low aspect ratio ultralight

[slowrotor]

After decades of thinking high aspect ratio wings were needed my view now is low aspect ratio is best for an ultralight. There is a small group of designers thinking about low aspect ratio, including Barnaby Wainfan who built the Facetmobile and also gives talks at the Oshkosh forums about the advantages of low aspect ratio (AR).
Some major advantages of low AR are:
1) much lower weight wing with less spar load.
2) very good stall/spin resistance.
3) more compact overall.
4) more manueverable.
5) less gust reaction from lower lift slope curve.

My question is:
Are the aspect ratio charts in textbooks like THEORY OF WINGS SECTIONS based on wings of equal area or wings of same span?
It is not specified in the book.
An aero-engineer told me it does'nt matter in a wind tunnel. But it sure matters on the full size. I am confused. Any thoughts are appreciated.
slowrotor

[barit1]

Here (http://www.nurflugel.com/Nurflugel/n_o_d/weird_04.htm) is a good starting point. Also this. (http://www.century-of-flight.net/Aviation%20history/flying%20wings/early%20US%20flying%20wings.htm)

Also Google around for Arup designed by a Dr. Snyder. A replica of one hangs in the passenger terminal at KSBN.

An original:http://www.ultraligero.net/aviones/modelos/arup_s_2.htm

[Mad (Flt) Scientist]

Are the aspect ratio charts in textbooks like THEORY OF WINGS SECTIONS based on wings of equal area or wings of same span?
It is not specified in the book.
An aero-engineer told me it does'nt matter in a wind tunnel. But it sure matters on the full size. I am confused. Any thoughts are appreciated.
slowrotor

If they are showing you variation of CL, or any other non-dimensionalised coefficient, with AR then the effect of wing area is removed and even if the tests were conducted on wings of different physical sizes the results are independent of wing area.

[chevvron]

Not a TP but I've flown microlights of both styles:
AX3 - 50hp low aspect ratio 390kg MTWA
Chevvron 2-32 - 32hp high aspect ratio 390kg MTWA
The AX3 was much more fun to fly as it's shorter wingspan allowed more responsive roll rates; in comparison the Chevvron roll rate was much slower to the point of being like a Wessex ie move the stick and wait for the response!
The Chevvron encountered ground effect when landing even with flaps or airbrakes (it had three types of wing, one of them clean and the others with either flaps or trailing edge brakes) but the AX3 was very easy to land with neither of these; I used to demonstrate how you could level off at about 2 feet and it would land itself when ready. The AX3 was also less prone to gust reaction and much easier to handle in a crosswind. Having said all that, the Chevvron had a much higher cruising speed in spite of lower power, and hence was a better cross country machine, especially if you climbed to about 4 or 5 thousand.

[slowrotor]

Interesting replies.
My plan is to build an ultralight with an aspect ratio 2.5 wing.
AR 2 or 3 is very common in RC models and the aircraft have amazing performance. But not much in full scale at AR 2.5 that I know about.

I have run many simulations with X-plane and it works well. But still one wonders why others have not built planes with AR 2 or 3.

The Wright brothers found that AR 6 was much better than their early AR 3 glider. But they had increased the span. And span is the major factor for induced drag.
I think an ultralight with low span loading and wide chord would have low induced drag. With sufficient span the low aspect ratio is really not a factor.
It is just the result of adding area.

slowrotor

[Confabulous]

Speaking of x-plane and microlights, I wondered what the maximum level flight speed of a 450kg VLA would be, so I designed a rough model. It has an 80hp engine (neglecting cooling drag for the moment, also with retractable gear and a NACA23-016 shaped fuselage (carbonfibre, 0.012 Cd without any antennae, pure aerofoil shape)) the wings are NACA23016 (root) and NACA0009 (tip) with 1 degree of washout. Horizontal tail surfaces don't exist, the elevator is part of the aerofoil-shape fuselage. It's taildragger until I can figure out how to curb the psuedo-groundloop behaviour of the nosedragger!

Some interesting results...


(All results are at MAUW)

212KTAS at METO (1800ft, ISA standard)

184KTAS at 75% (1800ft, ISA standard)

1700fpm ROC @ 86KIAS

Stall is elevator limited at around 55kts (power off, flaps up)

Stalls at 28kts (power on, flaps down)

Just an excercise, but an interesting one! :ok:

(And yes, I know the Cd is very low, should be around 0.017, but it's only a rough model)

[Genghis the Engineer]

Speaking of x-plane and microlights, I wondered what the maximum level flight speed of a 450kg VLA would be, so I designed a rough model. It has an 80hp engine (neglecting cooling drag for the moment, also with retractable gear and a NACA23-016 shaped fuselage (carbonfibre, 0.012 Cd without any antennae, pure aerofoil shape)) the wings are NACA23016 (root) and NACA0009 (tip) with 1 degree of washout. Horizontal tail surfaces don't exist, the elevator is part of the aerofoil-shape fuselage. It's taildragger until I can figure out how to curb the psuedo-groundloop behaviour of the nosedragger!
Some interesting results...
(All results are at MAUW)
212KTAS at METO (1800ft, ISA standard)
184KTAS at 75% (1800ft, ISA standard)
1700fpm ROC @ 86KIAS
Stall is elevator limited at around 55kts (power off, flaps up)
Stalls at 28kts (power on, flaps down)
Just an excercise, but an interesting one! :ok:
(And yes, I know the Cd is very low, should be around 0.017, but it's only a rough model)

Whilst a typical real-world microlight of that sort of size and shape will probably give you something like:

- 75-100kn CAS maximum level cruise
- Climb ~800-1100 fpm at sea-level
- Stall at ~33 knots power off, flaps down
- Stall at ~30 knots power on, flaps down
- Stall may be elevator limited clean, but is likely to be around 38 knots.

G

[Confabulous]

Bugger, so much for sim fidelity :} Thanks G, at least that saves me wasting any more time on the concept :O

[cwatters]

Interesting replies.
My plan is to build an ultralight with an aspect ratio 2.5 wing.
AR 2 or 3 is very common in RC models and the aircraft have amazing performance.

As an RC model builder with 30 years experience perhaps I can comment...

It depends how you define performance? Most of the low aspect ratio RC models are either deltas or similar. They are mainly highly aerobatic models with power to weight ratios approaching 1:1 or better. They have very high roll rates - sometimes able to roll 360 degrees in less than 1 second. Some are able to loop in just a few plane lengths. Part of their amazing performance is due to the prop wash - even the ailerons are in the prop wash!

The problem comes when the motor quits... You loose the prop wash over the controls (so some of your control authority vanishes) but the roll stability/instability is unchanged. It can also be tricky to get them to slow down without tip stalling.

[Confabulous]

As far as I'm aware models and full scale aircraft can't be compared due to scale effect, lower Reynolds numbers, huge props etc. Models can do things the real aircraft can't... its the same reason we can't make a Cessna size model of a hummingbird - scaling it up cancels out all the advantages. And as Genghis showed with my little concept, the simulator and real life don't always compare well.

(Although the MCR01 Ban-Bi shows 30% higher numbers then Genghis quotes, it's not really a typical microlight)

In theory, the higher the normal wing loading the faster the top speed, also you'd be able to make a low AR wing stiffer and stronger. Of course there'd be higher landing speeds and a rougher ride, but that's one of the many compromises :{

[Genghis the Engineer]

Well, technically you can scale from a model to a full-scale aeroplane (or vice versa), the problem is that the various characteristics scale at different rates. So, to carry out an effective scaling exercise you need a fairly high level of engineering knowledge - and even then the further you extrapolate data, the more potential for "gotchas".

G

[cwatters]

Can I propose a design for this low AR plane? It's an amphibious twin with retractable gear. It's quite old design though ;)
http://www.flyingmachines.org/pengau5.jpg

Edit: I forgot to add that it has forward swept wings although that isn't obvious.

[Mad (Flt) Scientist]

Well, technically you can scale from a model to a full-scale aeroplane (or vice versa), the problem is that the various characteristics scale at different rates. So, to carry out an effective scaling exercise you need a fairly high level of engineering knowledge - and even then the further you extrapolate data, the more potential for "gotchas".
G

In fact, if you COULDN'T translate from model to full-sized, there'd be an awful lot of wind tunnel engineers looking for new jobs ....

[slowrotor]

cwatters,
The RC models I referred to above are called "fun fly models" here in the U.S. They usually have an AR of 2.5 to 3 with rectangular wings. The deltas you mentioned would be extreme low aspect ratio (below 2) with quite different performance and possibly more inclined to tip stall than a rectangular wing.
This aspect ratio 3 seems to provide for extreme slow flight, lift off in one fuselage length and extreme maneuvering as you mentioned when fitted with large control surfaces. For a full size, the large controls would not be desired as it will not be designed for aerobatics.
But the rigid light structure is desired. Of course, with less power the climb would not be spectacular, but the stable slow flight is my primary interest. The models have thick cantilever wings and I would do the same. Most full size ultralights look something like your drawing with struts and wires everywhere. A thick cantilever wing is simpler to build and less drag. Thats why RC models are built with cantilever almost always.
Genghis,
I think the main consideration with scale is reynolds number. In that respect a low aspect ratio with wide chord wings has a favorable higher reynolds number. So what works for RC models should work well in full scale also, I think. If anyone has comments about why we don't see more aircraft of AR 2.2 to 3, please advise.
slowrotor

[cwatters]

If slow flight is your target then you need an efficient wing and low wing loading.....just like a glider. "Plank" style Funfly models manage slow flght by having low wing loading, a reasonably thick section and an excess of power to counter the drag that causes.