Airfoil selection

[Chrono]

Hello people!!!!

I am currently working on a sailplane design, an aerobatic slope soarer. I am using a trapezoidal wing for the design, and intend for the airplane to be lower wing model.

First of all, I've done a few designs in the past, but mostly of 2 meter class sailplanes, which worked wonderfully, but 'm still an amateur designer (I've got no formal studies of aeronautics, I do my designs using only personal research, and my experience as engineer and a r/c modeler. Also being the son of a fighter pilot helps a lot!!!! :D ).

Well the thing is that this is the first aerobatic model I design and I don't know what airfoil to use. I'm going for a NACA-00XX wing section, but I don't know if a semi-symmetrical airfoil would do better instead of a symmetrical (considering that its a sailplane). I want the model to be fast, with good response (as good as I can get) to controls and fully aerobatic. I'm also unsure in which thickness would be better.

Another problem I have is the wing reinforcements at the center. I always put the wing spars exactly at the point of maximum thickness, and giving the fact that the wing is trapezoidal, the spars join with an angle at the center of the wing. I don't know what kind of reinforcement to put at the joint. With the 2 meter class sailplanes was easy, just a plywood reinforcement on either side of the spar was enough. With this is also my first non-rectangular wing. I don't know what to do.

I'll appreciate a little help. Thanx a lot!!! :D

[Sparky Paul]

If by trapezoidal you mean equal taper leading and trailing edge, put the spar at the mid point of the chord for a straight spar joiner.
If you set up the taper so the 30% (or thickest point) is at the same location all the way out, then use that.

[skyflyer]

Sounds like a fun project. By trapezoidal wing I assume you mean a simply tapered planform. You can make the wing structure easy if you give the wing 0 degrees of sweep at the max thickness location (.3c for a NACA 4-series). Then you can make your spar a straight piece.

[Chrono]

yeah, I meant that. You gotta excuse me people, I'm not very familiar with the terminology, aside the fact that I'm not an english speaker.

I'm sorry, but I understood you just partially. What do you mean by "give the wing 0 degrees of sweep at the max thickness location"? maybe If you could provide me with some graphical aid, I could get a better idea.

What airfoil do you people think could work for the model?

Thanx again.

[Sparky Paul]

Use the "maximum thickness" point on the chord on the root airfoil as the position for the spar, and run it straight out to the tip.
Put the maximum thickness of the tip airfoil chord on the spar.
You'll have a tapered planform, with a straight spar.

[Chrono]

Ok, that was clear enough :D

[BMatthews]

If you're truly sure that it will spend close to 1/2 it's time upside down then I'd say go with a symetrical section. Instead of a NACA I'd suggest a Selig 8020. That's a fairly thin airfoil that was intended for stabilizer sections on all moving tailplanes. But it has a good low drag over a good range of angles of attack and lift coefficients. I think it would do the trick for you better than the NACA option.

If you're pretty sure it'll be a 75% upright and 25% inverted then some airfoil with up to 1% camber that is thin and slippery would be a good option. That sort of airfoil will let you do fast high G turns without bleeding quite as much speed. But it'll be a touch draggier when inverted if you push hard on the sticks. Some of the HN series of airfoils found in Profili2 would fit the bill for that option.

[Ricardo RW]

Hi 'neighbor'

With a group of friends here in my country, designed and built 4 slope racers, 60" span. We all went for RG-15 airfoils and the planes really love the slopes.

For fuses, some used wood pods and fishing pole booms.Other guy imported (though a bit expensive) a kevlar all molded fuse. Pod and booms fuses fly very well anyway.

V-tails for all models.

Foam wings (EPS) balsa-sheeted and covered with Monokote.

Saludos.

Ricardo.

[Chrono]

Hi man, I used to live in Santiago a few years ago, and liked it a lot. As a matter of fact I flew my very first model (gas engine trainer) in the Universidad Catolica Stadium. I remember that few inches of the airstrip and a crash into the dry bushes was assured.

I thought about using foam wings in my model, because of the added weight. Do you think that would be an issue? I'm going for a box plywood fuselage, and weight is a primary concern.

what do you think?

[Ricardo RW]

...I thought about using foam wings in my model, because of the added weight. Do you think that would be an issue? I'm going for a box plywood fuselage, and weight is a primary concern.

what do you think?It depends on the average wind speed you're plannig to fly. Although for vast majoritie of sloper you have to add weight as ballast to face high wind speed slopes.

I always try to build as-light-as-possible... and resistant if a plane is a sloper. In our case the plane weigh in at 24 oz (700 grs) and they flew ver well in 13 km/h (8 mph) wind speed.

I think foam wings would not be an issue for weight, but a plywood fuse... What kind of plywood are you thinking to use? Maybe a balsa and fiberglassed fuse would be OK. I forgot to mention that our balsa fuses were fiberglassed.

Saludos.

[Sparky Paul]

I've found "doorskin" plywood is good enough for sloper fuselages, with glass cloth on the forward half.
Builds easy, repairs easy, and doesn't hurt much $$$ wise to throw away when it's had it. :)

[Ricardo RW]

I've found "doorskin" plywood is good enough for sloper fuselages, with glass cloth on the forward half.
Builds easy, repairs easy, and doesn't hurt much $$$ wise to throw away when it's had it. :)Good tip Sparky. Thank you.

I think that the PLY is about 1/16" or 1/8" (2 or 3mm), or it's thicker?

}Take care.

[Chrono]

I'm doing it with 3 mm plywood

[Sparky Paul]

The US stuff comes from abroad, so it may be 3mm in reality.. I've never bothered to measure it.
There's many different woods used, so hands-on at the lumber yard probably works best.

[Ricardo RW]

I'm doing it with 3 mm plywoodHey Chrono,

Post some pictures of what you're doing!!

Saludos.