12% MPG Improvement from Flying in Formation

[tasdrouille]

You are right. I totally forgot about the other members of the equation. Thanks for setting the record straight! The Cd variation with distance is interesting nonetheless.

Originally Posted by raquatrac

It'd be pretty pointless drafting an aerocivic, for example ;-)

Doesn't it depend if you are also driving an aerocivic?

[doviatt]

Originally Posted by 2003protege

and good grief...3-5 miles?!? It really is hard to put your head around just how big the wake is from a commercial airliner--especially for being so aerodynamic.

Sorry for jumping in late here....I have a clarification from earlier in this thread.
Form drag is related to aerodynamics and is drag based on structure and shape. Airliner wakes are made of induced drag which is a byproduct of lift. A heavy clean plane makes the largest wake in the form of two wing tip vortices. Vortices are not fun or safe to fly in if you want to live very long. Drafting on the ground behind form drag wake is much safer in this context .

[raquatrac]

Originally Posted by tasdrouille

You are right. I totally forgot about the other members of the equation. Thanks for setting the record straight! The Cd variation with distance is interesting nonetheless.

Yes, it is quite a fascinating graph indeed, especially for the lead vehicle.

To be honest, I have second thoughts about what I have written, which is true in a strict mathematical sense but might not be what the author intended. The two reasons why my interpretation might have been wrong are:

1. It would mean that that in addition to measuring drag on the drafting car model (in the tunnel), the author has also measured the airspeed. (At which point in space? It's quite an important decision!)
2. It would mean that the graph is not very useful to the reader, since the air speed is unknown (yet both varying highly and of very high influence).

So, the author might have just taken the airspeed as "free stream speed" (ie, same for both cars) and I might have been over-zealous here ;-)

Doesn't it depend if you are also driving an aerocivic?

Not really. A very streamlined leader car would leave small wake turbulence. The air behind it would have almost no velocity relative to the ground, thus would not be very interesting to drive into, whichever car you are driving.

As a drafter, I'd wish to have a very streamlined car and an awfully blunt leader.
As a leader, I'd just wish to have a drafter whose nose is at the same height as my tail.

[slopemeno]

You'd be surprised how small an object can produce a substantial vortex. I used to fly an electric acrobatic r/c model airplane (an Electrostreak, for those of you that know) and if the air was still, I could fly a very accurate loop, and if I flew right through the right spot, the plane would visibly drop about 6" as it flew through it's own wake. This is a 44" wingspan 35 ounce plane, by the way.