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Why VTOL jets are generally a bad idea (but would an ESTOL fighter work?)

Avimimus

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The ability to operate without a runway is attractive. This allows dispersal, making aircraft difficult for an enemy to take out on the ground or predict.

There is also the possibility that a fighter could use a FARP - allowing it to operate in areas without airfields or to operate closer to the front lines (allowing faster responses to threats or calls for CAS).

Theoretical benefits (other than VTOL):

- Excess power (excellent acceleration) is theoretically possible. This is especially true if the lift engines can be engaged for short periods in forward flight.

- A reduction in wing area is possible (theoretically allowing for higher cruising speeds with less induced drag)


Why this doesn't help:

- Complexity and difficulties of design (e.g. correct spacing of thrust, problems with area ruling, control mechanisms at low speed, ground heating, hot air ingestion, the difficulty addressing engine failure).

- Fuel load is limited by the total thrust of the engines at takeoff. Furthermore, the engines add excess weight and volume. So range will fundamentally be much lower than a CTOL design. Similarly, any armament or equipment will be cutting into scarce fuel reserves.

- The actual excess power required for VTOL work isn't used for low-altitude as air-density and supersonic heating will keep your speeds below Mach 1.3 anyway (and then only for short periods of time). So it is basically useless for a low altitude aircraft.

- The excess power could allow high supersonic speeds at altitude. However, the fuel used in climbing to altitude means that interception range will be very limited. In general a CTOL interceptor of equal weight can patrol for longer, can afford a longer climb and can operate off of a conventional airfield further from the front.

As a result, almost every lift-jet design has been a failure.


So what works?

- The Harrier Jump-Jet obviously worked (although typically in a CTOL mode).

- A tail sitter (like the Sukhoi Shkval/Squall studies) might work as a point interceptor (similar to a Komet or Natter but with more efficient turbojet engines).

- What about larger aircraft? Any economies of scale (i.e. how was the YAK-133 a good idea?)


A better solution to the requirements?

A possible compromise would be an ESTOL design - using some lift jets or blown flap technology, but also able to operate as a high speed fighter or attack aircraft. Something like the Sukhoi T-6.

Could a high-subsonic/supersonic aircraft be designed that takes off and lands on an unprepared strip that is less than 200 metres in length (a supersonic OV-10?)


Anyone know of some designs along these lines?

What do you think of the overall problem?
 

Michel Van

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what stands abbr ESTOL for ?

The use of lift jets have one big problem Dead weight during flight
here was the concept of Harrier Pegasus engine brilliant.

and Harrier show in Falkland war some brillant feature thank it VTOL engine
like waiting on island until Argentines air force arrive, take off and attack them while enemy airplane is running low on fuel.
or transport of Harriers on container ship to battle field

oddly the concept of blown flap technology aka upper surface blowing (USB) was adapted to transporter, not to Fighters !
it would give a fighter bomber STOL capacity or allot payload to be drop on enemy.
i think it was Boeing who made studies about this kind of aircraft.
 

shedofdread

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I think possibly more could've been made of the Buccaneer concept (see 'Spitfire to Eurofighter'). That used blown flaps to great effect.


As regards the E(extreme?)STOL thing - the BAe P103 always looked interesting but ultimately, the Peggy in a Harrier was a better concept. Work of genius IMO...
 

Avimimus

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Neat to find out about the P.103

Michel Van said:
what stands abbr ESTOL for ?

Extreme short take-off and landing. Think 200-300 metres (or even less), rather than 500-800 metres.


Thanks for the thoughts. How would you guys go about designing such an aircraft? Lift-jets, blown flaps, thrust vectoring (at less than 90 degrees), a box/tandem wing?


I really liked the original LARA concept (OV-10 requirement) , but this is obviously a role for a much larger and faster aircraft.


Btw. I suspect that lift-jets might not be so bad in terms of dead-weight - once the aircraft is in forward flight the wings usually generate more than enough lift (at least if the aircraft has anything remotely like VSTOL power to weight ratios). It would seem that loss of volume and added complexity (multiple engines to maintain) are the real killers of such concepts.
 

JohnR

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Avimimus said:
- The Harrier Jump-Jet obviously worked (although typically in a CTOL mode).
Hardly CTOL, a short rolling take off to benefit weapons load and fuel usage, and a very short rolling landing or in the case of a sea borne operations vertical landing.

Whilst on the subject of Harriers/Vectored Thrust. Has the potential developments reach its end in light of the F35B and its lift fan? What happened with regard plenium chamber burning was it a dead end?

Regards
 

Sundog

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You'll see VSTOL advances, IMHO, in concert with electric propulsion developments. NASA is already flying a model using distributed propulsion for VTOL and flight. However, I don't know that you'll see it in a fighter type aircraft due to the volume distribution requirements. Maybe that "pulse jet" lift technology Boeing was working on could be used for a fighter type aircraft. However, to be efficient in vertical propulsive lift you want a low velocity delta distributed over a large area (Like helicopters, tilt-rotors, etc.) to be efficient and lift fans still require a large amount of internal volume. Eventually, lift fans will probably be driven by electric motors, but you're still going to have packaging problems.


As for blown flaps, those were also used on the TSR.2, F-104, and other aircraft. The problem with them is they're tied into engine speed. USB flaps are great at low speeds, but tend to have a penalty at higher cruising speeds. Which is why the C-17 just drops the flaps into the jet efflux to increase it's lift.


The problem with Harrier type installations, is once again, the packaging. For a supersonic aircraft that is area ruled, it's really better to have the powerplant in the tail (F-35B). The P.1216 style designs from BAe were probably the epitome of a harrier style engine layout for a fighter.


The problems with PCB were noise and heat effects on the aircraft structure. Once again, not so much of a problem if the engine is in the tail. Also, if you really wanted that style of engine, with all of the advancements we have made in turbomachinery technology, you could probable develop an engine today that would put out dry thrust equivalent to those older designs that were to use PCB. Though you're still going to have the packaging issues; which also goes into how difficult it is to change it out. Don't forget, to change the powerplant on the Harrier they had to remove the wing. With a conventional layout, you just pull it out the back, though the Navy would prefer if it could just be dropped down.
 

CiTrus90

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I'm not an engineer, but i think you would need to better define the role of such an aircraft.

Are we thinking along the lines of an air superiority fighter, a point interceptor, a CAS/COIN aircraft or a cargo?
What are the mission requirements, except for the VSTOL/ESTOL capability? Supersonic/subsonic speed, short/medium/long range, payload, etc.? What kind of threats is it meant to face and be able to survive?

If you need a point interceptor, that's what SAMs are for.
For a CAS aircraft instead, supersonic speeds would be unnecessary and even detrimental, in my opinion.
For a cargo, a part from the interesting designs for SOFTA, if VTOL is a must you could always revert back to helicopters and tiltrotors.

Other interesting concepts though, could be Lockheed VARIOUS, the already mentioned P.1216 and the AIT.320, but it all really comes down to what mission requirements such an aircraft is going to have.
 

pathology_doc

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Tailsitters are probably viable now that we have inherently unstable aircraft and electronic systems which can keep the thing stable and back them down under thrust. Perhaps it's time to revisit that concept. Ground erosion would still be an issue, but the other problems they had all seem soluble now.
 

Avimimus

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CiTrus90 said:
I'm not an engineer, but i think you would need to better define the role of such an aircraft.

Are we thinking along the lines of an air superiority fighter, a point interceptor, a CAS/COIN aircraft or a cargo?
What are the mission requirements, except for the VSTOL/ESTOL capability? Supersonic/subsonic speed, short/medium/long range, payload, etc.? What kind of threats is it meant to face and be able to survive?
I guess that is basically what I was trying to get at here:

What requirements does this technology actually offer an advantage for? Is jet VTOL technology actually appropriate for any role?

Do the design tradeoffs involved in making a VTOL jet (e.g. high thrust to weight ratio) benefit any other capabilities? Or will having VSTOL always lead to an overall weaker design?

Would an ESTOL design always be superior?
 

Jemiba

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Avimimus said:
... Do the design tradeoffs involved in making a VTOL jet (e.g. high thrust to weight ratio) benefit any other capabilities?
Am I wrong, or would for example the F-15 have been (theoratically) able to take off vertically,
given its thrust/weight ratio ?
- For quite a time there were thoughts about launching fighters from ships fitted just with heli
pads, probably unsuitable even for ESTOL.
- Several VTOL aircraft were designed for beingn launched from forrest glades, as giving them
the most secure cover, when on the ground.
- VTOL passenger jets were seen as a high speed replacement of helicopters for inter city routes.

....

What has to be taken into account are the reasons that were seen for real VTOL. So I cannot fully
agree to the title. Reasons and tactics are changing regularly and I wouldn't be that sure, that we
won't see pure VTOL coming up again one day.
 

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CiTrus90

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I suppose ESTOL gives and will give always overall better performances over VTOL with current technology.

The only role open for a VTOL jet now, in my opinion, is in close air support because that's the only role for which an aircraft is absolutely necessary in an all out war.

If we're to consider a peer to peer conflict, worst case scenario, i suppose every airfield close to the front line will be pounded to rubble in the first hours of it. This would mean game on for ground operations, and given the number differencies between NATO and opfor countries with peer capabilities to NATO's, it wouldn't be acceptable from a western point of view.

This means you need to be able disperse and launch tactical air support from pretty much any suitable space without giving away your position to the enemy. One way to do this, could be the one Sweden and Switzerland (just to name a couple of countries) have come up with: make your higways compatible with aircraft operations. The other option is to proceed with a VTOL attack aircraft.

What's needed in my opinion, is an aircraft with the same mindset that was behind the A-10 but mixed with Harrier capabilities. Nothing too technologically complex avionics wise, as much heavily armed as possible for a VTOL platform. Not at all necessarily supersonic.

The VTOL requirement arises from a need, and in any other role it would limit too much the design.
 

RyanCrierie

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The reason VTOL looked like a good buy in the early 1960s is that they were projecting in the future 100 to 1 thrust to weight ratios for liftjets; meaning that if you had a F-105/F-35 sized aircraft of 50,000+ lbs; then taking off vertically would with a T/W of 1.15 would need only about 600 pounds of parasitic mass for the liftjets.
 

mz

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Even though an F-15 can have T/W greater than 1, it would not be controllable at low speed. However with thrust vectoring and computer control a lot of things have become possible.
https://www.youtube.com/watch?v=N2o6Ks55xvw
https://www.youtube.com/watch?v=uxRhWiTlra0

I think with a modified gear and software you could take off or land a Raptor on really short runways? Probably not a lot of payload or fuel though.
 
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