Why must VTOL engines be larger than normal engines?

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Carno Raar said:
You're right in that it's rare, but it's not limited to fighter jets. There's the V-22 Osprey for example.
Yes, yes, and all helicopters of course. My comment was referring to aircraft that are not VTOL capable, which trivially requires thrust > weight.
 
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OCR said:
Well really the V-22 Osprey uses powered rotors, also sometimes called proprotors... they're not "true propellers", as used on a standard airplane...

The Osprey also uses turboshaft engines... Rolls-Royce Allison T406/AE 1107C-Liberty turboshafts, to be precise... though similar, they are not exactly the same as turboprop engines...

What, exactly, is the difference between "proprotors" and "very large variable pitch propellers"? It feels to me like a distinction without justification. The same is basically true of the engine distinction - there's really no fundamental difference between a turboshaft engine hooked to a propeller and a turboprop.
 
I did some research via the web. A turboprop bears the structrual load from the propeller (via a gearbox if required). The turboshaft doesn't, and some other structure takes the physical loads. The engines are basically the same, so what we're really describing is a combination of engine and transmission.
 
Carno Raar said:
I did some research via the web. A turboprop bears the structrual load from the propeller (via a gearbox if required). The turboshaft doesn't, and some other structure takes the physical loads. The engines are basically the same, so what we're really describing is a combination of engine and transmission.
A turboprop has a single shaft for both compressor and turbine stage, and that shaft drives a gear that drives the propeller. A turboshaft has a separate shaft for one (or more?) of the turbine rotors which directly drives the propeller. The equivalent of gearing can be achieved by the angle of attack on the turbine rotor(s) connected to the second shaft.

proprotors versus propellers
I'm not sure on this one. Helicopter rotors use a symmetrical airfoil since a cambered airfoil produces a twisting torque about the rotor even with a zero "lift" angle of attack, which creates an excessive load on the central bearings and also possible unwanted twisting type flex issues with the rotor itself.
 
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rcgldr said:
A turboprop has a single shaft for both compressor and turbine stage, and that shaft drives a gear that drives the propeller. A turboshaft has a separate shaft for one (or more?) of the turbine rotors which directly drives the propeller. The equivalent of gearing can be achieved by the angle of attack on the turbine rotor(s) connected to the second shaft.

Not necessarily. Some turboprops do run two shafts, one for the compressor and high pressure turbine, and a separate one for the power turbine. It's a more efficient arrangement, especially for large/high power applications.
 
OCR said:
A picture of a turboshaft engine... notice the "free power turbine".

A picture of a turboprop engine... no "free power turbine".

Those are hardly technical drawings. As I said, some turboprop engines will use two separate shafts, some won't. It depends on the gear ratios, the desired operating parameters, the tradeoff of complexity/reliability vs efficiency, and several other factors.

For example, here are some turboprop engines that use a separate shaft and turbine to power the prop that rotates independently of the compressor spool:

Europrop TP400: http://en.wikipedia.org/wiki/Europrop_TP400 (this one actually has 3 separate spools - it has the power turbine that powers the gearbox and propeller, an intermediate pressure turbine that powers the low pressure compressor, and a high pressure turbine that powers the high pressure compressor, all of which can rotate separately)

Progress D-27: http://en.wikipedia.org/wiki/Progress_D-27 (this has the same 3 shaft configuration as above)

Rolls Royce AE-2100: http://en.wikipedia.org/wiki/Rolls-Royce_AE_2100 (this one has 2 shafts, one for the gas generator section, and one for the prop)

There are several engines that use the single, common-shaft approach too though. It is simply a matter of design tradeoffs. It definitely isn't a part of the definition of "turboprop" though that the power must be extracted from the same shaft that runs the gas generator.
 
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cjl said:
Those are hardly technical drawings.
Yeah, you're right...

You did some very respectable research for that post... :oldcool:

Now... do the same for...
cjl said:
What, exactly, is the difference between "proprotors" and "very large variable pitch propellers"?

I'd rather read, than search... :oldsmile:
 
Apparently my idea actually works in theory. Or at least, aircraft designers at Georgia Tech have designed a STOL fixed-wing aircraft based on the same principle, which is that the apparent airspeed over the wing is what generates lift. They're calling it a "blown wing" design.

http://www.gizmag.com/nasa-cestol-research-jet-shorter-runways/16842/Edit: just for clarity...there are two different principles at work in the aircraft. The main one is a "blown flap" boundary layer control, which is not the principle I was thinking of, but the article also says that the exhaust from the jet running over the top surface of the wing increases lift, which is the effect that I was thinking about in my diagram. Although the article does make it clear that it's not as important to the design of this aircraft, the principle was sound.
 
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E'lir Kramer said:
Apparently my idea actually works in theory.

its nothing like what you were talking about in your original posts ;)

Dave
 
Carno Raar said:
What is the actual difference between turboprop and turboshaft?

The difference lies mostly in application. In all gas turbine engines, excluding turboshafts, thrust is produced by the engine and applied along the same axis as the engines rotating shaft. A turboshaft engine has an entirely different purpose. They are used on helicopters for the following reason. Turbo shaft engines apply their power to a shaft that leaves the engine and by gearboxes, the power is translated mechanically to something like a helicopter rotor. The thrust is being applied elsewhere away from the engine. But the power supplied by a turboprop engine rotates a prop attached directly to the front of the engines shaft. This is where the majority of the thrust is produced. I am not the best at explaining things, but I hope this helps.
 
Something like the Hawker Siddeley Harrier T4 uses a low bypass turbofan. The VTOL thrust comes from rotating outlets on the side of the aircraft and the thrust produced comes exclusively from the bypass duct. A traditional turbojet aircraft doesn't need the low bypass and so the engine is lighter.
 
Why do we not use VTOL these days? Harriers are being scrapped, as I understand.
Could it be because modern missiles can more easily deal with them?
I know nothing about this but it is an interesting topic.
 
sophiecentaur said:
Why do we not use VTOL these days? Harriers are being scrapped, as I understand.
Could it be because modern missiles can more easily deal with them?
I know nothing about this but it is an interesting topic.
It's because a plane that is optimized for VTOL is not well optimized for speed or other performance.
 
russ_watters said:
It's because a plane that is optimized for VTOL is not well optimized for speed or other performance.
Of course. But what is different now, from the situation when they were developed and used widely? I remember that, way back in the Falklands war, the Harriers could outmanoeuvre the Argentinian fighters by stopping dead in the sky and then the chase aircraft would have to fly past and become a target.
Vanadium 50 said:
Harriers are being replaced by the F-35B (at least for the USMC). One V/STOL airframe is being replaced by another.
OK. That answers the question. Thanks
Clearly, the Harrier is a pretty old design.
 
We shouldn't have scrapped the Harriers so early. We have been borrowing Harriers from the US every now and then, and I know we needed the money for the F-35's and the two new aircraft carriers, but jeeze.
 
sophiecentaur said:
Those danged politicians again.
Yup, lucky we aren't at war...Yet
 
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A VTOL aircraft must have an engine with sufficient thrust to lift the whole airframe with zero contribution to lift from wings.
In most conventional aircraft there is little or no contribution to lift directly from the engines, they only need to be powerful enough accelerate the plane horizontally until it's going fast enough for the wings to do the lifting.
 
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E'lir Kramer wrote: "Could an aircraft achieve lift by blowing a fast stream of air over an internal fixed wing?
I.e., by having an onboard wind tunnel with a wing suspended inside?"

David Lewis wrote: The basic idea is valid but the wind tunnel/internal wing is unnecessary.
Propwash blowing over a conventional wing, arranged properly, can reduce stall speed and increase rate of climb.
 
E'lir Kramer said:
Could an aircraft achieve lift by blowing a fast stream of air over an internal fixed wing? I.e., by having an onboard wind tunnel with a wing suspended inside?
Conservation of momentum tells us that what happens on board, stays on board. If the result of the internal 'wind tunnel' is not a net injection of downward momentum to some air then the lift can't be improved. We discussed the Bernoulli vs Newton III many times (I thought we had general agreement in the end) and, whatever the (important and highly relevant) local pressure situation happens to be, if there's no net down draught then there's no lift.
For 'wind tunnel' , substitute ' large box of flying canaries' and (like the wind tunnel) the idea doesn't hold weight.
An internal duct, which vents downward could provide more lift, of course, but that isn't the same as a totally internal system.
 
Vanadium 50 said:
A plane goes up when the sum of upward forces exceeds the sum of the downward forces.

This is true when the climb is initiated. When an airplane is in an ongoing climb, however, the sum of upward forces can be equal and opposite to the sum of downward forces.
 
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David Lewis said:
This would violate Newton's First Law. When an airplane is climbing, the sum of upward forces is equal and opposite to the sum of downward forces. The net force on a free-body is zero unless it's accelerating.
To start climbing it must accelerate upwards.
 
Point well taken.
 
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David Lewis said:
You are 100% correct that the aiplane will speed up, but this acceleration will be brief and transient, and mostly horizontal.
Only the vertical component of acceleration is relevant to climbing. How long the acceleration period lasts depends entirely on the situation.
 
You're right. Only the vertical component of acceleration is relevant when the plane initially begins to climb. However, the airplane can continue to go up when sum of all forces is zero.
 
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David Lewis said:
When an airplane is in an ongoing climb, however, the sum of upward forces can be equal and opposite to the sum of downward forces.

You're right. Only the vertical component of acceleration is relevant when the plane initially begins to climb. However, the airplane can continue to go up when sum of all forces is zero.

then it will hover, it won't climb
 
davenn said:
then it will hover, it won't climb

No - when the airplane is climbing, steady state, it is not undergoing any acceleration, so the forces can indeed sum to zero. To initiate the climb, the vertical net force must exceed zero temporarily, but this is just a transient state.
 
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cjl said:
No - when the airplane is climbing, steady state, it is not undergoing any acceleration, so the forces can indeed sum to zero. To initiate the climb, the vertical net force must exceed zero temporarily, but this is just a transient state.

doesn't sound right
need to see proof of that