This question kinda goes with the previous one:
Say you have a 48 in span and a 72 in span of the same area, will there be a noticable difference in speed due to less drag?

In a nutshell, yes, the longer the wing for a given area, the lower the span loading, good all the way 'round.

Jetts

As a rule of thumb this is normally true but in our model sized world we need to consider Reynolds numbers as well. In an old Soartech journal of mine there is a study of the optimum aspect ratio for model sailplanes. For 2 meter models this worked out to be a surprisingly low, and not very pretty, 6 or 7:1 as I recall. But for open class it was more like 12 to 14:1. This related the need for speed RANGE over pure sink rate to be an effective contest model. There's always compromises to consider in the real world, isn't there.

If you're talking pure sink rate then higher IS better as long as your airfoil is picked for efficiency at the resulting reynolds numbers. But you can then end up with a "one speed" model like the current range of FAI free flight models that all look like very high aspect ratio gliders regardless of what's pulling them into the air.

You will have less induced drag, which is a very small portion of the total drag at high speeds. So, no, it probably won't give you much more speed, especially considering that thrust decreases with speed also. However, in tight turns, where induced drag is much higher, you will see a benefit as the plane will not lose as much speed in the turns.

This particular application is open style pylon racing. The SA must be at least 500 square inches, and everyone is pretty much using the predator with a 52 in span. I thought if I used a higher AR with a tripple tapered wing (10in root, 5in tip), there might be a significant drag advantage. Higher AR means less induced drag, and lower sink rate (less energy to stay aloft, and more used for forward speed?). I didn't know there was more induced drag in tight turns, that's an added bonus.
So would this configuration be a noticable benifit over the std predator?

I doubt if you'll notice any change one way or the other on the straights but in the turns the higher aspect ration will certainly help as will the tapered planform. Just look at the FAI racers for guidance.

"Just look at the FAI racers for guidance." Where can I get more info?

I thought the simple answer would be to do a search on www.google.com but there doesn't seem to be a lot out there.

Found a page of links at....

http://www.pylonworld.com/toc.htm

and a few other pages using "f3d pylon racing" without the quotes. Start with that and see if you can find some racers to ask on the other high speed and racing forums.

Thanks!

In many cases, the profile drag increase from dropping the Reynolds number can outweight the induced drag benefits of a higher aspect ratio. This is part of what influenced the results on gliders quoted before.

For instance, if we start at Re=200000, and double the aspect ratio, we will recuce chord, and therefore Re, by a factor of 2.

For a sample airfoil, this increases profile drag (Cd0) from 0.01 to 0.014, which will significantly reduce top speed, as well as attenuating the turning benefits of the higher AR.

It's a very complex and mission-dependent problem, and increasing AR may very well help you, but I just wanted to point out the physics behind some of the drawbacks to high-AR wings on models.

Above about Re=300000, these evvects become almost negligible. Below Re=100000, they are incredibly important.

Starfire, I don't know if you noticed but he's designing an open class pylon racer so it shouldn't be an issue. Using a little Reynolds number calculator I found at...

http://www.mame.syr.edu/simfluid/redder/reynolds1.html

I get 1.03M for an 8 inch chord moving at 150 mph.

Whoops. Missed that bit. at those Reynolds numbers, jack that AR up!

There IS a mechanical consideration:
A high AR wing is subject to greater bending loads.
You can't win if you break the spar in the first turn.