Hello, my friends.

Has anyone heard of a flapped Selig's S1223 airfoil?
Would an external airfoil flap work with this airfoil?
What about leading edge slots? Re would be around 300000...
I intend to participate in the SAE Aerodesign next year
Thanks.

For SAE, it's a good time to be doing this. Actually, to do a good job, you will need to hurry a little. You should have the design of the plane finished in the next few weeks, so you can use the rest of the year to do the detail design of the parts and draw plans.

Construction would take up January and Februrary, if you are on schedule - leavnig March to get finished up before the competition. Don't procrastinate!

I know so from experience. I competed in 1990. SInce then I was the rules chairman for the SAE event for about 5 years, and I was the chief organizer for the East competition for about 6 years. Lately I just volunteer where I can, and pilot for teams that don't have their own pilot.

the airfoil

The Selig 1223 is currently the pinaccle of airfoil design for low-speed, high CL. However, your estimate of Re=300k is quite a bit off, unless you plan on having an unusually large wing chord. In the standard class, a large chord would limit your wing to a prohibitively low aspect ratio of less than 5. It would be better practice to assume a reynolds number of 150000 and use the corresponding curves, even lower if you have a highly tapered wing

As for flaps, if you look at this airfoil, it has a carefully managed camber profile - where there is alot of camber at the leading edge - then a very flat "ramp"area designed to delay flow separation. The last part of the airfoil has added camber, probably the most that could be added without seeing added drag or lost lift from separated airflow.

So, I believe the airfoil was designed with the maximum practical trailing edge camber. Adding more camber with a plain flap would not be good.

I have seen leading edge slots used in this competition with good results. On an airfoil like this, with the high camber up front, a slot would probably do very well to energize the airflow. However, it would be a gap right thru the thick part of the wing, where you probably want your spar to be. It could be done, but don't neglect spar strength. As for how much benefit it would be, you'd have to do your own tests.

Slotted flaps have been used as well, with good results. On this airfoil, they would probably have a good impact putting the slot right where the airfoil starts to increase camber it the trailing edge. THen camber could possibly be added for more lift. The problem is, this airfoil is so thin at the trailing edge, you might not be able to ( or want to) build a flap that is that thin. That's OK. Just build the flap using the same upper surface outline, and as thick as it needs to be for reasonable structure. Using a single slot flap, with hinge line below the chord line(DC-9 style) would be good. The web site below shows examples of this.

Here's a good web site about flap & slat design:

http://adg.stanford.edu/aa241/highli...liftintro.html

Thanks!
Your notes were most helpful, Johng.
I did the math again(Re=p.v.L/u): With a 12in chord (AR= 6.7) at 17m/s (38mph), I'd have Re around 300k - with a straight form.
Is the 6.7 Aapect Ratio too low for this application?
Maybe I could go the Cessna way and taper it half way the span to, say, 6in (here's our Re 150k), but that would certainly prohibit the external airfoil flap option, though.
What about those lift coeficients of 4.81 for the LI74?!?!!! Where do I read more about it?
Thanks again!

OK, I was assuming 10in chord, but you are using 12 in chord, which is a good thing. However, your takeoff speed is quite a bit higher than is realistic. Hopefully with flaps & a higher CL, it would be even lower.

If the airplane weighs 30 lb, Cl is 2.0, and the wing area you provide, the stall speed will only be about 11.6 m/s. So takeoff speed would be around 13 m/s resulting in your Re of 270k. Not as low as I first thought, but still lower than the 300k data you would like to depend on. Anyway, if you add high lift devices, the basic airfoil data becomes part of the background and not quite as important as before.

I like the wing Aspect ratio and non-tapered design you have in mind.

Never heard of the LI74.....

Wait a minute. Isn't there a 6ft wingspan but no area limit in this year's rules? If you have a 12" chord and AR of 6.7, your wingspan would be 80.4", which is outside the contest rules and as I understand, would disqualify you immediately. Or did I miss something?

You may be right. I haven't looked at this year's rules. The rules had been relatively consistent for years. On to sae.org.

Having looked at the rules at :

http://www.sae.org/students/aerorules.pdf

Oryx is dead-on. 6 foot wingspan limit. I didn't see any other limits on the planform. That makes things quite interesting. How much wing area to use...?.?.? Biplane, tandem wing, tri-surface, wheee!

Yes, I think it will make things quite interesting this year. In particular, I think it will give new teams more of a fighting chance against the more established teams than in the past. Because the rules were so similar in the past, I think a lot of teams just did more refinement each year, while new teams had to work from scratch. This year it should be less of a factor...

I will be at the West competition in 2003, but as I will be competing I don't think I will be able to give too many tips to Souza

Regards,
Bennie

Thanks for the headsup, Oryx!
I'm in Brazil and I was working with SAEbr 2002 rules...
This year's competition here will take place next weekend, by the way! Usually, the best two local teams go to the USA for the East competition... I'll most likely participate in SAEbr competition next year.
Thanks again.

PS.: The LI74 appears on the stanford.edu site above...

With a 6 foot wingspan limit a low aspect ratio sounds very tempting at first. But beware of the tip votices causing so much drag that you don't get the full benifit of the area not to mention that you can easily wind up with so much drag that the poor engine can't pull the model up to the expected speed.

Seems to me like winglets might just be in style this year...... hint, hint. Or did they short circuit that by specifying a limit on winglets?

If they are allowed I see a wing with a constant chord of about 14 to 15 inches with 14 inch tall winglets having a base chord of about 11 or 12 inches. The spill loading around the tips at the full weight will be quite strong so you'll probably have a lot of incidence at the base of the winglet with a very strong twist to the upper tip. You'd have to do the angle of attack for the wing and set up the root of the winglets with that much "toe-in". Meanwhile the upper tips would be "toe'd-out" to achieve the zero Cl angle of attack. Probably about a 7 or 8 degree twist at least. It would look like you had prop blades on your wingtips ........

The details of the winglets are a little fuzzy now as I read some articles about 15 or 20 years ago but the above is the general idea as I remember it. I think it was Herk Stokley that did a 2 meter glider with a large chord and large winglets to get around the 2 meter span limit without getting into the inefficiencies of very low aspect ratios. It worked very well for him and a couple of others that bult it and reported in the magazines at the time.

And if you try this you'll need a VERY large fin area. The winglets added a lot of spiral instability force that required a big fin to counteract. But on a positive note the winglets added a lot of dihedral effect.

Bruce, I reviewed the rules for '2003 and winglets are not prohibited...so far. I'll probably contact SAE to confirm this. If the only limitations are the wingspan, TO distance and cargo cmpt volume, then we'll have a VERY interesting competition next year!
I wonder how bad it is -- the induced drag...
My first estimates (wing Cd 0.06; fuselage, 1.2; tail, 0.1 and 'wheel' u=0.03; total mass=34 pounds ) indicate I'd need 0.36 BHP (~3.6 lbf) to get to 38mph within 200 feet! The FX.61 has theorically 1.85 BHP @ 16000rpm - at 70% efficiency, that's ~1.3 BHP available! ... Something is amiss here, don't you think?! -- An FX.61 does not get a 34-pound bird to 38 mph that easy!

I'll have to build a database with TO data from a couple of models and engines and extrapolate the data to my endeavour!!! On to the field...or the net...

Best regards. Souza.

Good catch on that LI74 graph. Didn't even consider that when i was reading the article. Yeah, yeah pressure plot, whatever...

I'm still not sure why you keep using the 38 mph figure. Takeoff speed should be substantially below that, for your given weight.

As for the more established teams, Akron already competed a few times with short span biplanes that did quite well, and might even qualify under these rules. So, even they probably have a head start.

I'm visualizing something that looks like a Vari-eze, but with a tractor prop and nearly equal front & back wings. Rutan built one of those one-off Scaled Composites planes a while ago for study as a special ops plane - Looked sort of like a cross between a Quickie "biplane" and a Twin Otter. I think it was called a dragonfly. That would be a good place to start.

PS - make sure the winglets & anything else you add don't break the 6 ft limit. I ran this contest for years & no student has ever won a "it's not part of the wingspan" type argument.

Drag estimation will become much more important, since as wing area goes up and AR goes down, induced drag limiting the ability to climb will be as important as the ability to actually take off. In the past, the competitive planes that could lift 20+ pounds weren't really limited by climb rate, but purely ground roll. Now it will be a balance of both - unless you have guaranteed flat, open space and want to build a ground-effects cruiser.

Why 38 mph? -- Good point!
I guess the first question was: how fast can I get in 200 feet and still be able to control the airplane ? With a 12 in chord, the only controllable variable I had to increase Re was speed, correct?!
Then my tortuous reasoning was:
1. FX.61 for max static thrust would require, probably, a 13X4 propeler;
2. FX.61 with 13x4 will turn @10-12k (?);
3. A 13x4 moves 4in fwd for each revolution, theorically. @10k, that's 38 mph.
4. Trainers do it (TO) around 24 mph (ref. Andy Lennon's book) - I'd probably be able to control that...;
5. Competitors from previous indicated they were working with Re around 250 ~ 300k.

Honestly, I never challenged that first assumption...it just seemed I was in the ballpark...and, you know, I want to lift as much as I...ehr...the airplane can!
Sometimes we want to believe stuff, you know, and we build reasons to support it...human factors, right?
There's even data for the S1223 @ 300k, etc. I wanted so hard to believe it...

Actually, I've been reading about aerodynamics for a while, but the idea of competing in SAE Aerodesign is new...and the airplane is in the conceptual design phase yet

And this forum is all I ever asked for!!! Thank you all for sharing!!!
I'm learning a lot!!! Rgds. Souza.

Hi all,

Just found this thread....ahh the memories! I've competed the last three years in a row, and I HIGHLY recommend that you go!

For construction hints, feel free to peruse: http://www.geocities.com/sunbirdz/Aerodesign_Home.html

or

http://www.geocities.com/sunbirdz/20...sign_Home.html

First off, I'd suggest the APC 14x4W as a prop. It seemed to develop the best thrust for our team on the OS engine.

Second, 30 mph is a better takeoff velocity estimate: http://www.angelfire.com/indie/aerostuff/sae2000n8.htm
I used that in our calculations.

Third, don't use the 1223 below Re=200K. Dr. Selig advised that it's prone to developing histeresis (sp?) at about that spec.

Fourth...pray! We were climbing out (easily) carrying 20 lbs last year when our engine quit. It had about 50 flawless flights on that engine, but chose that time to quit. Stupid nitro engines...I fly electric and sailplanes exclusively now

Trust me, at 70 oz/sq.ft, they don't glide very well! :stupid:

Good luck!
Adam

Yeah, and if I shave my legs, I could be a Sports Illustrated model.

Where you go wrong is by taking the power of the engine at 16k rpm. At what point of the takeoff run do you think it will be turning 16k?? Does OS supply a power/torque curve for the engine? If so, take the power developed statically -around 10k and the power developed around maybe ~12k rpm and you will get a better average power value for the takeoff run. The 12k figure is an estimate of the speed of the engine as the prop unloads when the plane gets some speed up. You could prop for max power, but then the prop efficiency would sink under the grass, as it would take a small diameter speed prop to let the motor turn that fast. Prop for max static thrust, then back-figure the available power. That will get you an efficient prop setup.

If you take a fixed weight, and assume the ability to accelerate past takeoff speed on the ground, the only thing that determines takeoff speed is:

L = Cl*q*S

For a fixed runway length, then you would need to go figure out what the max possible weight would be with a take-off distance equation and then figure out takeoff speed and determine reynolds number from that.


With no limit on wing area, you should be able to have GW of 40 lbs and take off at less than 30 mph. Then you need to think about having excess power available to climb.

John,
That was good! ...Perhaps doing also the nails and hair...)

My logic was what is called Forgetful Induction, according to a book I have... overlooking the obvious! (That data is an optimistic estimation for the best possible scenario for that objective- definitely not our case!!)

A team from UFSC-Brazil published their findings during tests with these engines (see www.aerodesign.usfc.br - in portuguese) at various speeds on a simple wind tunnel. They obtained 12k static, 12.2k at 40 mph wind speed, with the thrust varying linearly from 10.22 lbf static to 5.46 lbf at 40 mph... that's close to half what I expected in terms of thrust, isn't it?!
Still, it's enough to accelerate (no drag) 60 lb from zero to 30 mph within 200 ft in 9 seconds. Deduct drag and rolling friction and your 40 lbs GW estimate is pretty close...

Now the winglets: commercial airplanes' winglets are optimized for cruise (they do not need such a strong twist)... The one suggested above would be optimized for max Cl... and be toe'd in at the base the same angle as the wing aoa for max lift...
I can see clearer now the rain is gone...
Lots of learning! Thanks!

Souza.