A bit of question
- Thread starter Awang se
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Because engines are heavy.
And because they were basically all designed around the same first concept - the Pegasus engine of the Kestrel, which became the Harrier, and which the Yak-36/38 bears some resemblance too (other than using two extra lift engines because the nozzle systems of the Kestrel/Harrier are a rather complicated affair).
And because they were basically all designed around the same first concept - the Pegasus engine of the Kestrel, which became the Harrier, and which the Yak-36/38 bears some resemblance too (other than using two extra lift engines because the nozzle systems of the Kestrel/Harrier are a rather complicated affair).
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that's one point, but the power from 2 engines should be able to qvercome the weight right?
mind explain how the concept was created? i know Michel Ribault first created the concept in 1956, but what i don't get is what stop anyone from creating a multi engine VTOL. i don't mean a separate lift and cruise engine, but same engine for both lift and cruise mode, just like harrier, only there's more then one. furthermore, 2 engines that rotate on 2 different directions may eliminate the gyroscopic effect without resorting to complicated counter-rotating fan.
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I'd suspect it would be even more complicated to work out the flow to the nozzles (especially lift) between two engines. In particular if both engines aren't adjusted to each other perfectly in output.
Sure. One could also argue that two engines takes up more room, and 'deadweight' from having more than one set of engine management systems hanging off it. 2x engines = 2x the bits and pieces, from pumps and oil systems, right through to the jet cores and bodies that you have on the thing.
The smaller your aircraft to begin with, the easier it is to get off the ground. Adding a second engine makes your airframe bigger, which might allow you to carry more fuel, but it turns into a slippery-slope of needing more power to get the extra weight off the ground, so you need the extra fuel, but this adds weight, so you need to tap into that power....
Smaller is better. They seem to be meeting that rationale with one engine.
Manufacturing an identical engine that rotates in the opposite direction is just a huge cost that no-one would be willing to bear. It's a matter of cost-effectiveness in that respect, as you would need to design, test standardise and commission an entire production line dedicated to the new engine.
It sounds more complicated than it should be, but a different direction of rotation isn't simply a matter of putting the blades in the fan the other way. A lot of it is techno-legal, and so on, but you still have to build it from scratch, test it, ensure that the way you bolt on each and every part to it doesn't affect it in any way other than the complete opposite of the other engine.
In other words, way too much work and big bucks.
McTaff, you've stolen my thunder ! (lol)
(sat up till almost 3am this morning typing out a very similar reply, only for the system to ask me to re-enter my details, loosing all my witty comments !)
Anyways....
EVERYTHING you've stated is correct, more engines = more fuel, = more weight, which requires more thrust to get off the ground, etc, etc.
Adding another engine, plus a complete carbon copy of thrust nozzles, duct work = TROUBLE, as it's x2 things that can go wrong!
In addition, the controls needed to complete the task, while not impossible in the 21st Century, would be difficult at best, trying to balance thrust, direction of flow, speed of the engines, etc, etc.
All in all, not worth the effort !
Why, I hear you ask??
When designers looked at this in the 50's & 60's, lots of designs where tried & failed, many spectacularly !
The French even had at one point a 5 engined beast, 4 engines for hover, one for flight, but it was too heavy, too slow, would have cost too much & could never compete with the British Designs for the Harrier !!
Finally, we come to the F-35, the "Updated Harrier".
(I do use the term loosely)
They looked at the design, plus all the historical data they could find (one assumes), on VTOL. Having this as a baseline, they would have known that more than one engine = problems, thus settled for one LARGE engine.
So, it's the 21st Century, they can use exotic materials, improved combustion techniques, better design & manufacture process, but why did the stick with 1 engine, albeit that they added an additional lift fan, ran off the common drive shaft??
You don't re-invent the wheel every time you design one, you improve on it !
in other words...
"If it ain't broke, don't fix it !!"
Nuff said.
Systems Adict
(sat up till almost 3am this morning typing out a very similar reply, only for the system to ask me to re-enter my details, loosing all my witty comments !)
Anyways....
EVERYTHING you've stated is correct, more engines = more fuel, = more weight, which requires more thrust to get off the ground, etc, etc.
Adding another engine, plus a complete carbon copy of thrust nozzles, duct work = TROUBLE, as it's x2 things that can go wrong!
In addition, the controls needed to complete the task, while not impossible in the 21st Century, would be difficult at best, trying to balance thrust, direction of flow, speed of the engines, etc, etc.
All in all, not worth the effort !
Why, I hear you ask??
When designers looked at this in the 50's & 60's, lots of designs where tried & failed, many spectacularly !
The French even had at one point a 5 engined beast, 4 engines for hover, one for flight, but it was too heavy, too slow, would have cost too much & could never compete with the British Designs for the Harrier !!
Finally, we come to the F-35, the "Updated Harrier".
(I do use the term loosely)
They looked at the design, plus all the historical data they could find (one assumes), on VTOL. Having this as a baseline, they would have known that more than one engine = problems, thus settled for one LARGE engine.
So, it's the 21st Century, they can use exotic materials, improved combustion techniques, better design & manufacture process, but why did the stick with 1 engine, albeit that they added an additional lift fan, ran off the common drive shaft??
You don't re-invent the wheel every time you design one, you improve on it !
in other words...
"If it ain't broke, don't fix it !!"
Nuff said.
Systems Adict
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A major issue is the problem of what happens if one engine in a twin-engined VSTOL aircraft were to fail in the hover.
With a Harrier-type aircraft, if the engine fails in the hover the aircraft will start to descend as rapidly as the engine thrust dies way. It’s a case of make a rough landing (if the thrust decay is slow enough) or eject.
During its 1960s P.1154 studies, the UK looked as using two afterburning Speys as an alternative to a single engine. But if one Spey fed the nozzles on its side of the aircraft and the other Spey fed the nozzles on the other side, things would get very nasty very quickly if one engine failed in the hover – the result would be an uncommanded roll, with no chance of a rough landing and maybe no chance to eject. As a potential work-around, a system of cross-ducting was devised, so that both engines shared the task the task of feeding the nozzles on both sides of the aircraft.
Mercurius Cantabrigiensis