aspect ratio is a very interesting item. But it is tricky to write about it. The history
is tricky and the design is tricky. A hard job for me. But ... hey ... I like them a lot.
So here I go.
Aspect ratio or (or AR in many other books) can be defined as:
| = b2 / S (formula
= S / k2main (formula 9)
= b / kmain (formula 10)
|with = aspect ratio
S = wing area
b = span
kmain = main chord
other word: low aspect ratio's are airplane with a small span in relation to their
Oh, yes. Before I start I need to mention that first I will write about the design of a low aspect ratio. History will be mentioned as last.
To better understand this part you need to know some things. So I advice you to go to the theory pages and read about the Cl / Cd curve and its meaning, the relation between glide ratio and the needed power to keep flying, Reynoldsnumber and surely about induced drag. If you don't like to read this, no problem go to the history of these remarkable planes.
Low aspect ratio: the design
If you have a book where they mention in very short a low aspect ratio, you will read about two advantages.
First I need to say that those are not the
only advantages. But let examine them a bit. Looking at a low aspect ratio will help you
believing that it is less trouble to fit the cockpit, engines, fuel and cargo into the
wing. The result is a clean wing without humps or any extra external volumes as fuselages,
engine pods and stuff. A clean wing leads to less overall parasite drag. This could lead
to a better glide ratio if there wasn't something as induced drag. But there is. Just look
at the size of the wingtips. Can you imagine the air leak over this wingtip? I bet you
Using the a polar and the next formulas you will get the right curve. But in this example I will not take in count things like vertical tail, landing gear or any other items that normally also need to be added to this curve. If you like to do it anyway, go see the theory (basic wing design; theoretical glide ratio) to see how to do this.
D = Dp + Di (formula 5)
Now let's look at the curve. Point 1 indicated the stall. It is further away to the right related to airplanes with a higher aspect ratio. So the line drawn from the point (0,0) to this point makes a larger angle in relation to a vertical. This means that the low aspect ratio can fly with higher glide angles and still be controllable. And that can be handy during landings.
Charles H. Zimmerman did some experiments with low aspect ratio and he did reach a Cl of 1.85 for a Clark Y airfoil of aspect ratio 1.27 (circular) at a 45° angle of attack. Surely is impressive. Using a modern airfoil the results would even be better.
I found a Nurflugel-mailinglist-mail about Zimmerman's tests from Andre Martins: "I have with me a copy of the original first (I think) NACA paper by Zimmerman on the subject (1929), where he presents wind tunnel force measurements for a series of low aspect ratio wings. Each wing model was composed of a straight and rectangular center section, to which two kinds of wingtips could be attached: with "square" or with semicircular planforms. Zimmerman progressively reduced the span of the center section, until it reached zero. Obviously, in that condition only the wingtips remained, directly attached to each other and forming a circular planform wing in the case of "round" wingtips. Apparently "by accident", he noticed that the circular wing developed a maximum lift much higher than that for other aspect ratios, either with round or square tips. I may not remember the exact numbers (I don't have the paper here right now), but I think CLmax peaked from a bit more than1.0 for the other AR's to almost 2.0 for the circular wing, that is, maximum lift almost doubled. We're talking about Clark Y airfoils, under low Reynolds conditions in 1929."
There is some controversy about the origin of this idea, but that is for the history part of this page.
Now comes a part where I need to depend on the mails I got. In the mailinglist one member suggested that the low aspect ratio's are not stable in turns. He did mention that air bleeds of the wings in turns.
Carlo Godel, who make several low aspect ratio's in model, reacted that he didn't experienced this unstability with his models. They even were as steady as a rock.
Later I got a mail from John Mc Donald, the inventor of the TwinWings (a kind of tandem). He did write: "Low AR airplanes are more stable than high AR airplanes in turns and at low speeds. Usually they need less dihedral, or no dihedral, for stability. Especially Deltas and swept-wing airplanes. Sweep-back has the same stability effect as dihedral on a straight wing, so swept-wing airplanes and low AR often have Anhedral, (the wings hang down a bit) otherwise they are TOO stable, so they will wave their tails side to side as they fly (this is called Dutch Roll, can be dangerous). The F 104 Starfighter has very low AR and the wings have anhedral. A very good airplane, if you don't try to hang too much weight on it, like the Germans did. Then you can easily get high-speed stalls and things will break, including the pilot when it hits the ground!"
So I guess that the low aspect ratio are stable. But it can be that the mail about the bleeding air was right about the bleeding air, but it seems not to affect the stability of the low aspect ratio. And stable means good for beginners. So if you are new in the hobby and you like to build a weird radio controlled model, why not try a low aspect ratio?
Now, let's look at some reactions I got about low aspect ratio in the Nurflugel-mailinglist. Don Stackhouse did mail me that the biggest problem with low aspect ratio's is the friction drag of the wing. Due to the fact that the leading edge of the wing is much higher, there is a problem with the skin of the wing. It has to hold against the air hitting the higher leading edge. More area means more hitforce. So the skin of the wing has to be thicker if the airplane has to fly at higher speeds, which also increases the hitforce. Thicker skin means extra weight. He also did mention that low aspect ratio increases the length of the airfoil. And that does increase the Reynoldsnumber (if "huh?", go see the theory pages). So airfoils perform better. When closely watched these two (thicker skin and higher Reynoldsnumber) can compensate each other. (simplified version of original text)
This problem isn't a problem if your airplane is designed to fly slow. There are lots of types of airplanes who only do fly slow (ultra light, observers, trainers,...) and they benefit also of the higher Reynoldsnumbers. So they have a advantage in relation to "normal" slow flying airplanes. So the low aspect ratio still has a future.
Serge Krauss did mail: "low-aspect-ratio wings get their maximum lift at high angles of attack, most such wings must be allowed these high angles at lift-off and touch-down...or use higher speeds and more runway. Wings like the Snyder/Hoffman, Zimmerman, Hatfield, and R. B. Johnson types are capable of quite high lift coefficients, without high-lift devices, but at very high angles of attack." Later he did mail: "If you are referring to very-low-aspect-ratio (low-A/R) flying wings, flaps are not an option, since they would create a nose-down pitching moment and drag." So here you see a advantage in the simplicity of design (no need for high-lift devices), a warning (don't go experimenting with flaps on low aspect ratio's) and a disadvantage (high angles of attack). More on the disadvantage later.
Another thing Serge Krauss did mail was: "The downside is that you can only have this low sink rate at low speeds, because of the low value of L/D for low-aspect-ratio wings, an insurmountable disadvantage in sailplane competition." (L/D = ratio Cl / Cd of total airplane)
Norm Masters did mail: " Basically a
discoid planform is all tip. At high speed this isn't a problem but as you pull the nose
up for more lift the tips begin shedding huge vortices. At the AoA that would normally
stall a high AR wing the vortices cover a large part of the wing's upper surface and this
vortex action is what allows the plane to fly at a few more degrees AoA. Deltas are better
at exploiting vortex lift than other shapes.
Pity, seems that low aspect ratio are not so ideal for soaring (= gliding with the ability to climb in thermals). And that is not hard to believe. Gliders in thermals need to fly slow to stay inside the thermal. Low aspect ratio's can fly slow, but only at those high angles of attack and those create huge vortexes at the wingtips and that creates a high amount of drag. So... the glider doesn't perform inside this thermal. But there are still a lot of motorized airplanes where you can use low aspect ratio.
Another disadvantage I did find out myself. I did sketch several low aspect ratio's and in all my sketches the pilot is surrounded with wing. So his view is not excellent. The Vought V-173 has some glazing under the cockpit to improve the view. The XF5U-1 had a small extended cockpit which placed a bit more forward. This shows that the larger the airplane, the lesser the problem of putting the pilot in front of the wing. If your full-size project is small size than you might have a problem. But like everything in aviation design: "All is a compromise". Si if you think you can live with the not so excellent view, pleeeaase go ahead and design your low aspect ratio.
The thing that worries me the most is the landing gear. If you look at the picture of the V-173 than you will understand me better.
isn't it? Now, why do you need such a long landing gear on low aspect ratio's? It all has
to do with C l max. Pilots like to take of as quick as possible (only short
runway needed) and land as slow as possible (safe landing). How can you do this? By flying
at C l max (near stall speed). Then the wing gives as much lift as it can. Low
aspect ratio's have their C l max at high angles of attack. So the plane needs
to have this angle of attack on the ground to be in ideal position to take off as soon as
the needed speed is reached. When coming in for a landing the plane has this same angle of
What are the consequences of this long landing gear in aerodynamics? It adds drag, that is for sure. And extra drag leads to a lower top speed. That can be a problem of you like to fly fast. But I guess that most applications of a low aspect ratio can be found in slow flying. So it will not always be a problem in relation to speed. But it also adds weight. And that leads to a higher wing loading, a higher sink rate and more needed power. So climb rate will get lower. Now you can choose to learn to live with this or you can place a larger engine, increase weight more, increase wing loading and so on.
You can place a shorter landing gear if you can live with the consequences. A shorter landing gear will lead to a lower angle of attack on the ground and to a lower C l in this condition. The formula below will show that the needed speed, to get enough lift to compensate the weight, will rise.
will also happen at higher speed, because the airplane can not land at its slowest speed.
The tail would hit the ground first if landing at that angle of attack. And that would be
But again ... if you can live with this higher landing and take off speed, you may use shorter landing gear.
You could of course make the landing gear retractable (like the XF5U-1). It would reduce drag, but it would gain in weight. Again ... the choice is up to you, but first see if can agree with the consequences.
So, what do we have till now?
If you know any other one, please, tell me.
Low aspect ratio: the history
The first low aspect ratio's I heard of are the annular experiments of Lee-Richards in 1911-1914. I did see a model of his experiment in the Science Museum in London. It looks like a flying pancake but with a hole in the middle. A fuselage crosses the hole. The wing is supported by a bunch of cables.
it is not a flying pancake as most know it, it sure has a low aspect ratio. Some will see
it as a tandem with the wingtips attached. For that reason I will place similar designs
(annular wings or ringwings) in the tandem-section.
In 1932 flew the Arup 1. Its designer, Dr. C. L. Snyder, did patent his idea (filed 8 septembre 1930, granted 26 april 1932). His wing looks like a heal of a shoe when seen from above. Some call it a half moon, others call it a felt heel. With the help of Raoul Hoffman he did build the Arup planes. The Arup 1 began life as a glider. Later it did receive a engine. Several flights were done.
You can see some of the Arups on the Nurflugel-website in the section "Others". Go see.
The Arup S-1 was nicknamed "The Dirigiplane" as it was designed to hold buyant gas. It did use a classic Clark-Y airfoil. Another nickname I did find was "Monowing".
The Arup S-2 did fly in 1933. It had a enclosed cockpit, a engine in front and a single vertical tail. I did see two pictures of the Arup S-2 and both show a different placement of the ailerons. The first picture (seen on nurflugel-website) shows ailerons added to the wingtips as extra surface in the shape of a half moon. The ailerons were fully rotating, which means that this extra surface did completely turn around a axis and was not made in two separate parts (one fixed, the other rotating). The other picture shows ailerons placed next to the elevator at the rear of the airplane. The picture of the Arup S-4 on the Nurflugel-site shows the Arup S-2 in the background with the last mentioned ailerons.
I saw a Arup S-2 on TV and it sure was a beautiful sight. It was announced as the Ford T in aviation. Clearly it was planned for mass production. But it was a bad economical time, the "Great Depression", and no buyers were found. Pity... because it flew good. The S-2 flew hundreds of hours and Dr. Snyder took his young son and daughter with him on several trips. That is something you don't do if you are not sure about safe flying.
The airfoil of the Arup S-2 was the Munck-designed; reflexed NACA M-6 (about 12% thickness).
The Arup S-3 and S-4 did probably have the same airfoil as the S-2. They were further refinements of the S-2. Both now did have a classic tail. The S-4 had ailerons placed in the trailing edge of the wingtips. I didn't find reference which aileron configuration the S-3 had.
| I have no idea which type of
aileron had the best performance. I guess the last one, but I am not sure. If you know or
you can explain why, tell me, please.
Most of you will know the "Flying Pancake" of Charles H. Zimmerman, He did some windtunnel tests and with the result he did design the Vought V-173. It was a low aspect ratio with at each wingtip a propeller of large diameter. Zimmerman thought that the props could guarantee a good airflow over the wing. The V-173 was tested ... a lot. Besides Mr. Guyton and Richard Burroughs, several pilots did test it. Even Charles Lindbergh wanted to fly it. It did survive several forces landings, it even did a nose-over (no injury to the pilot and no serious damage to the plane). It did gather 131 hours. This may seem impressive, but I guess that the Arups flew even more.
Zimmerman did catch the attention of the Navy with this V-173. It was a safe airplane due to its slow landing speed. It would land easily on a carrier. The Navy ordered the construction of a larger version. The Chance-Vought XF-5U1 was a fighter with two radial air-cooled engines of 1 350 hp at 2700 rpm. Both did drive a large prop. If one had a problem the other engine would take its task over. So that was safe too. But it was not to be. The XF-5U1 never did fly. The age of the jets began and a fighter with props did look obsolete. It was scrapped, destroyed.
Looks like a bad end for a good idea, doesn't it. Snyder did even worse with his Arups. The Arup S-2 was abused in a air show. The S-4 was scrapped for the war-effort. And the S-3 felt victim to... arson (= being put on fire!!). Talking about a sad end.
Now comes a part where I have to trust on dates. There is some discussion about the true inventor of the "flying pancakes". Most think it is Zimmerman. I think (as well as several other Nurflugel- mailinglist members) that Dr. Snyder gets the credit for the flying pancakes. Just go see the next dates (source Serge Krauss).
I got another remark from Serge Krauss: "I think that Zimmerman got his inspiration from viewing Snyder's and/or Johnson's patents. The NACA research on Clark Y airfoils of low aspect ratio may have been done because (as I understand it) the NACA reviewed aviation patent applications in those days."
There were also other low aspect ratios than those of Lee Richard, C.L. Snyder and Charles Zimmerman. For instance: deltas and biplanes are low aspect ratios. Example: the small Dyke Delta JD-2 has a aspect ratio of 2,7. But because they are regarded as normal, they are not weird enough to be placed in my site.
Quick overview of the other low aspect ratio's I know:
N. Masters gave me this on Hoffman: "Hoffman left Arup in 1933 and went to Florida, in 1934 he built a machine similar to an Arup but more refined. Unfortunaly the Hoffman plane caught fire from a broken fuel line and development stopped there." Serge Krauss stated that the airfoil was a Munk-designed, reflexed NACA M-6 of about 12% thickness.
R.B. Johnson did built a low aspect ratio as well. His "Uniplane" did fly in august 1934. It made several crashes. Serge Krauss told that this would not be due to the aerodynamic design, but due to engine problems. The survival of the pilot in those crashes tells something about the safety of the airplane.
The Frenchman Payen did build a low aspect ratio as well before WW II.
Arthur Sack made in Germany, with encouragement of Ernst Udet, a manned aircraft based on his scaled models. It was a circular wing. The manned aircraft, Sack AS. 6, made a few taxi-tests and they noticed problems due to the place of the control areas. They were placed in the vacuum of the wing. The small span in combination with the torque of the engine caused also a problem. The plane never flew. The plane was straffed by the Allied in a air raid. After this attack the plane was scrapped.
There is a site that gives the complete story of the Sack AS.6 . Go see www.luft46.com sector prototypes.
Hmm, the Arup S-4 has a similar tail configuration as this Sack AS.6. But the horizontal tail is placed a bit higher on the Arup. Seems that the Arup didn't had the same tail problem as the AS.6. Did the higher placement of the tail solve the problem? I wonder. Maybe it was something different I didn't see.
Can anybody help me on this one?
William Horton (not to be confused with the German Horten brothers) made a "wingless plane". I can describe the plane as a straight wing with a very long chord. The wing was the fuselage or ... the fuselage was a wing. That was probably why he called it wingless, because it was all fuselage. At the end of the wing were endplates in airfoil style. The endplates had a vertical tail on their end. Behind these vertical fins were some control areas placed which did extend beside the wing. Viewed from above the plane did look a bit like a square. I would like to say more, but there is allready a excellent site about this airplane. Go see home.att.net/~dannysoar/Horton.htm. The story also tells about the dark side of Howard Hughes. It did dissapoint me to hear this about him.
I did find this Russian low aspect ratio. It is called Diskoplane. Somehow I think that it is not the same one as mentioned in the book "Ailes Volantes" by Alain Pelletier. The fuselage here is a pod under the wing. The one in "Ailes Volantes" had a fuselage nearly completely inside the wing. Well, I do think it is, but the picture is too blurred to be sure.I don't know if this one is a forerunner. No data available on this one. But it looks like it uses elevons.
Ralph V. Sawyer from California holds a US patent on his Skyjacker. You can describe the plane as a fuselage with on both sides a short wing with long chord. The wings extend behind the fuselage. A horizontal tail is placed between the wings like they do with twin-booms. At the trailing edge of each wing is placed a aileron. They line with the elevator. Endplates are placed at the wingtips. A drag rudder is placed near the leading edge of each endplate. The pusher prop is placed between the fuselage and the horizontal tail.
N. Masters did send me this info on the mailinglist of the Nurflugel-site:" Milt Hatfield flew the Arup S-2 and S-4 when he was in his 20s. In the late 1980s he built three planes derived from the Arups." Apparently the name for these planes was "Little Bird". Serge Krauss stated that the airfoil were 23012 derivatives.
At the website of TWITT (The Wing Is The Thing, a organization interested in flying wings) I found info about a Australian project called the Facet Opal. It was build and flown by Scott Winton. It was powered by a 40 hp engine. Scott broke 4 records with it, including climbing to over 30 000 ft with only his 40 hp. But Scott got killed in a accident with his Facet Opal. I have no data on the reason of this accident. If anyone can tell more, please contact me.
The following information on the Facetmobile is for interest and education only and not to be used for other purposes (on request of Barnaby Wainfan).
But I guess that the most recent low aspect ratio is the Facetmobile by Barnaby Wainfan. It looks like a combination of a F 117 and a low aspect ratio. The engine is in the front. The very large cockpit is also the wing. Endplates are placed at the wingtips. From what I did read, the project looks promising. It had a accident, but that was due a bad carburetor. Meanwhile the Facetmobile is in repair and will receive a new engine. Go see users.aol.com/slicklynne/facet.htm to find the official Facetmobile site.
Q: I keep trying to find out why this design doesn't have those very long legs. Can you explain this? All I can think of is that the landing speed could be higher or that the legs are high but don't look that way when viewing the plane (optical illusion).
A1: (Serge Krauss): "I was intrigued by your observation of the Facetmobile's shorter than expected landing gear legs, something I had not thought about. So I looked in my file for scale drawings and checked out the maximum pitch angle when on the ground. My protractor showed only about 10-13 degrees (depending on whether one uses the actual trailing edge, which is "reflexed" quite high on the bottom). So your expectation of higher take-off speed seemed valid. With a gross weight of 600 pounds and 200 square feet of area it does have the expected low wing loading. A pilot report by Peter Lert of Air Progress magazine confirms that take-off/landing speed is 55 KIAS - "limited not by aerodynamics but by the possibility of a tail strike". This was considerably higher than the minimum expected air speed. In a later Sport Aviation article, Wainfan reported a couple interesting facts. First, the plane would maintain altitude at 50% power at a speed of only 30 mph (which is less than 30 Kts) at 30 degree angle of attack. He estimated that the minimum flying speed was less than the speed where the angle of attack caused the pitot tube to stall. Second, he reported that holding the stick hard back would cause a mild buffet, after which the plane's nose would drop very slowly to about 10 degrees above the horizon, and the rate of descent would stabilize at 1000 ft/min at an estimated 20-25 mph. Controls were still fully effective. I wonder whether this is another example of Kasper's trapped vortex phenomenon."
A2: I got a other answer
from Barnaby Wainfan himself ! "The landing gear length decision on Facetmobile
was a considered gear length, gear weight and drag, and landing speed. Because of the very
low thing loading we were able to get the landing speed down to an acceptable slow number
(about 45 mph) without making the gear so long that we could land at maximum lift angle of
attack. This gave a good compromise between gear weight and drag, and landing speed. The
airplane will actually fly slower than the normal touch-down speed which is why there are
small tail skids built into the bottom of the fins."