5

I was thinking about a simple super-light aircraft powered with a little low-power piston engine. Is there any way that we can design a wing so the aircraft flies very slow (like 25 km/h as minimum speed) without stalling? I mean a suitable wing design for this kind of aircraft. (slow one) Other questions:

  • Which parameters in designing wings must be considered for lowering stall speed?
  • My thrust power is low so I accept lower cruise speeds.whats the effects of this on my design?
  • Can we choose thicker airfoils for wing and ignore the drag issues due to low speed?
fooot
  • 72,860
  • 23
  • 237
  • 426
Mohsen
  • 51
  • 1
  • 2
  • 1
    Basically you’re going to need a lighter weight or higher aspect ratio wings, or both (sounds like a glider, huh?). – Pugz Jan 10 '18 at 16:55
  • 6
    Any wing can have any arbitrarily low stall speed - if you ask it to generate (or support) a low enough load (weight). SO the answer is related to the Power-to-weight ratio. Look at human-powered flight aircraft for examples: https://en.wikipedia.org/wiki/Human-powered_aircraft & https://en.wikipedia.org/wiki/Power-to-weight_ratio – Charles Bretana Jan 10 '18 at 17:14
  • More than Power-to-weight ratio, I'd say key factor here is wing loading. – user721108 Jan 10 '18 at 17:50
  • (by "more than" I mean : every aircraft has to glide in the first place. balancing dead weight will be replaced by engine-propeller afterwards) – user721108 Jan 10 '18 at 18:17
  • It sounds like you're describing a motor glider – TomMcW Jan 10 '18 at 18:18
  • @TomMcW difference between a motor glider and an aircraft is quite like between a four teeth fork and a five teeth fork. – user721108 Jan 10 '18 at 18:21
  • You just need a large and light wing. – user3528438 Jan 10 '18 at 19:40
  • could you please enlarge effect of aspect ratio and power to weight on stall speed and generally wing design? – Mohsen Jan 10 '18 at 19:46
  • Why we need a large and light wing , user3528438 ? – Mohsen Jan 10 '18 at 19:49
  • And what about the shape of airfoil?(wing's cross section) – Mohsen Jan 10 '18 at 19:50
  • Wing Loading alone, without considering the power available, will not suffice. No matter how low the wing loading is, if the power is too low you cannot maintain aircraft speed above whatever the stall speed is. And although Wing Loading (Weight per unit area of wing surface) includes the weight, it omits consideration of the power available. Wing loading (to a degree) determines the stall speed. but power available determines whether that speed is sustainable. The Power available - drag (Excess Power) must exceed 0.) – Charles Bretana Jan 10 '18 at 21:33
  • In the USAF, there is a topic which Fighter Pilots are exposed to called Energy Maneuverability, https://en.wikipedia.org/wiki/Energy%E2%80%93maneuverability_theory which might shed some light on this for you. As an aircraft slow down, AOA increases, an the angle of the Aerodynamic force from Lift points further and further aft, increasing what is called induced drag. As an aircraft slows towards stall (maximum AOA), this progresses, until at a certain point, the slower you go, the more power is required. – Charles Bretana Jan 10 '18 at 21:45
  • If total power available is below that required to maintain flight at Maximum AOA, prior to that, you will reach a condition where specific excess power is less than zero, and the aircraft, even with full power, will continue to decelerate through stall.. – Charles Bretana Jan 10 '18 at 21:46
  • I doubt that an aircraft flying at 25 km/h is useful. Steady wind reaches 25km/h quite often (in Europe) and wind busts have even more speed. Such airplane would be hard to handle on the ground... – Timothy Truckle Jan 10 '18 at 22:05
  • @TimothyTruckle: the chase vehicle for Solar Impulse was a bicycle, and the guy riding it didn't look excessively sporty. The speed the OP is looking for is not that much slower than the take-off speeds of SI, Helios and other such very lightweight, very low-power planes. Worst case, he'll be flying backwards or taking off from a standstill. – Jörg W Mittag Jan 11 '18 at 01:24
  • @JörgWMittag Solar Impulse was a proof of concept and never meant to be used on a regular profile neither private nor commercial. -- "Worst case, he'll be flying backwards or taking off from a standstill." of what use is an airplane that cannot return to the place it took off? Also you always have to secure the plane by fixing it to the ground while not using it to avoid unintended "unmanned" takeoff and, in result, "uncontrolled landing" aka crashing. – Timothy Truckle Jan 11 '18 at 08:20

2 Answers2

2

The first thing you need is a large wingspan $b$. This will help to spread the lift over more air and reduce induced drag, which grows inversely with the square of flight speed $v$: $$D_i \propto \frac{1}{v^2} \text{ and } D_i \propto \frac{1}{b^2}$$

Large wings have a high root bending moment, which requires a strong and heavy structure. Next you should add bracing, at least on the lower side of the wing. For that to be effective, you need a high fuselage so the bracing will become efficient. Yes, those wires will add drag, but at low speed this will be a lot less than the added lift-induced drag from lifting a heavier structure.

Dedalus human powered aircraft

The single bracing wire can hardly be seen on this picture of the Dedalus human powered aircraft, but it is there and saves a lot of drag at low speed (picture source)

Next, pick the right airfoils. Depending on the local Reynolds number, use thin and highly cambered ones but make sure that the pressure rise on the rear upper surface is slow enough to prevent early separation. One example is the DAE31 outer wing airfoil of the Dedalus aircraft (the inner and mid-span ones are DAE11 and DAE21). At Reynolds numbers of 500,000 to one million, turbulators positioned 2-3% ahead of the separation line will trip the flow and make the airfoil tolerate steeper pressure gradients.

Pick wing chord such that the wing area gives a lift coefficient around 1, details depending on the exact Reynolds number. Lower values go with lower Reynolds numbers.

Peter Kämpf
  • 231,832
  • 17
  • 588
  • 929
1

Wings that would produce sufficient lift at that low of a speed would have an enormously high induced drag. So unless you are operating a model plane that weighs almost nothing - I don't think you will have any luck.

In theory...

You would need a high cambered wing to produce lift at low speed. The wing should have a very smooth surface and max camber should be placed further back to promote laminar flow which reduces air resistance. You could also use turbulators at the seperation point. Basically anything that helps to reduce weight and drag is your friend.

To reduce the risk of a stall you should also look at slotted flaps so that a higher angle of attack is possible.

To achieve a low Induced drag you should have an elliptical wing shape and a very high aspect ratio with winglets.

Such wing could be designed without much of a problem. Your plane could however not fly without an extremely powerful motor to counter the insane induced drag. Such motor would then weigh a lot. So you would need even more lift resulting in more drag thus an even more powerful motor and so on and so forth.

Therefore just use a helicopter ;) The slowest glider I have ever flown had a stall speed of around 60 km/h and it was an extremely light wooden aircraft. I am sure you could get a little better than that with todays technologies. But there is a reason why todays sailplanes fly faster rather than slower than they did 60 years ago.

blenderTyphoon
  • 125
  • 5