mvallyman

My Feedback: (32)

I have a plane that I would like to fly at a slower speed,and land slower without removing any weight. It has a semi-simetrical airfoil, cord is 10.5 inches wingspan is 61 inches. My plan is to make a new wing leaving the top airfoil basically the same but longer so it fits in the fuse and changing the bottom airfoil shape to be flat from the spar back and increase the cord by 1.5 inches. I believe this will give me light enough wing loading for what I want.
Will it be a problem leaving the highest part of the airfoil where it is even though Im lengthening the cord by 1.5 inches to the rear??

limeybob

My Feedback: (6)

have you thought about adding flaps?
bob

limeybob

My Feedback: (6)

Changing the airfoil won't make much difference, unless you went to a under cambered section.

you are increasing the wing area by 114%, the wing loading will decrease by 14% and the stalling speed the same.

Calculate here http://pages.sbcglobal.net/limeybob/stallspeed4.html

Bob

TFF

A flat bottom airfoil will make more lift so you should be able to fly slower, and more area is always better. The question is why do you need to do it and how much slower do you want it to fly to call it successful?

BMatthews

A flat bottom airfoil will allow the wing to go up to a higher angle of attack and make more lift.  But it's not a big amount.  If you're talking about only needing a small change for wing loading and lift then fine.  But if the model is just plain not fun to fly due to the wing loading being too high then your addon area and airfoil change will not make enough of a difference to transform the model from a dog to a kitten.

You may also run into some stabilty issues.  You're reducing the wing to stabilizer moment arm and making the chord bigger all at the same time.  You will need to run the new shape through a CG calculator to find out where it needs to balance.

Lnewqban

Mvallyman,

With those changes your plane will loose up to 33% of the current speed, but that depends mainly on the actual difference in CL values of the flat bottom and the current airfoil.

The wingloading will decrease as much as the area will increase: 14%.

A2/A1=(61 x (10.5+1.5)) / (61 x 10.5) = 732 / 640.5 = 1.1428

For example, if the current wingload is 22 oz/sqinch, the new one will be 19.25 oz/sqinch.

Now, the difference the airfoil modification will make:

See the attached schematic for a comparison between a symmetrical and an equivalent flat bottom.
I know your plane has a semi-symmetrical airfoil, but I couldn’t find a graph for that one.

Let’s say we keep the same AOA for both, make it 2 degrees.
The CL for the symmetrical is 0.20 for that angle (Let’s make it 0.325, since yours is not a pure symmetrical).
The CL for the flat bottom is 0.65.

That means that the CL gain of your change of airfoil will be around: 0.65 / 0.325 = 2.0

Now to the formula for velocity:

V1 = Square Root [ (Weight x 3519) / (Density x CL1 x Area1) ]

V2 = Square Root [ (Weight x 3519) / (Density x CL2 x Area2) ]

CL2 = 2.0 CL1
Area2 = 1.14 Area1

V2/V1 = Square Root [ 1 / (2.0 x 1.14) ]

V2/V1 = 0.66

Note that reducing the thickness of the airfoil, the strength to bending and flexing of the wing will decrease quadratically (strength is proportional to the square of the distance between the spars or balsa skin at the thickest section).

You will also need to increase some of the horizontal stab area in order to compensate for the increased nose down pitch of the flat bottom airfoil.

Best luck with your project!

mvallyman

My Feedback: (32)

Sorry for getting back to this late but thank you for all of your input.
The plane is a blast to fly aerobaticly (seagull sSpacewalker 40) but at the slowest speed it still looks too fast to be scale. I realize I wont be able to make it fly like my friends 1/4 scale Spacewalker but it looks like I can slow it down somewhat and use the original wing for sport flying. I re-covered the fuse on this ARF so I could add a stringer down the side for a better look, rounded the cockpits just because I like rounded cockpits and to put better looking red stripes on the fuse and tailfeathers as the original looked hokey to me. All in all I have too much work in the fuse to change the airfoil much. Building a new wing from scratch will be fun and will help me in building and/or disigning planes altogather.
Thank you again
Greg