Aspect Ratio
 

I'm gonna let my ignorance show and ask what is meant by the term "High Aspect Ratio" with respect to wing airfoil designs?

Ron Cowan :eek:

RE: Aspect Ratio
 

nothing to do with the airfoil. It is the ratio of span to mean chord

span * mean chord = area

aspect ratio = span / chord
or
span^2 / area

higher aspect ratio means less induced drag and reduced downwash = greater pitch stability, smaller tail required
greater lift per degree AoA (lift slope) = a little more sensitive to changes in AoA ( wind gusts, for example )

RE: Aspect Ratio
 

Thanks wellss. That would seem to explain why pylon racer wings are long and narrow while others such as aerobatic designs are much wider. I'll have to think one over till the implications are clearer to me.
Ron

RE: Aspect Ratio
 

Life is all about compromises. Gliders and our racing planes use high aspect ratios for the same reasons. Te reduce induced drag and tip vortex loses. They just do it at different G loads but the reasons are the same.

Aerobatic models suffer from the higher loses of low aspect ratios but suffer it willingly to gain in the far smaller momets of inertia that comes from keeping the center of masses of the panels in closer to the center of mass of the model so that it can accelerate in roll faster in order to accomplish the flight goals.

By the standards of aerodynamic efficiency the aerobatic designs are terrible ugle lumps that do not deserve to exist in a perfect universe. But they do very well at what they can do. And that's the point.

RE: Aspect Ratio
 

and just about the time you think you have it all figured out --
along comes designs with 3-1 aspect ratios - which trounce the high aspect ratio racers --
(See Steve Wittman designs --Bonzo -etc..)
the right compromise can be the best design
till someone else comes up with a better one -

RE: Aspect Ratio
 

Well, although the Bonzo did well in it's day I suspect that a racer with a higher aspect ratio running the same engine could have beaten Steve. Why didn't they? There were probably factors that we don't know about that prevented them using or building such a plane.

RE: Aspect Ratio
 

In Bonzo n Buster - the planes used tiny engines and a small man for a pilot (Bill Brennand) and very light construction.
The planes relied on extremely efficient use of power -being very light weight
they could turn tight - and not snap out .
Many of the earlier contemporary racers use large engines -big radials etc., and were forced to run a more open course.
I still have a page in a 1949 Airtrails with 3 views of all the Thompson Trophy F1 aircraft - showing these planes and the other F1's of that period . There was a very diverse approach to what would work at that time ---
Wittman showed that if you keep the aoa low (low wing loading) - the so called inefficient short stubby wing would do an excellent job (Kick asp)
The later 1949 Tom Cassut racer was much the same setup - and still is a very good machine .
For model builders - I might add that the new BLING by H9 - is almost a dead on planform copy of the Cassut - I have one and it is a extremely good performer - the usable speed envelope is very large. Mine is 1450 sq inches - on a 71 " span and 14 lbs all up - with apiped 40 ZDZ or the 50 non piped ZDZNG - either one - it is literally a VTO setup and has the ability to stop- or fly very slowly with easy, controllable recovery. High sink attitudes are fun to watch. A great big foamie---

RE: Aspect Ratio
 

The Wittman designs were excellent in their day for examples of light racing aircraft. Both were only about 10 lbs. over the 500 lb minimum weight, while many other examples were much heavier. Wittman had a lot of experience with small engined light weight racing planes from the 30's, and provided much input on the overall spec's used in the Goodyear event. The early races showed them to their best advantage, being held at low altitude airports.

However with races held at Reno, it has been shown that the higher aspect ratio wings work better due to the lower induced drag. Where the Bonzo's aspect was around 3.5, along with the early Cassutts, later designs (including Cassutts with the Wilson wing) have far greater span. One of the safety factors of full size racing is the g loading in the turns is not very high. Earlier races with tighter turns took the lives of several fliers, and more pylons were added to the course to loosen the turns.

When we to turn to RC pylon racers, wing span has been determined to the one of the most important design changes ever made to designs. This is because the g loading in the turns is an order of magnitude higher than the manned aircraft. While all the AMA events have been span limited by the rules, the FAI events have been free to evolve. If you compare the typical FAI design from before 1990 to todays design, you will see that the wingspans have increased by around 30%. This allows the aircraft to maintain a higher exit speed out of the turn, thus increasing the overall average speed. This change decreases the induced drag to about 60% of the shorter spaned model, and in the turns induced drag is the dominate drag.

RE: Aspect Ratio
 

Right as rain- again - the compromise for the conditions decides the "best' setup.
here are pics of early/later Cassut setups used.
The materials at hand also play a big role in which type wing can be used
There were no bagged composite wings in 1949---

RE: Aspect Ratio
 

I'd like to thank you guys for your inputs. Lots of information here and all I had to do was ask...

Ron