Couple of weeks ago, the Mythbusters studied the effect of flying in a goose-style V formation on airplane fuel consumption. They found that the trailing aircraft saved some fuel, due to flying through the vortex created by the plane ahead, which I can believe.
However, their data on one of the two passes also indicated that the lead plane in the V saved some fuel. And they accepted that as a good result.
But I can’t see how that could possibly be true. What force from the planes behind it could possibly be acting on the lead plane to reduce drag?
Just a guess, but…
The tip vortex is wasted energy for the lead plane. Perhaps the trailing plane, by riding on that vortex, transmits some pressure back ‘upstream’ that reduces the losses?
Imagine it like this: you’re blowing air through a tube. Now add a restriction to the far end. It will affect what you feel at the beginning of the tube. Pressure has been transmitted back upstream.
I don’t know planes, but I do know some NASCAR. Things may be able to be tied back somehow, though I don’t really know.
When drafting, both cars have an advantage. The back car has the obvious advantage because it isn’t having to break wind. The front car also has an advantage because the airflow over the car goes almost straight back over the one behind - it doesn’t drop into the dead space behind the car and create a suction-y kind of drag.
The goose in front does experience less drag than normal because the other birds dissipate the downwash vortices from it’s wingtips. The lead bird works harder than all the rest, but not quite as hard as if it were flying alone. This should also apply to planes, but I didn’t see their experiment.
The Mythbusters routinely botch things. Jamie Hyneman, for one, has a really poor understanding of Physics. This segment is a classic example.
Note that they were internally contradictory: At one point they say that the trailing birds beat their wings fewer times, thus proving they were burning less energy than the lead bird. Then later, they say that their lead plane using less fuel is just like what people found out about birds, that the lead bird also used less energy.
Pardon?
The biggest gripe I have about it is that they didn’t seem to take into account lower fuel loads in their tests. Test B following Test A will always be more fuel efficient since some fuel has been burned off. They showed one batch of tests has happening one after another without landing and fueling up again.
A much better explanation of why a vee formation works is the disruption of air flow prevents laminar flow off the wing tips which strengthens wing tip vortexes. (Laminar flow is great … until it stops being laminar when it leaves the wing. Then the drag is worse.)
Whether their test “proves” the lead plane saved fuel or not is far from clear.
I agree. It could be that the lead bird (and plane), used less energy than if it were completely alone, but still more energy than the fliers that followed it, which use the least energy of all.
I’d have to ask how close the planes were while formation flying. I understand the drafting effect falls off very rapidly as distances between the craft increases.
They did tests with the planes close to each other, then they went back and did them a lot further apart. There was a big drop off in fuel savings as they spaced to planes apart, but it was still (slightly) better than the plane by itself.
The same way that the lead car in a NASCAR drafting train benefits from having cars behind him. It makes the air spilling off the lead car (or bird) less turbulent, causing less drag.
Thank you, but that answers the question without really answering it. In simple terms, how does reducing turbulence in a parcel of air that is behind/downstream of a vehicle impart an effect to the vehicle in front? How is that air still coupled to the vehicle?
(I’m not disputing that it is, I’m just asking how)
Note also that there are a number of reasons why birds fly in a V, not all related to efficiency. Navigation and the fact that birds have eyes on the sides of their heads also forces a V-formation.
Related question: which birds are known to fly in a V? Around where I am we have Canada Geese everywhere, but other than them I don’t think I’ve ever seen another bird flying in a V formation.
Why do eyes on the side of the head force a V? Eyes in front of the head I can see (npi) but with eyes on the side they could all be in a straight line and still see just fine. This assumes that eyes on the side of the head preclude forward vision, which I highly doubt.
This is easier to understand if you think of a car racing alone down the highway. If you draw the streamlines of air passing over the car, you’ll notice that they curve upward at the front, over the roof, and then back downward at the rear of the car.
Those streamlines are curved by a force; in the front it’s the force of the car striking the air, in the rear its the natural tendency of a fluid to fill the lower-pressure area behind the car. Now draw arrows showing how these forces act on the air: The streamlines at the front of the car will have arrows going up and toward the front of the car, while the arrows in the back are pointing down and also toward the front.
By Newton’s third law, the action of these forces must be counterbalanced by a reaction, so an equal and opposite force to each of these is applied to the hood and trunk of the car. In the front the force on the car is down and back, in the rear it is up and to the back. In any case, both forces–one from plowing into the air, the other from the suction of the viod left behind–contribute to the drag on the vehicle.
This is why two vehicles traveling one after another in close proximity will move faster; the slipstream travels effectively over both (hopefully without “dipping” in between), so each vehcile only has one of these two drags to contend with. I assume the same is true of birds.
When I was doing the bicycle racing thing the conventional wisdom was that the lead guy in a paceline got a bit of help by having someone on his wheel.
Drafting vehicles are related but not exactly the same thing. With cars, you’re trying to tuck into smooth air and reduce drag by not having your nose smashing through the air. In the case of flight, the wingtip vortices have a low pressure so they essentially suck the bird/plane back a little bit. Having another object (another bird or plane’s wing) right there in the middle of the vortice breaks it up and reduces the low pressure that is sucking the lead bird back.
While there may be other benefits to the formation, this effect can really only be gained with a V because that’s where the trailing birds/planes have to be to get the benefit. The vortices are like little tornadoes that produce a downwash behind the bird and an upwash just outside it’s wingtips. The trailing birds need to put their wings in the upwash and keep it out of the downwash, so they don’t really want much more than the wingtips to overlap.