T-crafty

Hey Guys,

I Have generated a 3 view drawing from the side view of what "they" are calling the future Gee Bee R3. I was hoping someone with a little more experience in aircraft design could take a look at the views and let me know if I have made any critical design errors. The fuse will be 61" and the wingspan 72". The airfoil shown in the side view is the airfoil at the fuse/wing intersection and the chord length gets a little longer outwards from the fuse. I will eventually design the airframe in 3D to make sure all my parts fit nicely together. I may even have the parts laser cut.

I need help with the airfoil design also. I understand that a barn door will fly with enough incidence and velocity but I do not know how to efficiently design an airfoil. If the top is longer than the bottom it will generate lift right? Is that all there is to it? I want to use a semi-semetrical wing. I want 1° of incidence, 3° of dihedral, and I like the top down profile I have come up with. Anyway here is a pic of what I want to build. Thanks for looking! Any coments are welcome!

Aaron

HighPlains

My Feedback: (1)

The "R3" is an abomination. The aerodynamics of the design are terrible, certainly nothing that the Granville brothers would have ever built, since they designed airplanes that were fast and efficient.

On the other hand, this model design is sexy from the curves and paint of the prototype. If viewed as an artistic exercise, it is interesting. Wing airfoil section really won't matter all that much. Just make it thick enough for strength.

BMatthews

You probably do not want to "design" your own airfoil. I do for some models but the shapes are closely inspired by the "proper" shapes that have been wind tunnel tested.

For your new wave racer I'd suggest something like the Selig 8036 or if you want to keep it thinner the Selig 6060. The 8036 has a good reputation for being a bit more tolerant of heavier wing loadings so if you tend to build a little heavy then this option will possibly give a little more performance to a heavier model. The SD6060 is more of a sailplane airfoil but on my advice a buddy used it on a Quickie 500 and he was very happy with the performance.

T-crafty

HighPlains - I can understand your comment "The "R3" is an abomination." I do not particularly like the title of "Gee Bee R3" but the design of the aircraft. I also only took the side profile of the fuse in my design, I "made up" the rest. Thank you for your input on the airfoil, but why do you think it will have little effect on the performance of the airframe?

BMathews - Thanks for the input. I will look up those airfoils and let you know what I think when I have time. Why do you recomend I do not design my airfoil? I was going to go at it in a very simple way - two elipses, the top being taller than the bottom and tangent straight lines leading to the trailing edges then I would pitch it at 1° if incidence along the chord. I understand that there are many pre defined, and tested airfoils but I sort of wanted something of my own. Again thank you for your input! Some real good info here so far.

HighPlains

My Feedback: (1)

The airplane built to the plans as shown will have a huge drag penalty due to several things:

The sharply contracting fuselage width over the chord of the wing.

The angle of the fuselage to wing junction.

The upsweep of the fuselage aft the wing where there is downwash from the wing.

T-crafty

Hmm, ok - so I will narrow the fuse closer in the front in the top down view.

Change the front on profile of the fuse to better suite the angle of intersection between the wing and fuse and I will rais the wing a little for this purpose too.

And I will modify the cross section of the fuse to be more elipse like than triangular - this should also help with item #2. maybe the new profile will be slightly larger on bottom rather than on top. Would this help with the drag problems you see? I will post a new image when I am finished. Thanks for the help!

HighPlains

My Feedback: (1)

GeeBees were all about curves, so a nice fillet between wing and fuselage would both look good, and help topreventing flow separation (drag). You might also want to consider a few degrees of dihedral.

T-crafty

Those too are good hints for my model, but is the drag really that big a problem as I have it drawn? I am not questioning weather or not there is drag created by this design but I am more thinking of the connection between the wing and fuse of like an extra 300 and it's lacking of wing fillets, does this aircraft just deal with the additional drag created by this intersection? Also I have 3° of dihedral designed in - you think I need more?

tom czenthe

My version is evolving 86" wingspan

T-crafty

Hey thats really cool! Can I ask how you go about your design process? Also will yours be a model of the "R3" that seems to be floating around or will yours be more "one off" than that? What size motor will you be using? Sorry about all the questions, thanks for the pictures!

BMatthews

"desinging" your own airfoil is not a good idea for many reasons. First off the curves are not elliptical. The old NACA ones used a mathematically produced curve and others have used trial and error. More recently the airfoils are shaped with a close eye kept on the pressure distribution and the curves altered to attempt to reduce any sudden discontinuities or slopes in the pressure map over the range of Reynolds numbers and lift coefficients that the airfoil is intended for. Pressure peaks and steep slopes tend to produce separation bubbles that result in nasty performance issues and nasty flying results.

As I said, I've done my own based on makeing shapes similar to the "brand name" airfoils but with specific minor alterations to make building easier or for some other specific changes. However when performance really matters I use an unaltered and proven airfoil as it was intended because even a minor alteration can have serious drawbacks.

If you want to persue this more then I recomend that you download Profili 2 from www.profili2.com and pay the paltry amount he wants to unlock the Xfoil option. Draw up whatever shapes you like and import the coordinates into the Profili and run them through the Xfoil software to generate a lift drag polar set and pressure distribution prediction see how they compare to the ones in the program.

Or make up a simple Stik like model that you can use to mount simple constant chord wings onto and try making wings with different airfoils.

tom czenthe

My design is basically the same shape as the R3......The airfoil is an Effinger 205 ....I had good results with this airfoil in the past.....I am not an aircraft designer !!!!! Trial and error, cut away anything that doesn't look like your goal and most important learn from your mistakes.......I have been designing and building scale competition planes, the R3 is nice break from the 100% accuracy................Engine is going to be a G62.....target weight25lbs......Some of my scale designs

T-crafty

BMatthews Wow, thanks for the in depth description of the airfoild design problems - I think I will heed your advice and use an existing airfoil. I would very much like to buy the Profili program however I just bought a new house and I don't think I can afford much of anything for the next month or two, but I have to get on with my designing. Thank you for all your help so far. If you think of anything else you think would help me I would love to hear it!

tom czenthe Those are some cool planes! I hope your "R3" goes together well!! Would you mind if I look up that airfoil and posibly use it on my airframe? Thanks for all the info, it looks like you do some great work!

BMatthews

The writer of Profili is offering the basic program for free and to unlock the extra helpful features (including Xfoil) he's requesting a 10Euro donation which works out to $12.35 US.

Start off by downloading the freebie and as you move around in the program and play with it you'll come across the parts that are locked out for now. It's still pretty good but for less than $15 you can have something that'll provide literally hours of knowledgable fun and frolics...

T-crafty

BMatthews WOW! $12.35??? Thats less than a pizza! Maybe I will try it, thank you!