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enter image description here

(Source)

Inspired by the now banned late 70's F1 ground effect, I wondered if it can be adopted by airplanes. So, I came up with this:

enter image description here

(Own work)

The system

A belly door that opens at slow speeds and leads to a narrowing cavity—inverted diffuser.

In essence, the same as the F1 image above, but enclosed since there's no ground. And inverted since we need lift, not downforce. An F1 car's ground effect is the opposite of a plane's ground effect. The door is to stop its operation at high speed.

Theory of operation

The narrowing cavity will speed up the air, the air in the cavity thus will lose pressure (potential energy tuned into kinetic), the air outside the plane and underneath the cavity will be higher pressure, forcing the plane up.


There's no mission, just an experimental racing Howard Hughes kind of thing.

The engineering why is higher top speed, and low angle of attack slow flight.

  • The way I see it at slow speed the center of lift will shift aft, thus lowering the required angle of attack, I'm not sure about the longitudinal control authority and stability though.

  • Also, there'll be no need for high-lift devices and their associated systems, and possibly shorter wings, thus shedding weight and increasing top speed.

Can it fly? Are the assumptions above correct?

Engine location, nose gear location, etc., are not part of the question, just a pure aerodynamics question if we can make a cavity produce lift.

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    $\begingroup$ They'd probably want it back. $\endgroup$ – GdD Oct 21 '16 at 7:48
  • $\begingroup$ Well, every light aircraft pilot learns to use ground effect in takeoff (especially grass & dirt strips, where you get the wheels off the ground as soon as possible and accelerate in ground effect before climbing) and landing - the flare. And those of us who have e.g. large dry lakes handy sometimes use it for fun. But it's called ground effect because you have to be close to the ground to use it :-) $\endgroup$ – jamesqf Oct 21 '16 at 18:07
  • $\begingroup$ @jamesqf: The GE here is not one that would be favorable to an aircraft taking-off. In automotive the name is used for the Venturi effect that sucks the car down to the bitumen (or whatever it is) and allows a better and more continuous contact between the tires and the track. $\endgroup$ – mins Oct 21 '16 at 19:13
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The reduced pressure on the car not only exerts a downward force on the car, but an equal upward force on the ground. Mythbusters did an episode where they drove an Indy car over a manhole to show that it does lift the manhole cover slightly.

The force on the ground is irrelevant because it doesn't move and is not attached to the car. But in your drawing the same thing is happening. The reduced pressure exerts an upward force on the bottom of the channel and an equal downward force on thing top of it. They cancel each other.

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    $\begingroup$ The Mythbusters episode showed that the lift on the manhole was very slight (at least with a modern Indy Car where ground effects are minimized by rule); nowhere near enough to lift a 30-pound steel manhole, and only just enough to lift their plexiglass test manhole with no additional weight. With older cars with full ground effects, it probably had more effect, which is what likely led to the practice of tack-welding manholes in place on street courses (that and making sure they would not move, under any circumstance of a car rolling over them). $\endgroup$ – KeithS Oct 21 '16 at 16:57
  • $\begingroup$ @KeithS Indycar aero is built for speed and stability, not cornering. A formula one car can happily lift a manhole cover and has done quite notably around Monte Carlo in the last few years. All ironwork on street circuits is welded down for this reason. skysports.com/f1/news/12517/10294890/… $\endgroup$ – Gusdor Aug 18 '17 at 10:25
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The issue here is that ground effect requires there to be another body in which the airfoil is in close reference to (i.e. the ground).

The wiki article sums it up nicely

A substantial amount of downforce is available by understanding the ground to be part of the aerodynamic system in question

You cant have ground effect occur inside an airfoil and generate lift in reference to that airfoil.

In a more general answer, there have been "aircraft" of sorts (generally known as Ground Effect Vehicles) that have taken advantage of ground effect and the reduced drag benefits. Generally speaking they were never terribly successful as they have a limited mission profile due to their operational requirements.

enter image description here (source)

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  • $\begingroup$ Wouldn't that craft be using the exactly opposite kind of ground effect from the one the OP mentions. That craft is trapping air, building pressure and going up. He's talking about evacuating air, lowering pressure, and being pulled towards the ground (well, another part of the wing...which we would agree is impossible. You can't push or pull on another part of the same craft and have a net effect on force.) $\endgroup$ – Jay Carr Oct 21 '16 at 2:37
  • $\begingroup$ Jay, you are correct that what he describes is not possible, I was simply discussing an "aircraft" that took advantage of ground effect which seemed to be the over arching topic. $\endgroup$ – Dave Oct 21 '16 at 3:07
  • $\begingroup$ @JayCarr The OP is proposing a mechanism in which "An F1 car's ground effect is opposite of a plane's ground effect... forcing the plane up " $\endgroup$ – Cody P Oct 21 '16 at 16:57
  • $\begingroup$ @CodyP Right, the OP is talking about inverting a suction force to pull a plane up (which won't work in the way he describes, but that's what he's asking for.) The ground effect that the answer's plane is using doesn't use the same mechanism. Rather, it traps air under it's body and wing to build pressure. In essence, one is building air pressure, the other is decreasing it. $\endgroup$ – Jay Carr Oct 21 '16 at 17:09
  • $\begingroup$ @Dave It's what I figured, I was just adding the comment to add a little more information to the answer. No offense intended. $\endgroup$ – Jay Carr Oct 21 '16 at 17:10
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No, this doesn't work: it's directly analogous to trying to fly by grabbing your shoelaces and pulling upwards.

Ground effect works for the car because it increases the force between the car and the ground, over and above the car's weight. What your device would do is to increase the force between the top part of the channel and the bottom part of the channel, but those are both parts of the aircraft so there is no net effect on the aircraft as a whole.

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One of the reasons the lift is increased in the ground effect is the ram pressure, which produces an upward force on the wing/ aircraft when it is close to the ground- this works only when the aircraft is moving relative to the ground. The aircraft exerts an equal and opposite force on the ground.

Now, the same force is exerted on the lower part of the aircraft- so in effect, the net lift produced is zero.

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  • $\begingroup$ @ymb1 You asked about changing the cavity into an airfoil; then its upper surface would be the lower surface of nozzle. If that's not the case, you're adding an extra nozzle. $\endgroup$ – aeroalias Oct 20 '16 at 23:25
  • $\begingroup$ @ymb1 There was a typo in the comment. i've corrected it :). The basic premise is the same- you'll need a reference frame, if you will for lift by ram pressure. Look at it like this- the vehicle in ground effect is pushing ground down for lift; in this case, the lower part of the body is being pushed down, cancelling the lift produced. $\endgroup$ – aeroalias Oct 20 '16 at 23:31
  • $\begingroup$ @ymb1 For ground effect, you'll need a stationary ground so that it can 'absorb' the force exerted by the vehicle- here that's not the case. $\endgroup$ – aeroalias Oct 20 '16 at 23:42
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In short: The mechanism on F1 is a Venturi tube. This tube and the principle of a wing have similarities between themselves, but the significant difference is the absence of downwash in the Venturi tube. Due to this absence, lift is not created.


Details

The car wall and the ground create a Venturi tube. In a Venturi tube air pressure is reduced where the section is constricted (according to Bernoulli principle):

enter image description here
Source: Wikipedia

This suction effect is the one also at work on a carburetor (where it can create icing due to temperature being lowered in the low pressure section) or an airbrush:

enter image description here
Source

It works in F1 because there is a tube created with the ground, the walls of the tube have a tendency to move closer, so the car is maintained close to the ground. There is no lift per se, the vertical air motion (downwash) necessary to produce lift isn't created. So your design principle...

A belly door that opens at slow speeds and leads to a narrowing cavity—inverted diffuser. In essence, the same as the F1...

... won't create lift, because air mass, velocity and pressure are equal at inlet and outlet.

And, as soon as you adjust the Bernoulli/Newton effects so that there is an acceleration and a downwash, then you have created a wing:

enter image description here
Source

In a wing, there is somehow a Venturi tube too: The upper wall is actually created by the viscosity and mass of the atmosphere above the wing, outside the boundary layer, and the lower wall is the wing upper surface:

enter image description here
Source

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  • $\begingroup$ I'm not sure I follow your explanation of how the Venturi tube applies to F1 (though that may actually just be off topic, since I think a Venturi is what the OP is suggesting for the plane.) I thought with F1 cars you used a front splitter with a trailing diffuser. Is that what this is and I'm just not understanding? $\endgroup$ – Jay Carr Oct 21 '16 at 18:25
  • $\begingroup$ @JayCarr: I understand Venturi (or "ground effect") features are forbidden for F1 since 1982 (it was too easy), but the picture of the Lotus 79 visible in the question shows two green tunnels with a constricted cross section in the middle, which happen to be the famous Lotus venturis from the old era. The lower pressure in the constriction is what sucks the car down "in ground effect". Is that what you didn't understand? Better explained here. $\endgroup$ – mins Oct 21 '16 at 19:04
  • $\begingroup$ Ah, no, I follow now. I was thinking current tech, not late 70s tech. I keep forgetting about all the techs that have been banned. $\endgroup$ – Jay Carr Oct 22 '16 at 2:27
  • $\begingroup$ @mins - very late comment, but more than typical Venturi effect is involved for the old F1 cars. A "diffuser" at the back of the tunnels is required. The diffuser provides an ever expanding path for the air in the tunnels to flow into, so that air velocity returns to closely match that of the car. There is a low pressure area at the back of the car due to drag and the air flow directed upwards by the wing and in some cases the exhaust, the combined effect helps "pull" the air through the tunnels as well as whatever ram effect helps push air into the tunnels. $\endgroup$ – rcgldr Mar 24 at 23:26
  • $\begingroup$ @mins - a wing does not act like half a venturi. Instead the air tends follow a convex surface to fill in what would otherwise be a void swept out by the upper surface of wing (if the flow detaches, then vortices fill in what would otherwise be a void). The air flow has to curve as it follows a convex surface, and this curvature of flow coexists with a pressure gradient perpendicular to the flow, lower pressure on the inside of the curve, greater pressure on the outside of the curve. The lower pressure on the inside also coexists with acceleration of air in the direction of flow. $\endgroup$ – rcgldr Mar 24 at 23:31

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