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Why must VTOL engines be larger than normal engines?

  1. Jan 3, 2015 #1
    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.
     
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  3. Jan 3, 2015 #2
    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.
     
  4. Jan 3, 2015 #3
    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.
     
  5. Jan 3, 2015 #4

    NascentOxygen

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    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)
     
  6. Jan 3, 2015 #5
    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?
     
  7. Jan 3, 2015 #6

    russ_watters

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    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.
    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.
     
    Last edited: Jan 3, 2015
  8. Jan 3, 2015 #7

    SteamKing

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    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.

    You are correct.

    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.

    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.
     
  9. Jan 3, 2015 #8

    sophiecentaur

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    TGF wings!
     
  10. Jan 3, 2015 #9

    russ_watters

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    I think you misread -- it wasn't correct.
     
  11. Jan 3, 2015 #10
    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).

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

    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 :).
     
  12. Jan 3, 2015 #11

    russ_watters

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  13. Jan 4, 2015 #12

    A.T.

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    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.
     
  14. Jan 4, 2015 #13
    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?
     
  15. Jan 4, 2015 #14

    sophiecentaur

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    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.
     
  16. Jan 4, 2015 #15
    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?
     
  17. Jan 4, 2015 #16

    sophiecentaur

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    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!
     
  18. Jan 4, 2015 #17
    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.
     
  19. Jan 4, 2015 #18
    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!
     
  20. Jan 4, 2015 #19

    sophiecentaur

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    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.
     
  21. Jan 4, 2015 #20

    russ_watters

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    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.

    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.
     
    Last edited: Jan 4, 2015
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