fixed external leading edge slats

[minifly]

does anyone have experience with these?

i am thinking of putting them on my 40" buzzdog to increase drag and reduce neccessary landing area.

thanks in advance,
julian

[BMatthews]

Far better to just use a high cambered airfoil. Something like a NACA 6409 would be good. Or if you're REALLY serious about slow flying then then airfoils from free flight A-2 gliders would be a good choice. Fixed low speed aids are mostly bandaids. Far better to use the right tool (airfoil choice in this case) than to use a fix.

Now if you want a good speed range between slow and fast then you need movable aids. Flaps is the first thing to use and then go to fancier stuff from there.

[Bill Harris]

The "standard" text on STOL model aircraft is "RC Model Airplane Design" by Andy Lennon (1986), ISBN 0879382384.

--Bill

[minifly]

thanks, bill
i've already got it.

the reason i was asking is because my current airfoil is something like this....
it's built out of a sheet of F3.

[Karl B²]

... to increase drag and reduce neccessary landing area.
Fixed slats increase the CL at high AOA, effectively lowering the stall speed. However, a highly undercambered section like you're using already is pretty effective at that.

Slats on a semisymmetrical wing allow for both a low stall speed and less drag at high speed, low AOA flight. The price you pay for this increase in operating speed is increased structural complexity. Your section won't do well at high speed, and slats won't help you there.

I have AL's book (original small blue cover) too, btw, and from the microscopic reproductions of his plans, it looks like he attaches slats simply by a properly contoured former glued directly to the LE sheeting of the main wing structure.

[Sparky Paul]

Undemicroscoping the slat images from AL's book.... :)

[BMatthews]

As you can see proper slats require a lot of homework and building accuracy. Also the numbers used on most slat designs come from full sized aircraft work and it's my guess that at model sized speeds the air will require some adaptation of the numbers. That old Reynolds number business.

Just adding any old slat design will not guarantee a positive effect. In fact I'd guess it would be the opposite.

Simple arced airfoil shapes can work as shown by many generations of indoor rubber duration models but most of that performance comes from very low weight. Make it light enough and almost anything will fly.

As for you F3 arced airfoil I would say that if you could increase the curve towards the leading edge slightly and reduce the overall height of the arc at the high point then what you have will work as well as anything that is a curved flat plate. Right now you have most of arc at the high point and it's a very high % arc. The high point of the arc should be around 6% of the airfoil. Much more than that and you'll get too much drag for the lift gain. Ideally, and in absence of any truly valid wind tunnel data, your arc should be more of a true circle segment. The leading edge should be rounded as well. And if you can manage it the trailing edge should be tapered to a point. Frank Zaic published some wind tunnel data on simple thin arced curves and the outcome was that simple circular arcs were best and the max % should be close to 4%. But the wind tunnels of the day were not set up for super low speed flow so his data should probably be only used as a trend and not gospel. But the trend was simple arcs.

All of which goes against the simplicity of F3. At some point you'll come to realize that you can only do so much about making a silk purse from a sow's ear. F3 is simple, quick and fun. Trying to push the limits with such a medium is probably counter productive. If you're truly looking for the next level of performance and aerodynamics it sounds like it's time to exchange your F3 for a more advanced medium?

[Sail 'n Soar]

Someone at the Toledo model show a while back was demonstrating/selling add-on slats. They worked well in his demos at the show. Don't have the reference, but others have used relatively simple and effective add-on slats with their models. You might want to scan some of the on-line plan catalogs such as RCM's to see if any are listed, which you could use as a template. In general they have been added to modest camber foils and are just aimed at reducing stall speed. The benefit of slats over increasing airfoil camber, especially aft of centerline, is that they will result in lower / less negative moment coefficients than are generated with highly cambered foils of equal CL max. The more negative moment coefficient will result in greater travel of the center of pressure and will require a greater horizontal stab down force ot counteract the nose down wing moment. The wing with slats, even non-optimized ones, probably will have overall lower CD's at low CLs than will the highly cambered foil, since the airfoil will not be at as large a negative angle of attack at the cruise speeds. This won't be an issue if your model is intended to spend it's entire life flying near stall in calm wind conditions.

All that said, I wouldn't recommend them at all for your single surface, highly cambered foil.

Gerry

[50+AirYears]

Andy Lennon has had a number of designs published in MAN, and they may still be available. He has a couple designs at least with lots of methods for STOL planes.
By the way, the earliest article I have by him was from a 1962 Americn Modeler. He has been writing about Model Aerodynamics for a long time.