Why must VTOL engines be larger than normal engines?

[E'lir Kramer]

I am studying to be a pilot and something I've read in the book confuses me. It is that Vertical Takeoff and Landing (VTOL) engines need to be heaver than their corresponding traditional takeoff/landing counterparts.

Wikipedia says the same thing here.

I am going to interpret "larger" and "heavier" as actually meaning that VTOL engines need to produce more thrust in order to achieve takeoff than a corresponding runway takeoff requires. But why?

Suppose that a laden plane masses x kg. Then the force of gravity acting on the plane at Earth's surface is ~9.8x N. This is true whether or not the plane is taking off vertically or traditionally. So my intuition about forces tells me that the plane's engine must produce > 9.8x N in order to counteract the force of gravity in either case.

When considering the VTOL, it's pretty clear.

What about the traditional case? I thought that the lift on the airfoils must be counterbalanced by a drag force that acts in a direction approximately perpendicular to the lift and greater in magnitude. If the thrust does not counterbalance this drag force then the plane looses speed and will fall.

If anything this analysis would have me guessing that VTOL should require less thrust, since the takeoff speed is lower, and thus there is less parasitic drag.

 

[jerromyjon]

Try comparing apples to apples and picture a fighter jet being suspended vertically at ground level next to a harrier "jump-jet". The work of fighting gravity 1:1 and accelerating is more work than launching a fighter with the ground counteracting gravity until "lift-off". Also the engines need airflow to function more efficiently complicating things further.

 

[E'lir Kramer]

I understand that this is the assertion. I am hoping someone can come in here with a more convincing analysis.

At the moment that liftoff occurs, the lift on the airfoils is 9.8x N. Thus the drag is > 9.8x N. If the engine is not producing 9.8x N of thrust, then the plane slows down and the lift drops accordingly until the plane settles back to earth. At the moment of liftoff the plane must be producing at least 9.8x N of thrust.

 

[NascentOxygen]

In an ordinary plane, it's the wings that support the weight of the craft, so a plane with a puny engine just requires a very long runway to attain the necessary speed for that lift. ("puny" being a relative term)

 

[E'lir Kramer]

Let me put this another way.

I don't understand how a plane can take off when its thrust to weight ratio is less than one. Yet, many planes do so. The 747-400 has a T/W of 0.27.

Can someone explain how a plane can take off (and a fortiori stay aloft) when its T/W is less than one?

 

[russ_watters]

There are several separate issues here:

The first was described briefly in the wiki: if you use a regular jet engine for VTOL, the high velocity/temperature jet blast will damage the runway.

Second, engines don't produce the same thrust or work at the same efficiency at all speeds. A large, low velocity fan will procude the needed thrust for takeoff at a much lower power than a small jet of air. That's why helicopters use such large rotors and why turboprop planes (P-3 Orion) can fly for a very long time on one tank of gas. But a large, low velocity fan won't produce thrust as well at high speed, so it doesn't work well for a fighter aircraft.

So those two explain why the engines are physically larger for VTOL planes. That's a separate issue from whether or not they produce more thrust (some do, some don't).

Now, for normal airplane flight vs VTOL -- the engine isn't supporting the plane if it is flying horizontally; the wings are. The only thing the engines have to do is provide enough thrust to overcome drag.

At the moment that liftoff occurs, the lift on the airfoils is 9.8x N. Thus the drag is > 9.8x N. If the engine is not producing 9.8x N of thrust, then the plane slows down and the lift drops accordingly until the plane settles back to earth.

No. The ratio of lift to drag is on the order of 10 or 20 to 1. There is much more lift than drag. So at 10:1, the thrust needs only to be 1x N.

You can test this by sticking your hand out the window of your car while it is moving. Start horizontal. Slowly give it some angle of attack: your hand will lift up more than it will be pulled back.

Doesn't your flight training include some instruction in aerodynamics? How lift and drag are generated? I thought that was important for pilots...

In the Air France crash into the Atlantic a few years ago, the co-pilot held the plane's control stick back all the way, for pretty much the entire duration of the event, which caused the plane to fall out of the sky, despite the nose pointing up. Not understanding that the engines can't/don't support the plane could lead to such a disastrous course of action.

 

[SteamKing]

I am studying to be a pilot and something I've read in the book confuses me. It is that Vertical Takeoff and Landing (VTOL) engines need to be heaver than their corresponding traditional takeoff/landing counterparts.

Wikipedia says the same thing here.

I am going to interpret "larger" and "heavier" as actually meaning that VTOL engines need to produce more thrust in order to achieve takeoff than a corresponding runway takeoff requires. But why?

In a conventional plane, the engine produces thrust horizontally in order to get the aircraft to a sufficient speed where the lift generated by the wings is greater than the weight of the aircraft, and the aircraft can then unstick from the ground and fly.

In a VTOL, like the Harrier, the thrust of the engine is diverted perpendicular to the ground, while the aircraft has little or no forward speed. The wings are not generating any lift at this point with which to support the aircraft, which is done entirely by the thrust of the engine.

Once sufficient altitude is gained, then the thrust produced by the engine is redirected to cause the aircraft to start moving in a forward direction. As the aircraft picks up speed, the wings generate lift in a conventional manner, which allows the aircraft to stay aloft and fly. When the aircraft lands, this process is reversed.

Suppose that a laden plane masses x kg. Then the force of gravity acting on the plane at Earth's surface is ~9.8x N. This is true whether or not the plane is taking off vertically or traditionally. So my intuition about forces tells me that the plane's engine must produce > 9.8x N in order to counteract the force of gravity in either case.

When considering the VTOL, it's pretty clear.

You are correct.

What about the traditional case? I thought that the lift on the airfoils must be counterbalanced by a drag force that acts in a direction approximately perpendicular to the lift and greater in magnitude. If the thrust does not counterbalance this drag force then the plane looses speed and will fall.

Your thinking is confused here.
The motion of the wing during takeoff and flight generates drag as the wing separates the airflow. It's not that the aircraft will not fly if the thrust of the engine does not counterbalance the drag, the thrust of the engine must be greater than the drag, otherwise the plane cannot increase speed to a point sufficient for the wings to generate enough lift to allow the plane to fly.

If anything this analysis would have me guessing that VTOL should require less thrust, since the takeoff speed is lower, and thus there is less parasitic drag.

During takeoff of a VTOL, its speed is almost entirely in the vertical direction, and the wings are not capable of generating any lift. Drag is only coming into play in a limited amount here, but the engine must continue to provide thrust to keep the aircraft aloft.

This conclusion would be true, but for the fact that the engine must produce thrust continuously in order to counteract gravity until the plane reaches altitude before it is safe to redirect the thrust to generate forward speed.

This video:



shows a Harrier jet executing a vertical take off from what appears to be the deck of an aircraft carrier. You can clearly see that vertical thrust is maintained until the jet is aloft maybe 30 meters or so, after which the thrust is gradually transitioned from the vertical direction to a more horizontal direction, which causes the plane to start moving forward. During the initial phase of the takeoff, there is little or no lift being generated by the wings of the aircraft.

 

[sophiecentaur]

TGF wings!

 

[russ_watters]

You are correct.

I think you misread -- it wasn't correct.

 

[E'lir Kramer]

There are several separate issues here:

No. The ratio of lift to drag is on the order of 10 or 20 to 1. There is much more lift than drag. So at 10:1, the thrust needs only to be 1x N.

Ok, this is clearly the source of my confusion. And, I suppose I still don't understand how that is true. It feels like violates a conservation law, but actually I see now that it doesn't. (Though I don't understand how it's true, I can see that it's not impossible).

You can test this by sticking your hand out the window of your car while it is moving. Start horizontal. Slowly give it some angle of attack: your hand will lift up more than it will be pulled back.

That's true! I didn't realize how magical that effect really is until just now.

Doesn't your flight training include some instruction in aerodynamics? How lift and drag are generated? I thought that was important for pilots...

Flight training is mostly practical (hours spent maneuvering), but there is a knowledge component. The knowledge portions are split between weather, navigation, and physics of flight. Of the three, physics of flight gets the least focus, but it's there. I'm not a pilot yet, so don't impugn the certification based on my ignorance :).

 

[russ_watters]

No worries. Some links for more info:
http://www.grc.nasa.gov/WWW/k-12/airplane/lift1.html
http://en.wikipedia.org/wiki/Lift_(force)

 

[A.T.]

It feels like violates a conservation law

You mean the Law of Force Conservation?

Such law doesn't exist. Just like you can push a mass upwards on an incline with less thrust than its weight, so can you fly with less horizontal thrust than weight.

 

[E'lir Kramer]

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?

Couldn't that be used to achieve fixed-wing VTOL without a T:W ratio greater than one?

 

[sophiecentaur]

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?

Couldn't that be used to achieve fixed-wing VTOL without a T:W ratio greater than one?

This idea is due to a misconception about what the wind tunnel shows you. (There are an awful lot of people who seem to think that all of flight is explained just by the Bernoulli Effect in a wind tunnel and that Newton's Third Law, somehow doesn't apply, so you are in good company**). Firstly, there just has to be a force keeping your wind tunnel up. Sky hooks? For an object to be supported in air, there must be an upward force, balanced by a downward force on the surrounding atmosphere. In normal flight (level flight) the air that the plane passes through must be deflected downwards by the wing. This involves a lot of air moving down at low velocity and people tend to ignore this. In your ('ideal') wind tunnel, any air, deflected by the wing will be constrained by the tunnel wall and go no further. There will be no action / reaction on the surrounding air and the machine will plummet in free fall. If the air, passing through the tunnel could be deflected downwards in some way then you could have vectored thrust and, given the right design, enough fast enough air could be forced downwards to balance the weight force. But the presence of the wing inside the tube wouldn't be achieving any external lift.
** If you support a wing in a wind tunnel by an external arm (not connected to the tunnel, there is a lift force on the arm but the downwards force on the tunnel body would increase by the same amount. It's like the 6cwt of budgerigars in a 5cwt van joke.

 

[E'lir Kramer]

In your ('ideal') wind tunnel, any air, deflected by the wing will be constrained by the tunnel wall and go no further.

I understand that my question leaves this interpretation open, but this is what I had in mind:

https://docs.google.com/drawings/d/1WhC7r3491KIbeSNgewC9NnJqyDCJpwl46nwnKOSw6WY/edit?usp=sharing

The air would be deflected out of the back of the craft, not touching the walls. The difference between this idea and a regular "puller" aircraft is that in this case all of the lift is generated by the compressed airstream flowing over the wing very quickly rather than air flowing over the "external" wing at apparent air speed. (If this design actually worked, it would be very convenient, because the lift generated by this design would not be dependent on the apparent airspeed , and thus VTOL should be achievable. )

If the air, passing through the tunnel could be deflected downwards in some way then you could have vectored thrust and, given the right design, enough fast enough air could be forced downwards to balance the weight force.

That's what I thought. The purpose of this thought experiment is me trying to understand the difference between vectored thrust and lift. My diagram makes lift and vectored thrust look the same to me. What's the difference?

 

[sophiecentaur]

I understand that my question leaves this interpretation open, but this is what I had in mind:

https://docs.google.com/drawings/d/1WhC7r3491KIbeSNgewC9NnJqyDCJpwl46nwnKOSw6WY/edit?usp=sharing

The air would be deflected out of the back of the craft, not touching the walls. The difference between this idea and a regular "puller" aircraft is that in this case all of the lift is generated by the compressed airstream flowing over the wing very quickly rather than air flowing over the "external" wing at apparent air speed. (If this design actually worked, it would be very convenient, because the lift generated by this design would not be dependent on the apparent airspeed , and thus VTOL should be achievable. )That's what I thought. The purpose of this thought experiment is me trying to understand the difference between vectored thrust and lift. My diagram makes lift and vectored thrust look the same to me. What's the difference?

I don't think there's any real lift in your system. Afaik, lift is what you get as a shape moves through free air. Your picture doesn't include any mechanism for lift of the craft itself so it has to be a vectored thrust system. - if I interpret your picture right.
Alternatively, you could say that there must be some degree of lift from any air that doesn't actually go through the tunnel. But we're just talking semantics really. You wouldn't build an aircraft that looked like a 'brick' when it could be aerodynamic and so, once it was moving, it would have a degree of 'real' lift, in any case.
Also, it's not only the 'solid bits' of a wing that contribute to the lift generation. Sails have no thickness but they behave like aerofoils with a finite thickness. (It's bit like the way an infinitely thin wire antenna has a very finite equivalent cross section when it comes to gathering EM energy out a transmission. Things just ain't that simple!

 

[E'lir Kramer]

I don't think there's any real lift in your system. Afaik, lift is what you get as a shape moves through free air.

How does the foil know that the air which is moving over it is compressed air and not "free" air? It just sees air moving over it. We already agree that a foil in a stationary tunnel experiences lift. What's the difference here? From the frame of reference of the foil there is no difference between the two cases.

Your picture doesn't include any mechanism for lift of the craft itself so it has to be a vectored thrust system. - if I interpret your picture right

The theory is that the foil is rigidly attached to the rest, so if it experiences lift it will push up on the aircraft as a whole.

Things just ain't that simple!

That's for sure. The deeper I dig the more mystifying it is.

 

[jerromyjon]

The point of horizontal takeoff is that momentum maintains lift. If your windtunnel moves you forward like any prop plane the tunnel part doesn't add anything. If you move enough air around a plane's wing chained sitting still it will cause lift. Taking off in a head wind occurs at a slower velocity wrt the ground but the airspeed is equivalent and so is the drag. I think the edge of an aircraft carrier shows dramatically what happens gradually as you lift off the ground that the air is pushing against as well. When the ground disappears you drop so just keep on accelerating and hope you catch up!

 

[sophiecentaur]

How does the foil know that the air which is moving over it is compressed air and not "free" air? It just sees air moving over it. We already agree that a foil in a stationary tunnel experiences lift. What's the difference here? From the frame of reference of the foil there is no difference between the two cases.The theory is that the foil is rigidly attached to the rest, so if it experiences lift it will push up on the aircraft as a whole.

That's for sure. The deeper I dig the more mystifying it is.

A s soon as there's motion through the air, the speed and pressure changes. The wing doesn't need to 'know'. The air from the surface, right out to 100m away and more, moves differently and that's what causes the air round the wing to be different. When the wing is in a tunnel, the air, immediately round it, behaves much the same ( but not the same) as in the open. The difference is that, in the tunnel there's a force on the floor. That force needs to be counteracted if the whole plane is to stay up. Your other point describes the difference between flying in 'ground effect' and free air. And there is a considerable difference in the 'lift'. There would also be a measurable difference in the air pressure measured as the plane flies over a point, depending on how close to the ground it is. The tunnel situation is an extreme one.

 

[russ_watters]

I understand that my question leaves this interpretation open, but this is what I had in mind:

https://docs.google.com/drawings/d/1WhC7r3491KIbeSNgewC9NnJqyDCJpwl46nwnKOSw6WY/edit?usp=sharing

That looks like normal vectored thrust to me. Yes, it works. But that air flowing out of the back of the engine is "thrust", so it is what it is: if you lift a plane with it, it's a thrust to weight ratio >1.

How does the foil know that the air which is moving over it is compressed air and not "free" air? It just sees air moving over it. We already agree that a foil in a stationary tunnel experiences lift. What's the difference here? From the frame of reference of the foil there is no difference between the two cases.

The theory is that the foil is rigidly attached to the rest, so if it experiences lift it will push up on the aircraft as a whole.

The airfoil doesn't know, but the structure of the tunnel that is holding onto the airfoil certainly does. All of the force is internal to the structure. All you are doing here is stretching the tunnel below the airfoil and compressing it above. As I said to sophie it is the "pulling yourself up by your bootstraps" fallacy. If you reach down and pull up on your bootstraps (shoelaces), you don't reduce your weight, you just strain your back.

 

[russ_watters]

This idea is due to a misconception about what the wind tunnel shows you. (There are an awful lot of people who seem to think that all of flight is explained just by the Bernoulli Effect in a wind tunnel and that Newton's Third Law, somehow doesn't apply, so you are in good company**).

This particular discussion doesn't have anything to do with Bernoulli's principle -- misconcptions or otherwise (as evidenced by the fact that after citing it here, you never referred back to it). What you are interpreting here (and you may be right) is just the "pulling yourself up by your bootstraps" fallacy.

 

[sophiecentaur]

As I said to sophie it is the "pulling yourself up by your bootstraps" fallacy.

I remember reading that but not on this thread, I think.
[Edit: Time warp at work here. The post was posted after this one. (twilight zone?]
And I agree with you, of course.
It's such an old chestnut. Pleeease can we not start on the 'how planes fly' thing again. It's more boring than 'how bicycles stay up'.

 

[NascentOxygen]

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?

Couldn't that be used to achieve fixed-wing VTOL without a T:W ratio greater than one?

Two things are going on simultaneously: a force and a reaction. A wing pushes air downwards while at the same time the wing itself is pushed upwards by the air going under it. So, yes, a wing fixed in an enclosed air tunnel can experience an upwards force, but at the same time the air it pushes down will push downwards on the floor of the tunnel. The overall result: equal forces cancel. A lot of fuss, but no net lift.

 

[russ_watters]

It's such an old chestnut. Pleeease can we not start on the 'how planes fly' thing again. It's more boring than 'how bicycles stay up'.

Yes, that was the point of my reply: please don't start a discussion of that.

 

[Vanadium 50]

There's a lot of confusion here; here is my attempt to sum up.

1. There is no "conservation of force". If there were, common household objects, like nails, would not work.
2. A plane goes up when the sum of upward forces exceeds the sum of the downward forces.
3. A wing is, at its core, a device that converts thrust to lift.
4. In a VTOL aircraft, the wing isn't doing this at takeoff, so the thrust needs to be vectored to provide lift.
5. The thrust needs to increase to compensate for the lack of wing lift.

 

[FactChecker]

None of the early airplanes had nearly enough thrust to lift the airplane straight up. It is much easier to push a weight up a gently sloped inclined plane than to lift it straight up. Likewise, it is much easier for an airplane to take off in a gentle slope, effectively using the wings and lift as an inclined plane. To fly level, an airplane only needs enough engine power to counteract its gliding, zero power sink rate, which is much less than the acceleration of gravity. Any more engine power will allow it to climb. The extreme case would be a glider. It would only take a small engine to turn a glider into an airplane.

 

[A.T.]

None of the early airplanes had nearly enough thrust to lift the airplane straight up.

Even today it's rare, and restricted to a few fighter planes, eventually not when fully loaded.

 

[Carno Raar]

Even today it's rare, and restricted to a few fighter planes, eventually not when fully loaded.

You're right in that it's rare, but it's not limited to fighter jets. There's the V-22 Osprey for example. This has propellers (although they are turboprops, so really they are jet engines with a prop on the front) that can be rotated from horizontal to vertical for vertical take-off.

 

[OCR]

You're right in that it's rare, but it's not limited to fighter jets. There's the V-22 Osprey for example. This has propellers (although they are turboprops, so really they are jet engines with a prop on the front) that can be rotated from horizontal to vertical for vertical take-off.

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...

The F-16 can, however, climb "straight up"...

It has a thrust-to-weight ratio greater than one, providing power to climb and accelerate vertically.

Most of the time...

PS: Kind of a neat video...

 

[Carno Raar]

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 is the actual difference between turboprop and turboshaft?

 

[A.T.]

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.

 

[cjl]

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.

 

[Carno Raar]

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.

 

[rcgldr]

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.

 

[cjl]

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.