Help, flat bottom wing to better airfoil

[milesperpound]

I was hoping to get some help with changing a wing airfoil on a Guillows P-47. After converting a couple of these small kits to electric I was hoping to get a little better performance rather than the FF bob. The last one I did I still used the flat bottom and corrected the wing incidence. Fly’s OK but I wonder if I change the next conversion (P-47) to a more rounded profile, would that improve the flight characteristics?

So far I traced the ribs and copied them on the bottom creating what appears to be too thick an airfoil.

I guess my question is do I just add material to the bottom tapering to the TE? Find a wing rib profile more appropriate? Copy the top profile to the bottom? Are there plans or airfoils for this plane better suited for E flying? How does on one accomplish this?

Thanks for your help

Mark

Video of Guillows mustang here: http://www.rcgroups.com/forums/showthread.php?t=766304

[MarkusN]

Fly’s OK but I wonder if I change the next conversion (P-47) to a more rounded profile, would that improve the flight characteristics?
That depends on the flight characteristic yopu want to improve. Going to "semi symmetrical" will give you more performance at the high-speed end of he flight envelope, improved inveted characteristics, but less at the high lift end.
In fact good old flat bottomed Clark Y (the original, not the "all flat at the bottom" knock-offs) is pretty good right down to zero lift (high speed flight).

[BMatthews]

If you're spending almost as much time inverted as you are upright then yeah, going to a symetrical airfoil like an SD8020 or good ol' NACA 0012 (12% symetrical) would be a good idea. Note that the leading edge shape is quite important so I'd also suggest that you basically change the whole way the wing is made other than the outline and rib spacing. Use a proper leading edge and build up a proper I beam spar using top and bottom caps with vertical grain webbing. 1/32 sheet to form a D box leading edge would not be inappropriate either.

[milesperpound]

Thanks for the responses. I think the wing design is based on the Aquilla airfoil. Flat bottom and the leading edge really tucks down. I think that is why my plane once it loses thrust (2 degrees down) it pops up. As it fly’s, it is like when you put your flat hand out a car window, up down hard to keep stable.

Anyway I am going build up the wing and see if I can pull the leading edge up more D like. I plan on sheeting and will either put a main spar or do the vertical webbing. I do like to roll the plane and thus far it is a bit nail biting rolling a flat wing. At least the one I built. So a little more than just flat on the bottom and I guess I will see what happens. The variables can be overwhelming.

Thanks for the insight.

M

[Texas Buzzard]

Millerspond, you do a nice finishing job, real "spiffy" P-51.

Did you say you converted a controline ship to RC Electric? That is asking a lot in my opinion. Usually controline ships do not build with weight as a main concern. Now I think you are asking if you can make the P-51 fly better by changing the airfoil. Am I correct? With just a 27" wingspan you have about 150 square inches of wing area. That is about one sq. foot area. If the plane weighs 10 to 12 ounces then the wing loading is up close to 10 or 12 ounces per sq. foot area. For a plane loaded that much you are going to have to fly faster than the average elec. plane flies. So from the video weight accounts for a lot of the speed required to just stay in the air. Too small a plane and too much weight will cancel out any refinements you make to the airfoil in my opinion. :cool:

I don't think a change in the existing wing will make a difference. But a new wing with more area ( increase area by 50%) will allow slower flight. And if you fly below 22 mph a flat plate will fly the plane with enough area. But then the very pretty 51 will look like most other elec. plane....eh? Ha, ha I once heard, " You can dress a donkey up like a racehorse, enter him in the Kentucky Derby - and he will still run like a Donkey". Seems to me that you need a new plane. Make it fly first then later add P-51-like decorations. :)

As was mentioned before, in the video it looked like the CG is too far aft ( it's tail heavy). Move the battery forward a bit - 1/4 to 3/8ths an inch.

One thing that Reynolds Numbers and the wind tunnel reveals about very small planes like ours ( less than 48"w.s.) is that we cannot expect a small wing to have the same characteristics as a full-sized wing. This mainly is because of the small chord. Flying at slow speeds fewer molecules of air pass over the wing/ time than in full-scale wings. The Univ. Of Ill. wind tunnel showed that a flat plate of less than an 8" chord at speeds of less than 22 mph has similar lift characteristics as a "normal" airfoil. Build a new plane designed for elect. flight is my opinion. But KEEP BUILDING ... it's fun. :p :p

Check this - GWS has a P-51. Look at the specs. Go see the video, it flies like it should. 220 sq.in, 14 oz, & the address is:
<http://www.gws.com.tw/english/product/airfly/p51.htm> copy and paste.

[milesperpound]

Texas B

Thanks for the reply and compliment. Yes these Guillows kits do ask allot in reference to there size and that is half the challenge. I will say, that it flys much better as an E conversion than it did as a CL gasser.

The P-47 is aprox. 31 inch WS and I have not calculated the surface area yet but should be Significantly more than the p-51.

I do continue to move the CG forward and is almost at 20% already. I guess with the Free flight design that would make sense. What I have gathered so far is that moving the leading edge up and making the bottom a little "less flat" modding more toward a true Clark Y should improve inverted / rolls. That was more or less what I was looking for. Again just going to experiment since I will be building the wing anyway.

Thanks again for your input; I am learning a ton on building. almost as much fun as flying!

Mark

[BMatthews]

Actually you're going the wrong way with the CG. A forward CG requires more up trim. More up trim makes the model far more sensitive to speed variations and that can show up as a "nervous" model for pitch. Try going back instead. It should be balancing at around the 30 to 33% mark.

Do a search for how to set the CG using the test dive method.

Switching to a symetrical or low camber airfoil such as the NACA 1410 or similar would go a long way towards making this size and style of model fly a lot better.

And the reduction in Reynolds numbers shows up in the need for a far lighter wing loading on the small models. What would be considered as a good wing loading for a gas bag floater 100 inch glider at 8 oz/sq foot is just barely light enough to fly decently on a 30 inch span model and would be considered as heavy. 6 oz/sq foot is a far more successful loading on this sort of size.

The Guillows basic flat bottom shoe outsole airfoil has been around a LOT longer than the Aquila.... :D A ClarkY would help but from the way you're describing your flying a symetrical or low camber value 10% section would be far, far better for you.

[milesperpound]

Bruce,

Let me make sure I have this right. Up trim meaning trimming so the nose goes up, and down trim means the nose heads down? If that is the case, and my elevator currently has down trim as this is the trimmed out setting for level flight, wouldn’t that mean I still need to move the CG more forward for a neutral elevator setting assuming the wing incidences are 0,0? Also not a dive test but power off in glide slope and she balloons up. Not sure I have a clear answer but I did move the CG Forward from the first flight and it was more friendly. Still think couple of more mm forward would help. Thoughts?

The Guillows basic flat bottom shoe outsole airfoil has been around a LOT longer than the Aquila.... A ClarkY would help but from the way you're describing your flying a symetrical or low camber value 10% section would be far, far better for you.
Does this mean I could glue my size thirteen shoes on and get a better airfoil? :eek:

I think I will try the low camber value 10% on the bottom. Does this also mean thinning the top camber? If I don’t this wing is going to get pretty thick.

thanks again,

Mark

[BMatthews]

...Does this mean I could glue my size thirteen shoes on and get a better airfoil?....

Depends on which part you trace around for the airfoil... :D

Down trim on a model of this sort for level flight means nothing outside of that's where it has to be to let the model fly properly at the speed it is trimmed for. The designed in free flight wing and tail incidence settings are set for a lot of positive stability to deal with the far lower speed of rubber powered flight. Far too muchfor your case. At high speeds like your case where an engine is involved the airfoil is still producing lift even at fairly high negative angles of attack. And to hold the wing at this lower lift coefficient value negative angle of attack the elevator will most likely have to have a lot of down trim.

Another way of looking at it is that your stabilizer and elevator are ONE SURFACE. Forget up and down trim and instead look at the whole surface angle of trim compared to the wing. If you do that in conjunction with the likely negative angle of attack of the wing for the lower lift coefficient of the wing needed for higher speed level flight you would find that the elevator with a lot of down trim is actually NOT trying to pitch down but instead is quite happily stable for level flight at this particular airspeed.

Confused yet? Stop and think about how a model will act in level flight and where the tail ends up being trimmed for this case. With your extremely far foward CG even with the negative trim I can assure you that the tail is lifting downwards despite the elevator having a lot of down trim. Moving the CG back to where it should be will result in even more down angle in the elevator. But this is just more ammunition to suggest that you need to swap airfoils over to something more in tune with the model's new flying speed and to retune the wing to stablizier incidence angles. Guillows didn't design it to fly the way you're forcing the model to fly.

Actually I wonder if Guillows intended many of their designs to fly at all but that's a whole other topic... :D

Let's move on...

wouldn’t that mean I still need to move the CG more forward for a neutral elevator setting assuming the wing incidences are 0,0?

I touched on this above but to cement it more you need to realize that 0-0 isn't always what you think it is. If you set it to 0-0 relative to the flat bottom of the wing then it isn't 0-0. To set it that way you would have had to make the stabilizer and elevator 0-0 to the zero lift coefficient angle of the airfoil. Which as noted above is actually quite a negative number and likely in the neighbourhood of -2 to -4 degrees compared the the flat bottom. This is why you've got a lot of negative elevator angle to fly level at your much higher flying speed than the model was ever intended for.

Also not a dive test but power off in glide slope and she balloons up. Not sure I have a clear answer but I did move the CG Forward from the first flight and it was more friendly. Still think couple of more mm forward would help. Thoughts?

At first glance it may appear that this is a classic case of too far back a CG. But in your case with it already around the 20% point this is impossible. It is far more likely that you set the motor thrust line as per the rubber thrust line and you've got a ton of downthrust. The excess downthrust is forcing the nose down and it's only when you back off the throttle that it reduces and the nose comes up as a result. Again it comes up due to the forward CG and APPARENT high incidence angle differnece between teh wing and stabilizer/elevator surfaces due to the factors noted above.

Or if you're saying that you are flying level and you push over into a dive and it "balloons" up right away and very aggresively then that is the "dive test" results telling you that the CG is much too far forward. You need to stop and realize that if the CG is far forward that the model naturally wants to tuck and dive and it's only the stabilizer holding the back down that prevents that. This take up trim. Move the CG back and you need less up trim so when the model speeds up, like in a dive, there's less up trim to lift the nose so the "ballooning" is less harsh. Nose heavy is constant, the download at the tail to hold the nose up is SPEED sensitive. That is why your model wants to balloon up when you push the nose down.

What you need to do if you're keen on future Guillows kit mods to adapt to higher powered options is swap airfoils to something with only 1 to 2% camber (learn about airfoil specs if this is a mystery) or go full symetrical. Then zero out or set to about -1 degree the stabilizer to the zero lift angle of the airfoil and set the downthrust to 0 compared to the airfoil's chord line. Only then will you have a model set up for true overpowered high speed flying.

[milesperpound]

Bruce,

Thank you very much for the detailed explanation. The more I learn the less I know. Time to go off to airfoil school or trace that old shoe! :rolleyes: I have a much deeper appreciation for the scratch builder/ kit designer. Thanks for taking the time to help me out. :)

Easy way sounds like symmetrical. However, part of the fun is getting a Guillows kit to fly. I just hope that it does not become too easy with the proper wing design. ;)

I will let you know where I land so to speak.

Thanks again.

Mark

[BMatthews]

..... However, part of the fun is getting a Guillows kit to fly. .....

40 + years of model building, flying and designing and I've yet to find the intestinal fortitude to deal with such a daunting challenge.... :D

I once went back and made one of the small Spitfire kits that is around 16 inch span using the plans but my own wood and lightened it up tremendously. It still didn't fly worth a darn but it was now for aerodynamic trimming reasons instead of pure excess weight. If I knew then what I know now I think I could have made it fly decently. But I didn't and it was the end of the Guillow's chapter of my modeling history.

Well... aside from the Guillows Fairchild 24 that I got given to me years later. But that kit was a whole other enchilada. It had good wood in it and a delightfully lighter built structure. I actually had it flying decently for a while until something happened to it. Can't remember what but it wasn't around long once I got it flying.

[JamiePhoto]

Please pardon the interuption, but...

I must say that I'm impressed with the wealth of knowledge on these forums.
Complex principles and I'm able to follow along, thanks to your ability to articulate. Way to go Texas Buzzard and BMatthews.
I feel like I just exercised that muscle between my ears. Keeps me young! (at 53 years old).
Thanks and keep up the good work,
Jamie

And now back to your regularly scheduled program....