Reduce stall speed

[FoxLeader]

Hello everyone,

I'm currently designing a small scale EDF plane (about 1 m in lenght and ~65 cm wingspan). The topic I made on it is there: http://www.rcgroups.com/forums/showthread.php?t=825337
Please note that those are early drawings and that work is left to do.

The field I'll be flying from is a little small and people at the Club brought to my attention that I may not have a big enough security margin at both ends of the (grass) runway. Maybe.

So I wondered if the techniques applied to real life planes/very large scale models could apply in this situation: vortex generators or "suction holes" (I don't know if there's a specific word for that).

Any other suggestions to improve stall speed (and takeoff/landing distance) which are not to lighten the plane or enlarge wings are welcome.

Thanks for your help and tips!

Regards,
FoxLeader

[MarkusN]

Since that plane is basically a delta wing, (which naturally delivers high lift at high AOA even though the flow may be detached) there is not much you can do in terms of high lift devices besides installing ample power. A strake / LERX can help with developing that high lift vortex.

The planform is close enough to a straight trapezoidal wing that it might benefit from Flaps and slats, but those are complex solutions to install them out of the idea that they "might" improve the situation.

Changing to a canard configuration à la Saab Viggen can help with controllability at high AOA.

Since you mention a grass runway: The landing gear will play a role. Large wheels, preferably.

[JetPlaneFlyer]

Flaps may help a little but really by far the most effective way to reduce stall speed are the things you have ruled out...i.e. build lighter or increase wing area.

[Brandano]

Well, with a delta you can also increase thrust. The plane can keep on being controllable at high AOA even after a good part of the wing is stalled, and at that point part of the weight can be supported by the engine thrust. I believe that several delta's could approach at a slower speed than what they actually use, but re limited in maximum landing AOA by the undercarriage design.

[coosbaylumber]

Maybe I ought to throw yet another wrench in here too. For many EDF designs to not generate gobs of thrust immediately. They take time to "spool-up" and get going. Thus why an external prop works better.

I did a few plans for Kress jets, and they had lotsa problems in making the balance work out on their initial designs, for the prototypes were originally glow powered, and now with the weight of electric, batteries, controllers, etc. it no longer added up and C.G. was always a problem then. They had to scrimp for weight right off. John and friends were sharp and it took only three prototypes to get a flyable first, not the usual seven.

Don't expect your first try to be a world beater.


Wm.

[BMatthews]

If you remove the lighter and bigger wing options then the only one left is high lift devices. Basically you want to increase the camber for landing and takeoff and go back to flat for fast flying.

Using flaps on a model of this short a tail moment will likely result in unwanted side effects that require more movement from the stabiliator than you're willing to live with. So I'd suggest a combination of drooping leading edge and flap that are linked to a single servo. The forward 1/2 to 3/4 inch of the leading edge being hinged to droop down and the inner portion of the trailing edge being set up as flaps to depress. At the same time you'll also want the ailerons to droop as well but not by as much as the flaps. This will add in some washout to help avoid tip stalling.

Now all this comes with its own weight penalty. First you need an extra servo for the flaps. Then there's hinges and linkages to hook up the flaps and leading edges. Then there's a second aileron servo since if you mix the ailerons with the flaps you need to run separate aileron servos.

So all in all you may well end up adding enough weight that the extra lift is used up just lifting up the extra parts and the stall speed is still the same.

So the bottom line is just try to build it as light as you can short of blowing up in mid air. Besides, keeping it lighter will help with the EDF since they are not known for their powers of acceleration or thrust for climbing.

[FoxLeader]

Thanks for your answers.
MarkusN >> That kind of configuration (but more Gripen-style) is in my plans as an Ales II version. The Gripen rocks so much! For now, I'll stick with this as canards makes a different type of flying I'm not going to start so early. The big wheels is an important thing to remember. I'll have to see how's the ground this year at the field before. Or maybe I'll be able to find a paved runway? I hope so, but this wouldn't be at the Club.

coosbaylumber >> Those are things I need to remeber for sure... so I'm working as much as I can to improve the design and get more chance to have good flight qualities. I'd be happy if it can takeoff, make a circuit and land! We'll see for the rest.

Finally, I think I'll stick with BMatthews' answer as definitive. In short, the only effective way will really be to make it as light as possible without having a paper-like solidity. And to find the best motor around.

Thanks a lot for all your help, that's much appreciated!

Regards,
FoxLeader

[eflightray]

The top of the fuselage opens, (a bit like an upside down bomb bay), and a rotor head and blades unfolds turning the plane into an autogyro, alternately you could power it to turn it into a helicopter.

Or even more simple, look at the foamie jets videos that are built light, they can almost land in their own length, (gentle stall). It partly comes down to the aerodynamics of a thrown brick versus a feather, or what is now referred to as wing-loading.

To me the problem of adding 'gizmo's to reduce stall speed with a small heavy model is that when it does stall................you need to build another one.

Ok a simple view, but then I'm a simple modeler. :D

[Majortomski]

Funny how the oldest aviation adages have surfaced

Build in more lightness

I always like the sign that was supposedly on Ed Heinemann’s door (the designer of the A-4 Skyhawk)
If you can’t:
Build it lighter
Build it cheaper
Build it smaller
Build it faster
Then GO AWAY!


You’ve gotten about the best answers with respect to the design you can expect in this thread.

One thing not mentioned is technique. You can also learn to slow fly the plane into the field for a landing.

Best of luck with the plane

[Brandano]

put an arrestor hook on it and practice carrier landings on a string held between two sandbags :)