Aspect Ratio and Wing Loading
04-27-2002, 11:49 PM
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Aspect Ratio and Wing Loading
I have a couple of questions regarding aspect ratio and wing loading. I am hoping someone will be able to help so here goes.
1. Aspect ratio: At what point is a wing considered a high aspect ratio wing. I know that a "typical" sailplane is going to have a high aspect ratio wing and then on the other hand a Piper Tri-Pacer is going to have a low aspect ratio wing, but is there a spot where a wing becomes "High Aspect" vs "Low Aspect" or is it just a term to describe the wing.
2. Wing Loading: Is there some desired wing loading numbers to keep in mind when designing? I know that an airplane with a high wing loading is going to fly different than one with a low wing loading, but is there an ideal wing load I should design for.
Thanks,
Dan
1. Aspect ratio: At what point is a wing considered a high aspect ratio wing. I know that a "typical" sailplane is going to have a high aspect ratio wing and then on the other hand a Piper Tri-Pacer is going to have a low aspect ratio wing, but is there a spot where a wing becomes "High Aspect" vs "Low Aspect" or is it just a term to describe the wing.
2. Wing Loading: Is there some desired wing loading numbers to keep in mind when designing? I know that an airplane with a high wing loading is going to fly different than one with a low wing loading, but is there an ideal wing load I should design for.
Thanks,
Dan
04-28-2002, 01:25 AM
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Aspect ratio and Wing Loading
The term "high aspect ratio" is relative and there is no defined boundary between it and "low aspect ratio."
Aspect Ratio is very important because of its affect on drag. Induced drag is inversely proportional to the aspect ratio. At low speeds (below the best glide angle) the induced drag is more than half the total drag of the aircraft. In models there is a limit to the improvement in performance due to increasing the aspect ratio. For a wing of given area and wing loading, the aspect ratio increase results in a smaller chord and the smaller chord results in lower reynolds numbers. As the reynolds numbers decrease the profile drag of the airfoil increases. When the profile drag increase equals the induced drag decrease, the benefit from aspect ratio increase reaches its limit. This limit will be somewhat different for each airfoil. See:
http://soaring.cnde.iastate.edu/calcs/frames.shtml
for an airfoil comparison program that takes all this into account when comparing the sinking speed.
Generally speaking, the lower the wing loading, the lower the sinking speed. Free flight models benefit from reduced wing loading without limit. R/C models which rely on slope lift or thermal lift must have a speed range that allows them to cope with wind conditions. The higher the wind speed, the higher the wing loading necessary to have enough airspeed to be able to penetrate successfully into the wind. Many R/C model sailplanes have provision for ballast so that the wing loading may be adjusted to the wind conditions.
Mark Drela's Allegro Lite has the highest strength to weight ratio of any model known to me. See:
http://www.charlesriverrc.org/articl...egrolite2m.htm
It is capable of surviving a maneuver of over 100 G's.
Aspect Ratio is very important because of its affect on drag. Induced drag is inversely proportional to the aspect ratio. At low speeds (below the best glide angle) the induced drag is more than half the total drag of the aircraft. In models there is a limit to the improvement in performance due to increasing the aspect ratio. For a wing of given area and wing loading, the aspect ratio increase results in a smaller chord and the smaller chord results in lower reynolds numbers. As the reynolds numbers decrease the profile drag of the airfoil increases. When the profile drag increase equals the induced drag decrease, the benefit from aspect ratio increase reaches its limit. This limit will be somewhat different for each airfoil. See:
http://soaring.cnde.iastate.edu/calcs/frames.shtml
for an airfoil comparison program that takes all this into account when comparing the sinking speed.
Generally speaking, the lower the wing loading, the lower the sinking speed. Free flight models benefit from reduced wing loading without limit. R/C models which rely on slope lift or thermal lift must have a speed range that allows them to cope with wind conditions. The higher the wind speed, the higher the wing loading necessary to have enough airspeed to be able to penetrate successfully into the wind. Many R/C model sailplanes have provision for ballast so that the wing loading may be adjusted to the wind conditions.
Mark Drela's Allegro Lite has the highest strength to weight ratio of any model known to me. See:
http://www.charlesriverrc.org/articl...egrolite2m.htm
It is capable of surviving a maneuver of over 100 G's.
04-28-2002, 01:46 AM
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Aspect Ratio and Wing Loading
Ollie is right. There isn't "a number" to shoot for as requirements vary with applications and flying conditions.
Having said that you will find that aspect ratios over 6:1 are usually considered high while below 5:1 is considered low. For average size R/C planes wing loadings between 21 and 25 oz per sq ft are considered ideal for sporty aerobatic flying while 3D maneuvers work out better if your wing loading is less than 20 oz per sq ft. For airplanes with less than 500 sq in of wing area you need to shoot for lighter wing loadings while giant scale planes fly fine with much higher wing loadings.
Having said that you will find that aspect ratios over 6:1 are usually considered high while below 5:1 is considered low. For average size R/C planes wing loadings between 21 and 25 oz per sq ft are considered ideal for sporty aerobatic flying while 3D maneuvers work out better if your wing loading is less than 20 oz per sq ft. For airplanes with less than 500 sq in of wing area you need to shoot for lighter wing loadings while giant scale planes fly fine with much higher wing loadings.
04-28-2002, 06:09 AM
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Aspect Ratio
My previous post was written from the point of view of unpowered flight.
In the case of powered flight, aircraft that fly with minimum thrust, such as long range aircraft, tend to high aspect ratios to minimize drag which in turn minimizes the thrust required for level flight at a constant airspeed and maximizes fuel economy. The 10 ft. span aerosonde that flew nonstop 1700 miles across the Atlantic used a wing from a model sailplane with an aspect ratio of 15 to one.
On the other hand, aerobatic aircraft require enough thrust to overcome the weight of the aircraft in vertical flight. Aerobatic competition aircraft score better if they fly the pattern at a constant speed and in this case, drag limits acceleration and makes it easier. Because drag is a virtue and excess thrust is available, there is no advantage to high aspect ratio in an aerobatic aircraft. High aspect ratio is a disadvantage in aerobatic aircraft because it also limits roll rate and acceleration in roll.
Very low aspect ratio wings fly entirely in their own tip vortex. The downwash over the wing due to the tip vortex requires the wing to assume a higher angle of attack to produce a given lift coefficient. This additional angle of attack is called induced angle of attack and is inversely proportional to aspect ratio. At slow speeds the angle of attack can be high enough so that the thrust is helping the wing appreciably to support the aircraft. This effect can also be seen in knife-edge maneuvers where the fuselage and vertical tail are acting like a wing to produce the lift. Unfortunately, very low aspect ratio aircraft glide only a little better than a brick when they loose power.
To sum it up, the desired aspect ratio is entirely a matter of the purpose of the aircraft.
In the case of powered flight, aircraft that fly with minimum thrust, such as long range aircraft, tend to high aspect ratios to minimize drag which in turn minimizes the thrust required for level flight at a constant airspeed and maximizes fuel economy. The 10 ft. span aerosonde that flew nonstop 1700 miles across the Atlantic used a wing from a model sailplane with an aspect ratio of 15 to one.
On the other hand, aerobatic aircraft require enough thrust to overcome the weight of the aircraft in vertical flight. Aerobatic competition aircraft score better if they fly the pattern at a constant speed and in this case, drag limits acceleration and makes it easier. Because drag is a virtue and excess thrust is available, there is no advantage to high aspect ratio in an aerobatic aircraft. High aspect ratio is a disadvantage in aerobatic aircraft because it also limits roll rate and acceleration in roll.
Very low aspect ratio wings fly entirely in their own tip vortex. The downwash over the wing due to the tip vortex requires the wing to assume a higher angle of attack to produce a given lift coefficient. This additional angle of attack is called induced angle of attack and is inversely proportional to aspect ratio. At slow speeds the angle of attack can be high enough so that the thrust is helping the wing appreciably to support the aircraft. This effect can also be seen in knife-edge maneuvers where the fuselage and vertical tail are acting like a wing to produce the lift. Unfortunately, very low aspect ratio aircraft glide only a little better than a brick when they loose power.
To sum it up, the desired aspect ratio is entirely a matter of the purpose of the aircraft.
