thickness vs chord
 

hey, i have a quick question, that will probably have a long answer.
I am scaling up a stubby from rcm plans to about 130% the original size. Instead of a "flat bottom" wing, i want a symmetrical wing. I found an airfoil with a google search, and the one i picked said ultrasport 1000. I printed a pic of it off, and scaled it up so i would get a chord of 15.25" and ended up with 3" thickness. The wingspan will be 80". The problem is, if any. is that the airfoil looks really thick. Some reason just doesnt look "right" . I read on a sight that ideal thickness should be 15-18%. the airfoil i have is 19%. i probably shouldnt be sweating about this, but what type of flight characteristics could i expect? (i figure lots of drag, slow speed) I kinda want a plane that can be flown on the edge of a stall, but also able to do a good clip if i want to. I expect this plane to weigh about 12 lbs and it has a wing area of about 1200 sq" I intend on using a 25cc gas engine.
any input is greatly appreciated
thanks,
andrew

RE: thickness vs chord
 

Andrew, the thickness can be varried to cord length. 3" on a full symetrical airfoil on a 3D plane with a 80 wing span isn't far out of the ball park. My Ruperts Dad has a 60" wing span and the airfoil is 2.5" thick. The plane fits your requirements. My cord is 17" at the root and 15" at the tips. The main wing is a rectangle with tapered ailerons that are about 25% of the cord at the root. The thickness is 2.5" so your 3" for you longer wing sounds OK to me.

Don

RE: thickness vs chord
 

There ain't no ideal thickness-
Start with a rolling pin
(a cylinder shaped wing)
The lift is almost nil but the limit to angle of angle attack is ALSO also nil
smash the rolling pin to be 1/2 as high as it is wide
now we get lift at some angle of attack but still the angle is not critical
Also the lift possible is pretty low
Obviously - there is a relationship between chord thickness and critical angle of attack.
So- from cut n try (experience) most modelers have found that a airfoil shape of 10% to 20% is a workable parameter
It also (very important) allows for a light structure
Best of both worlds for a strong ,light, slow speed aerobatic airfoil is a fat one.
Once we get wingloading waaaaay down we find that the thickness can really be reduced -much further than seems practical
I have used airfoils which are only 2-4% thick! (flat Depron foam) and the shape does not stall out unpredictably. Itis simply because the craft is so light and strength is adequate..
These shapes also "fly" predictably when fully stalled - (tho stalled they are maneuverable )
Sounds impossible but in the area of super low wing loadings the rules can be badly bent.

RE: thickness vs chord
 

Thicker airfoils are stronger to flexion and twisting, but they produce more drag.

Switching from cambered airfoil (flat bottom) to uncambered (symetrical) will give less lift and less drag at the same speed; hence, your model will have to flight, to stall, and land faster.
Also, the relative angles of stab and wing will change, since your new wing will not have the pitch torque that the original wing produces.

RE: thickness vs chord
 

ok so it seems as though my airfoil will work. most the flying i will do is big loops, cuban 8s ect... now according to what inewqban is saying, i will need a little bit of positive incidence on the wing and maybe some negative incidence on the stab. as far as weight, i should have a pretty light wing loading, b/c i am going to use foam ribs with cap strips, and foam formers. should be alot lighter and pretty strong. now, on the tail, should i build it flat like most other kits, or should i use an airfoil shape as well (symmetrical)

RE: thickness vs chord
 

Just build something strong enough and keep th weight down
camber etc., ,really not important on this type project but WTH- build it and se what happens.

RE: thickness vs chord
 

I think i might build two stabs for this plane. one flat and one airfoiled. I could have the stab halves bolt on like the wings, and make the joining system the same on both stabs. It would be really interesting to see the effects. I was thinking of cutting the tailfeathers out of foam and sheeting and fiberglassing, so it wouldnt take much to build them.

RE: thickness vs chord
 

Andrew symmetrical airfoils of 15 to 17 percent thickness work great. The Ultrasport series of models have been proven in flight so those airfoils shouldn't be a problem. 19 percent isn't all that much different. You will need about 1/2 to 1 degree of wing incidence with 0 tail incidence to fly level with no up elevator trim. You can't get level flight without the wing lifting some. Look over the designs that are published and you'll find similar setups. The airfoiled tail would be ideal, flat surface is less than ideal but works OK as we have seen in a lot of airplane over the years. You can compromise and use a diamond shaped airfoil on the tail. It's not that much more work to do and works fine. I have all three in my hangar of airplanes.

I respect Dick but his comment of, "Sounds impossible but in the area of super low wing loadings the rules can be badly bent." isn't quite right - sorry Dick. All things that fly do so in accordance to laws of physics and aerodynamics. When you think of super low wing loadings and flat foamy types of flying surfaces all you have done is move into a lower Reynold's number flight regime and as a result you certainly don't have the same flight conditions as a .60 powered airplane or a Boeing 747. Each is flying in it's flight regime. Some have money implications and thus are studies more than others but all are working. Think of a Fruit Fly. There is a real low Reynold's Number flight condition but things work fine and according to physics. The rules aren't bent, they just are usually not understood - or studied that much. Years ago I saw some interesting studies about the different ways that wasps, flies, dragon flies, regular flies and so on fly. Interesting stuff to an aero nerd like me.

Good Luck

Ben

RE: thickness vs chord
 

sounds like i should scrap the flat stab idea all together. I wonder if kit comanies dont make flat stabs to keep costs down. Since im going to cut foam for the stab, it will be a piece of cake to "build"
thanks for all the input.

RE: thickness vs chord
 

A 3 inch thickness for a 15.75 chord works out to around 19% thick. That sounds like an airfoil from one of the old school fun flyer models from a few years back.

It would fly OK but you're right, it would look a bit silly. As mentioned something more in the 12 to 15% range would be just fine. Download Profili2 from www.profili2.com and use the freebie version to do a NACA 0013 or so and call it good.

RE: thickness vs chord
 

Not at all.

The reason our designs keep using flat stabs is because they work great for our models. There actually is a performance difference but it is so often insignificant that it doesn't come close to mattering.

Very often the aerodynamic results are far less than the structural results. We get a tiny bit more or less aero at some parts of the envelope, but wind up with a lot more or less weight or strength.

A properly sized and built flat stab compared to a properly shaped airfoiled stab is like the comparison of an orange to a tangerine.

RE: thickness vs chord
 

Sorry -Ben - I did not say"breaks a rule" -just bends it.
The configurations for a honeybee are not very good for a 747 and the configurations for a Piper Cub -tho more easily understood -are really not that grea for a basic model trainer
you have to know how to bend em -mend em and send em.
It's like playing good jazz.
You have to know the rules in order to move around the edges .
Example : Eb blues is played in Eb- but all the notes are not in a Eb major chord.
Dig it?

RE: thickness vs chord
 

as far as strength, a foam stab that is sheeted will me much stronger than a built up flat stab. If the gains are marginal, at least there is a bit of a gain by having an airfoiled stab. A few small gains here and there, will likely add up and make a noticable difference in the end.

RE: thickness vs chord
 

On my Clark Y tails & wings, I do know the stall speeds of the plane are slower and flatter. I do use a "fatter-thicker" Y in the tail surfaces so only the nose drops first. That way I never have a tail STALL & the complete loss of control. So far, so good.

RE: thickness vs chord
 

Dick Hanson

I just can,t keep quiet on this.

Love the way you think:):):):):):)

Donny

RE: thickness vs chord
 

The only reason I happen to know this is because I benchmarked UltraSport specs a couple of years ago. While thickness is 19% at the root, it thins to 16% at the tip. It also has a 25% chord taper from root to tip. Joe Bridi put just enough dihedral in to make the upper surface of the wing flat. Thus, looking at the small thin wing tip and flat surface creates an optical illusion that the wing is thinner than it really is. Pretty darn clever. Oh yea, it flies awesome too!