Which is Faster?

[skidoo]

Due to weather and being busy, I am behind on attempting any events this year. Summer weather seems to be here now and next week is a fly in with an Air Race. I am considering participating. So, I am thinking about the best way to tweak more speed for any given power level. So, which do you think would result in a faster speed?

(A) Pilot only - 81% of Gross Weight - CG about 2" forward of center. Or,
A + Rear Passenger - 85.4% Gross Weight - CG right in the center.
(C) B + Rear Baggage - 87.5% Gross Weight - CG about 2" aft of center.

 

[Henning]

Pilot only, if you can move the seat back and fly with your feet it's best, or you can cheat with a good autopilot and climb back to the luggage compartment. Leave as much fuel behind as possible.

 

[Richard]

C....

 

[Ted]

Letting the autopilot fly while you end up in the luggage compartment is probably ill-advised for many reasons. But, Henning is correct.

If you want to be up front, I'd say keep it solo and leave as much fuel behind as you can.

 

[BillTIZ]

Glider Racing pilots tried to keep the CG as far aft as possible, pushing the aft limit. A study showed that maintaining 85% to the limit was best.

 

[gil_mor]

I'm guessing aft CG and low weight.
Both means lower angle of attack for a straight and level flight.
Lower angle of attack = less air "pushed down" by the wing.

Ok, these are all guess, now educate me!

 

[Steve]

Doing Three Forks?

Keeping the plane light as possible with aft CG will give you the best speed, but you'll probably gain/lose more in how well/poorly you make the turns. I beat a Bo in my tank of a plane because he mis-entered a turn point in the GPS in the GLS race I ran a few weeks ago.

 

[Greg Bockelman]

I'm guessing aft CG and low weight.
Both means lower angle of attack for a straight and level flight.
Lower angle of attack = less air "pushed down" by the wing.

Ok, these are all guess, now educate me!

You are pretty much right on.

For a given weight, an aft CG is more efficient in normally configured airplanes. (IOW not a Burt Rutan design. LOL)

When you add weight, the airplane needs a larger Angle Of Attack to lift the extra weight, hence less efficient. The examples in the poll will for the most part result in the same relative efficiency due to the increasing AoA.

 

[Henning]

You are pretty much right on.

For a given weight, an aft CG is more efficient in normally configured airplanes. (IOW not a Burt Rutan design. LOL)

When you add weight, the airplane needs a larger Angle Of Attack to lift the extra weight, hence less efficient. The examples in the poll will for the most part result in the same relative efficiency due to the increasing AoA.

Right. As you move CG further aft in the envelope, you reduce the drag caused by the tail pushing down to hold the nose up (also adding the equivalent of the downforces weight to the wing and increase requirement in AoA) until you get back far enough that the tail is neutral or supporting weight (typically from what I have studied the back 20% of the envelope is where it becomes load neutral which is as good as it gets). However, you will never be able to "get ahead" in drag reduction by adding weight in the cabin because the AoA and drag increase from the added weight will be greater than the drag reduction from the tail. If you add weight in the very tail though, you have enough leverage that your total drag can go down.

 

[skidoo]

Doing Three Forks?

Yea, thinking about it..