gelsthorpe

HI,

I am currently interested in building an A-10 in 1/10 scale, this would be the same size as the new Freewing model which I am led to believe will be available April onwards this year.
Now most of the A10 video's in the smaller scale's seem to be way over speed. and do not look convincing when compared to full size.
Just to put some numbers in ::

Full size wingspan 57' 6", wing area 506 ft^2 root aerofoil Naca 6716, wingtip Naca 6713, cruise speed 340 mph, max speed 440 mph
Model wingspan 69" , wing area 5.06 ft^2, root aerofoil ? wingtip aerofoil ? cruise speed 34 mph, max speed 44 mph.

I would like to produce a "sport type design" what I mean by that is simplification , i.e. electric power via either a single pusher prop on fuselage tail or 2 pushers located in the turbine pods i.e. not a D.F approach. In other word by dropping a D.F. system gain more practability / increased efficiency, the resulting prop/ props would hardly be visible in the air. Ideally I would prefer to omit wing flaps so choice of aerofoils must be made on basis of no wing flaps for a quicker building wing and less weight.
I have done some calcs, and then there is some guess work,
Known quantities are wing Aspect ratio at ~ 6.53, taper ratio 0.68, wing tip chord 8.2 ", max chord 12.04", mean chord 10. 84" Reynauld's no's at 20mph are wing tip 127560, mean chord 169000, root 188000.
The guesswork is the final weight, now with a light build (some use of Depron) 8 to 9 lbs all up weight should be feasible , this results in a wing loading range of 25.3 to 28.5 ozs/ft^2.
I cannot find any aerofoil polars for the above Naca foils so cannot just trust to luck with them, should I use Clark y at root and tip with 1 or 2 ' of washout or does any body have a better suggestion please ?

Henry

2walla

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The designer of the jetarrows a10 did a lot of research on airfoils for his model and posted it .. It was on a german website. I have two of them and they fly wonderful... I don't remember where it was- i think there used to be a link on the jet arrows website to his data. If you are going to go single prop design.. just keep it light and it will fly fine with just about any airfoil..

gelsthorpe

2waller,
Thanks for the reply, I wasn't aware of the JetArrows site, checked it out
Looks like the A10 is no longer manufactured, and the aerodynamic insights have now been lost .
Still as you say if light enough most aerofoils should work.
As Clark. Y has some good data on the UIUC website (vol 3 download) I will
Use this.

Henry

Bax

My Feedback: (11)

The later Top Flite Gold Edition sport-scale kits have used a Selig airfoil for their wings:

Airfoils: Root S8036 Tip S8037

These are expressly-designed for scale model use, and have turned out quite well. For the speeds models fly at, they are good ones.

gelsthorpe

HI Bax,Thanks for taking the time to reply, the S8036 and S8037 are indeed mentioned in rather glowing terms in UIUC vol3 in relation to P47 warbird as you say Topflite.As I have done quite a few calcs using Clark Y I am now going to stick with this choice.Henry

BMatthews

The big claim to fame for those Selig airfoils is that they handle higher wing loadings with some tolerance. But they don't make high wing loading models fly any slower.

The big thing for the A-10 is the seemingly slow lower range flying speed and tight maneuvers. For that you want some camber and a lighter sort of model weight. If you are after that appearance in the air you'll want to aim at a flying weight of more like the 7 to 8lb range instead of 8 to 9 lbs.

It might be worth looking at some model airfoils with higher camber as well. Something like the Selig 4233 is a thicker airfoil for more stiffness and strength combined with good low speed characteristics and a fairly high maximum lift coefficient. Another good airfoil that worked well for me on an old school electric glider back when these things were all fairly heavy was the Eppler 201. It slows down nicely yet still allows for some scoot all while carrying a higher than optimum wing loading. "Higher" in this case being higher by glider standards. Not by big scale model standards. There is no free lunch to THAT one.

Given that the props set up as pushers would be eating the "dirty" air off the nacelles I'd suggest twin tractor props sitting just in front of the "air inlets" of the nacelles. And as much as possible make the nacelles flow through style tubes with the motor mount on fairly thin supporting "wings". The props in this case will still see some dirty air off the wings and forward fuselage but it'll affect them far less than trying to eat the turbulence off the nacelles directly in front of the props. The payback for this selection should be more pull from the same amps of power or same engine power. And if running engines and not electric it means more effective cooling for the engines.

All in all an ambitious and interesting plan. I wish you luck with it.

gelsthorpe

BMatthews,

Thank you for your contribution, I compared my choice of Clark y against the Selig 4233 using Soartech vol 8 data. The Selig is 13.6 % thick vs Clark Y at 11.72 % so a win for Selig in terms of wing strength with like for like wing spars.
Surprisingly Clark Y is slightly more cambered (3,55% vs 3.26 %) and drag coefficients are on a par at Reynaulds no.s around 100K and above, the Selig has a slightly greater max Cl of 1.25 vs 1.2 .
From a practical build point of view however the ability to build the Clark Y version direct without some form of Jigging is the clincher,
You also gave some sound advice re engine/ motors being tractor mounted vs pusher on engine pods, I have given this some thought and have taken some inspiration from here http://www.outerzone.co.uk/plan_details.asp?ID=6948 a Mike Pastro design with a single pusher motor. This has the advantage of only 1 motor and speed controller required and the thrust line will not require any offset whereas the high mounted pods would.
This of course limits the size of prop I could use, perhaps go to a 3 Blade?, not too worried about motor weight at rear as possibly 2 motors on the pods might have worked out the same moments around the CG.
The Key thing is to build it 8lbs or less flight ready, Mikes version was 7lbs with fixed gear, 8lbs with Pneumatic retracts for a 57 " span version.

Henry

BMatthews

I see that Mike's version uses a ClarkY style airfoil too. And there's no doubt that it would be easier to build.

Mike's version is a fair bit smaller than your version. In his case being able to use a larger prop might play a significant part. But I suspect it is primarily because he chose to use a glow engine instead of electric motors. If you are thinking of a glow engine for power than it does provide the benefit of only one engine to start. On the other hand if you are considering electric power I still feel there is some considerable advantage to going with a twin motor setup.

First off the mass of the motors will be a lot closer to the CG location. So even though there would be two motors the more forward location should eliminate the need for some amount of nose weight due to the closer distance. And that means an overall lighter model. Another bonus would be that centralizing the mass will also result in a smoother handling model in response to pitch inputs. And finally you would not have the same limit on the flaring angle during landing that the wire whisker forces to avoid the prop striking.

But that's only if you are planning on electric power. If you are going with a glow engine than a pusher makes a lot of sense. Or even to mount it in the nose since both are about the same distance from the CG location.