dispense with all the weight of an engine that can provide weight x 1.2 thrust without reheat downwards. A classic example is the F-15 Short Takeoff and Landing/Maneuver Technology Demonstrator (STOL/MTD). You could probably fly one of them from a ski jump carrier with a few discrete modifications.

 

[Tony Williams]

What I had in mind was a development of the Harrier which replaced the rear nozzles with a single, straight-through nozzle at the back end which could be deflected downwards by (say) 45 degrees.
Thoughts?

There are a truck load of problems from such a design such as how would you keep the nose up when you move a lot of the downward thrust to the rear. The aircraft will pitch forward on its centre of gravity.

That would be true if the rear nozzle were deflecting the thrust downwards to anything like the same degree as the front nozzles. Which is why my proposal doesn't do that - I suggested a maximum of 45 degrees downward angle when the front nozzles are at 90 degrees (the exact maximum angle would depend on various details and would be determined by someone with a much better knowledge of aeronautical engineering than mine).

If you want a STOL fighter the best point is to start with a conventional fighter. The advantage of such is you can dispense with all the weight of an engine that can provide weight x 1.2 thrust without reheat downwards. A classic example is the F-15 Short Takeoff and Landing/Maneuver Technology Demonstrator (STOL/MTD). You could probably fly one of them from a ski jump carrier with a few discrete modifications.

Undoubtedly true for a STOL fighter, but my proposal concerned super-STOL - much shorter take-offs and landings than any modification of a CTOL design could deliver, so it could still be used from small carriers as a Harrier replacement, and from very short landing strips (you could probably fly a modified F-15 from a Harrier carrier, but landing it would be a different matter).

I started with the Harrier because it seemed to me that the design lends itself to the kind of layout I proposed. If that approach proved successful, then the next model could have a new airframe optimised to get the the most from the capabilities of the system, e.g. with more internal capacity for avionics and fuel.

 

[SteveO]

I think it's been said that designing for Super STOL carries the same penalities as STOVL without the benefit of the vertical option.

I guess Super STOL would work for lower speed transports or CAS aircraft but if you're designing a high performance supersonic fighter you are probably better off optimizing the airframe for high speed and let the STOVL engine and reaction control system handle the low speed stuff.

The P.1216 is an obvious example. http://www.secretprojects.co.uk/forum/index.php/topic,6661.0.html

 

[Abraham Gubler]

That would be true if the rear nozzle were deflecting the thrust downwards to anything like the same degree as the front nozzles. Which is why my proposal doesn't do that - I suggested a maximum of 45 degrees downward angle when the front nozzles are at 90 degrees (the exact maximum angle would depend on various details and would be determined by someone with a much better knowledge of aeronautical engineering than mine).

Sorry mate but that’s still >5,000 lbs of downforce located ~4m aft of the CG. It is going to pitch the nose down in a major way and <10,000 lbs of downforce located ~1m forward of the CG will not counter it. It’s the law of the lever. In order to balance these forces out you would have to completely redesign the aircraft. All that would be left of it is your modified Pegasus engine.

Undoubtedly true for a STOL fighter, but my proposal concerned super-STOL - much shorter take-offs and landings than any modification of a CTOL design could deliver, so it could still be used from small carriers as a Harrier replacement, and from very short landing strips (you could probably fly a modified F-15 from a Harrier carrier, but landing it would be a different matter).

The F-15 STOL/MD could probably land on a Harrier carrier if it could land within 500m on a non-moving fixed landing field. Add in 30 knots of WOD and some high end brakes and that 500m landing distance will be slashed. You could also do some more things to the conventional aircraft to improve its STOL capability that weren’t applied to the F-15. Like blown wings for improved low speed lift. Maybe even variable incidence wings for higher angle of attack approaches. The F-15 STOL/MD was already rotating at under 40 knots (albeit clean) so that’s pretty much super STOL already.

 

[sferrin]

That would be true if the rear nozzle were deflecting the thrust downwards to anything like the same degree as the front nozzles. Which is why my proposal doesn't do that - I suggested a maximum of 45 degrees downward angle when the front nozzles are at 90 degrees (the exact maximum angle would depend on various details and would be determined by someone with a much better knowledge of aeronautical engineering than mine).

Sorry mate but that’s still >5,000 lbs of downforce located ~4m aft of the CG. It is going to pitch the nose down in a major way and <10,000 lbs of downforce located ~1m forward of the CG will not counter it. It’s the law of the lever. In order to balance these forces out you would have to completely redesign the aircraft. All that would be left of it is your modified Pegasus engine.

Undoubtedly true for a STOL fighter, but my proposal concerned super-STOL - much shorter take-offs and landings than any modification of a CTOL design could deliver, so it could still be used from small carriers as a Harrier replacement, and from very short landing strips (you could probably fly a modified F-15 from a Harrier carrier, but landing it would be a different matter).

The F-15 STOL/MD could probably land on a Harrier carrier if it could land within 500m on a non-moving fixed landing field. Add in 30 knots of WOD and some high end brakes and that 500m landing distance will be slashed. You could also do some more things to the conventional aircraft to improve its STOL capability that weren’t applied to the F-15. Like blown wings for improved low speed lift. Maybe even variable incidence wings for higher angle of attack approaches. The F-15 STOL/MD was already rotating at under 40 knots (albeit clean) so that’s pretty much super STOL already.

I don't know if this has been mentioned (didn't see it) but the SMTD also had thrust reversing.

 

[overscan (PaulMM)]

That would be true if the rear nozzle were deflecting the thrust downwards to anything like the same degree as the front nozzles. Which is why my proposal doesn't do that - I suggested a maximum of 45 degrees downward angle when the front nozzles are at 90 degrees (the exact maximum angle would depend on various details and would be determined by someone with a much better knowledge of aeronautical engineering than mine).

Sorry mate but that’s still >5,000 lbs of downforce located ~4m aft of the CG. It is going to pitch the nose down in a major way and <10,000 lbs of downforce located ~1m forward of the CG will not counter it. It’s the law of the lever. In order to balance these forces out you would have to completely redesign the aircraft. All that would be left of it is your modified Pegasus engine.

P.1216 layout puts the main engine nozzle near the CG, that would work much better.

 

[Tony Williams]

Sorry mate but that’s still >5,000 lbs of downforce located ~4m aft of the CG. It is going to pitch the nose down in a major way and <10,000 lbs of downforce located ~1m forward of the CG will not counter it. It’s the law of the lever. In order to balance these forces out you would have to completely redesign the aircraft. All that would be left of it is your modified Pegasus engine.

P.1216 layout puts the main engine nozzle near the CG, that would work much better.

Quite so. There are two variables to play with here: the location of the aft nozzle relative to the CG and the downwards angle it adopts. The further aft it is located, the smaller the downward angle required to balance the downwards thrust of the front nozzles.

 

[Tony Williams]

The F-15 STOL/MD could probably land on a Harrier carrier if it could land within 500m on a non-moving fixed landing field. Add in 30 knots of WOD and some high end brakes and that 500m landing distance will be slashed. You could also do some more things to the conventional aircraft to improve its STOL capability that weren’t applied to the F-15. Like blown wings for improved low speed lift. Maybe even variable incidence wings for higher angle of attack approaches. The F-15 STOL/MD was already rotating at under 40 knots (albeit clean) so that’s pretty much super STOL already.

I have highlighted the key words here: clean performance is one thing, performance (both in take-off and especially landing) with a substantial payload is another. Apart from the issue of whether or not the F-15 could fit into the lifts and hangars of a Harrier carrier.

To introduce complexity like blown/variable incidence wings would effectively mean a new design, and I'd be surprised if even then it could compete in landing performance with a plane which has more than half its thrust directed downwards.

 

[Spark]

Hi
Just a thought, but woud the final canard Avro728(Navy) c1955 variant have been the first jet stol Fighter project.?
I guess it could have been rather like a marine Viggen but some years earlier.

The F-15 STOL/MD could probably land on a Harrier carrier if it could land within 500m on a non-moving fixed landing field. Add in 30 knots of WOD and some high end brakes and that 500m landing distance will be slashed. You could also do some more things to the conventional aircraft to improve its STOL capability that weren’t applied to the F-15. Like blown wings for improved low speed lift. Maybe even variable incidence wings for higher angle of attack approaches. The F-15 STOL/MD was already rotating at under 40 knots (albeit clean) so that’s pretty much super STOL already.

I have highlighted the key words here: clean performance is one thing, performance (both in take-off and especially landing) with a substantial payload is another. Apart from the issue of whether or not the F-15 could fit into the lifts and hangars of a Harrier carrier.

To introduce complexity like blown/variable incidence wings would effectively mean a new design, and I'd be surprised if even then it could compete in landing performance with a plane which has more than half its thrust directed downwards.

 

[Abraham Gubler]

I have highlighted the key words here: clean performance is one thing, performance (both in take-off and especially landing) with a substantial payload is another. Apart from the issue of whether or not the F-15 could fit into the lifts and hangars of a Harrier carrier.

Well when I first mentioned the F-15 STOL program in this context I did so an indication of what can be done to a conventional aircraft in order to make it takeoff and land in short distances. I did so to explore what is possible for this concept of a naval STOL in place of STOVL by working from conventional aircraft not as an actual proposal for flying F-15s from Harrier carriers. For example the canard and thrust vectoring of the F-15 STOL could be applied to an F/A-18 or a F/A-50 or whatever.

As to the trial performance of the F-15 STOL as I mentioned in one of my posts above it never actually flew from a ship. Its performance on land was very impressive and would be even more so from a ship. Since ships move through space while launching and recovering aircraft this provides wind over deck (WOD) from their speed and natural wind. This provides more lift to the wings of the aircraft. So significantly shortening the length needed to launch and recover. Also for launch when the aircraft is heavily laden with fuel and stores the simple ski jump can provide a huge boost to takeoff performance.

To introduce complexity like blown/variable incidence wings would effectively mean a new design, and I'd be surprised if even then it could compete in landing performance with a plane which has more than half its thrust directed downwards.

Actually adding blown wings is not so difficult and a number of aircraft have had it done to them during their development life. Like the F-8 Crusader. Variable incidence is more complex but in some aircraft the wing is an add on structure and would be quite easy. Other aircraft it would be impossible. These are just ideas for the grill not concrete proposals.

As to competing with an aircraft with thrust deflected downwards this isn’t so cut and dry. Because thrust deflection aircraft tend to have a lot of extra weight and design compromises to achieve this. For example the Harrier. If you were to convert a Harrier to STOL from VTOL all you would be able to remove is the hover control system. Which wouldn’t change the Harrier’s performance in any significant way. It would have excellent STOL performance but would still be a short legged, sub-sonic aircraft.

 

[Tony Williams]

As to competing with an aircraft with thrust deflected downwards this isn’t so cut and dry. Because thrust deflection aircraft tend to have a lot of extra weight and design compromises to achieve this. For example the Harrier. If you were to convert a Harrier to STOL from VTOL all you would be able to remove is the hover control system. Which wouldn’t change the Harrier’s performance in any significant way. It would have excellent STOL performance but would still be a short legged, sub-sonic aircraft.

You are forgetting that not only would a straight-through aft nozzle release more thrust, it would allow an afterburner to be fitted. The extra take-off thrust would allow a higher MTOW = a useful improvement in payload/range.

 

[Abraham Gubler]

You are forgetting that not only would a straight-through aft nozzle release more thrust, it would allow an afterburner to be fitted. The extra take-off thrust would allow a higher MTOW = a useful improvement in payload/range.

Tony I don’t think you’ve made an acceptable case that such a Harrier version could exist. With 50% of the aft exhaust thrust (25% of total) directed downwards this would have huge effect as a lever on pitching the aircraft forward. Looking at the Harrier in a bit more detail this new aft nozzle would be around 6m aft of CG compared to the forward nozzles being 1m forward. So the aft nozzle would have around three times more pitch force on the aircraft than the forward nozzles. This Harrier is going to have a nose extension to fit in the displaced aft avionics bay and to try and balance the CG of the longer exhaust pipe and afterburner. But you would need to add lift jets in the nose to keep the pitch up countering the aft nozzle. It’s going to be a Frankenharrier.

If you were building a P.1216 then you wouldn’t have this problem. But you would then be back to building a VTOL aircraft with just the hover controls removed. You have to build a massive engine (more powerful than the F135), you have to build a supersonic separate tail boom aircraft and all sorts of other challenges. For what gain compared to the VTOL version? A few pounds of the hover control system removed.

 

[overscan (PaulMM)]

Actually, a better solution might be a fixed single rear nozzle with limited vectoring (say +-10 deg) only. This allows use of afterburner for takeoff without melting the deck too much.


For takeoff, you would have restrainers like on a STOBAR design, engage afterburner, with all three nozzles pointed directly rearwards, disengage restraints then as you accelerate off, the vectored front nozzles rotate downwards and rotate you rapidly to high AOA like a ski jump. You could then balance the front nozzles' vertical thrust with minor deflection of the rear exhaust downwards, with the vertical thrust component helping to keep the aircraft from falling into the sea/runway.

 

[Tony Williams]

Tony I don’t think you’ve made an acceptable case that such a Harrier version could exist. With 50% of the aft exhaust thrust (25% of total) directed downwards this would have huge effect as a lever on pitching the aircraft forward. Looking at the Harrier in a bit more detail this new aft nozzle would be around 6m aft of CG compared to the forward nozzles being 1m forward. So the aft nozzle would have around three times more pitch force on the aircraft than the forward nozzles. This Harrier is going to have a nose extension to fit in the displaced aft avionics bay and to try and balance the CG of the longer exhaust pipe and afterburner. But you would need to add lift jets in the nose to keep the pitch up countering the aft nozzle. It’s going to be a Frankenharrier.

I suggest that you look at post #13, when I said: "There are two variables to play with here: the location of the aft nozzle relative to the CG and the downwards angle it adopts. The further aft it is located, the smaller the downward angle required to balance the downwards thrust of the front nozzles."

The rear fuselage of the plane would need to be redesigned, and the decision as to where to place the aft nozzle would determine the downwards angle it could adopt.

 

[Tony Williams]

Actually, a better solution might be a fixed single rear nozzle with limited vectoring (say +-10 deg) only. This allows use of afterburner for takeoff without melting the deck too much.


For takeoff, you would have restrainers like on a STOBAR design, engage afterburner, with all three nozzles pointed directly rearwards, disengage restraints then as you accelerate off, the vectored front nozzles rotate downwards and rotate you rapidly to high AOA like a ski jump. You could then balance the front nozzles' vertical thrust with minor deflection of the rear exhaust downwards, with the vertical thrust component helping to keep the aircraft from falling into the sea/runway.

Yes, that's pretty much how I imagine it working on take-off. My main concern is with landing, which needs to be fast enough to generate the wing lift required - but no faster. Once this speed is determined, that will govern the rearward thrust required of the aft nozzle to maintain that speed while descending to land. In turn, that will determine the downward angle of the aft nozzle to deliver the required percentage of aft thrust while the engine is running at full thrust. That in turn will determine the location of the aft nozzle relative to the CG.

A characteristic of this design is that the engine needs to be delivering maximum thrust on landing, instead of throttled back as in a CTOL plane. That will require a significant downward angle of the aft nozzle, otherwise it will produce too much rearwards thrust and the plane will go too fast. That suggests that the aft nozzle needs to be located well forward, with the tail running above it (like a single-engined version of the F4 Phantom II - e.g. the F3 Demon - only even more so).

Compared with simply making this a VTOL capable design, the main advantage is that it could be designed from the start for significantly higher gross weights for both take-off and landing since downwards thrust would always be augmented by wing lift, which would enable an improvement in payload/range (and bring-back of weapons). It would also of course be simpler, as it wouldn't require all of the gubbins (and training) required to control the plane in the hover.

 

[Mike Pryce]

No surprise that I am biassed in these matters, but every study I have seen looking at 'ultra STOL' has shown all the penalties of STOVL, without the advantages.


As an e.g., the BAe P.1242-3 and -4 were STOL deflected thrust versions of the P.1216-52, but both were just as complex, with added lifting surfaces such as Krueger flaps (needed for trim) replacing the reaction control system.


However, there was an actual attempt at a naval STOL aircraft with deflected thrust, the Saro SR.177, which allowed a full throttle landing while deflecting some of the thrust downwards. Full throttle was good for the flap blowing too:


http://www.secretprojects.co.uk/forum/index.php/topic,1186.msg99623.html#msg99623

 

[shedofdread]

Oh well, no one else has mentioned it so I suppose I ought to. The BAe P.103 was a sort of super STOL platform aaaand yes, I know; what happens if you lose an engine in the hover (at least the Harrier *just* falls vertically)?