looking for low speed Airfoil for scale flying boat - RC Groups
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Oct 14, 2008, 02:24 AM
meatbomber's Avatar

looking for low speed Airfoil for scale flying boat

Hello guys,

i`m looking for an airfoil for a Martin P-5M2 Marlin Seaplane.

The Specs are as follows:
(Imperial units in brackets)

Wingspan 200cm (79in)
Wing Area 44 dm˛ (682sqin - 4,74sqft)
Mean aerodynamic chord 22cm (8,66in)
Project All up Weight 1500g (53oz)
Wingloading 34g/dm˛ (11,18 oz/sqft)

I've already built this plane in a smaller version with a open bottom Jedelsky type airfoil as per Philippe Jamets (GGRN on rcgroups) plan.
The Problem is that the real aircraft was very much a slow flyer with a cruising speed of only 150mph which translates to only 2 fuselage lengths per second to obtain a scale speed.
Now the Fuselage Length is about 1,4m so it would have to be able to cruise in the 3-4m/s range.

I have Profili V2 which contains a database of several thousands of airfoils, i just need a pointer where to look for the right airfoil

Any suggestions ?

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Oct 14, 2008, 04:36 AM
Registered User
John235's Avatar
I think it will be hard to find practical airfoils that have max lift co-efficient of more than 1.2 or 1.3. Based on a CL of 1.2, your specs require the stall speed to be around 24km/h or 6.66m/s. If you want it to fly slower, I think you need to reduce the weight. If you can halve the weight the stall speed would drop to 4.7m/s.

P.S. I hope my maths is right. Maybe someone can check it.
Oct 14, 2008, 04:44 AM
meatbomber's Avatar
well the 1,5kg Weight estimate actually includes a bit of a margin... right now if i take the allready built up components and estimate the rest that`s still missing plus teh weight of the RC components we'll look at about 1,2kg but then i have alwys put up extra weight in the finishing phase.

The Plane design also includes Flaps for slowfligth on take-off and landing. but she should be able to cruise as slow as possible in a clean configuration.
Oct 14, 2008, 05:14 AM
An itch?. Scratch build.
eflightray's Avatar
Isn't slow flying not just about the airfoil but the decalage and the way it's flown?

The usual way to slow a model, example for landing is to get the nose up so that there is plenty of drag. The motors then do more of the work to keep the model in the air.

Admittedly it probably wouldn't look all that scale like flying nose high, but is there a compromise where the incidence angle of the wing to fuselage could be increased, without spoiling the scale appearance?.

Just a thought.
Oct 14, 2008, 05:21 AM
Registered User
John235's Avatar
Maybe the Clark Y airfoil is worth looking at, but it isn't going to achieve the low speed performance you asked for, even with flaps.
Oct 14, 2008, 05:22 AM
An itch?. Scratch build.
eflightray's Avatar
Also the wing section could be one of the 'old' under cambered sections as used on vintage models. Perhaps a fairly thick section to keep the drag high.

I'm not into the science side of model building, I still have to rely on, "Mmmm, how would I do a...........".
Oct 14, 2008, 05:42 AM
German Engineering.......
HugePanic's Avatar
to fly slow, you have to build a very lightweight aircraft! Everything else (my point of these things) comes second.

what material are you using??
Oct 14, 2008, 06:00 AM
meatbomber's Avatar
it's a depron build. The whole Fuselage Assembly including the Tailplane weights about 280g

incidence of the wing is pretty high to faqciuliitate water takeoffs (i had to increase the incidence on the smaller models wing to 5° to get decent performance, and oh wonder researching the original it also had a 7°root and 2° tip incidence so overall 5°.

I would say that 1,5kgs or 3lbs is quite light for a 2m wingspan model no ?
Oct 14, 2008, 07:48 AM
Build, fly, learn!
As John wrote I would try a simple-to-build Clark Y airfoil. Anything you build from Depron is not going to achieve much better than a Clark Y will (concave wing bottoms are nearly impossible with Depron), and as an added bonus, it has gentle stall characteristics.
You'll probably want to add some wash out at the wingtips, too. Tip stalling that big, heavy model in slow flight wouldn't be nice!
Oct 14, 2008, 09:16 AM
meatbomber's Avatar
well guys.. you can build ANY airfoil in depron. Sorry to say but if you think otherwise you never tried. Of course to build a undercambered bottom is more time consuming needing a 2 step build process but in all not different than a fully sheeted balsa ribs wing. You can also use the same building tabs approach as on a sheeted balsa wing that get cut off after closing the wing.

Clark-Y doesn`t answer the question... seems i`ll have to look elsewhere
Oct 14, 2008, 10:07 AM
Registered User
Look for an airfoil with high camber (around 4 to 5%). Unfortunately many of the really high camber airfoils have pretty nasty stall characteristics (The FX 63-137 comes to mind; Hysteresis over several degrees of AOA, i.e. after a stall you have to reduce AOA by several degrees before you get the flow to reattach. For maximum lift it is hard to beat, though.)
Oct 14, 2008, 10:27 AM
Registered User
vintage1's Avatar
If using flaps, use a clark Y, as it's more benign when stalled. If not using flaps, nick and old vintage model undercambered one..Goldberg G-6?
Oct 14, 2008, 03:45 PM
Registered User
Originally Posted by HugePanic
to fly slow, you have to build a very lightweight aircraft! Everything else (my point of these things) comes second.

what material are you using??

This is your answer!

For a high drag airframe like your Flying Boat, operating at typical model reynolds numbers, it's all about the wing loading. After all, this is the reason indoor foam models with flat airfoil sections can fly as slow as they do.

Most of the finer points of airfoil design are about reduction of drag, while producing the desired amount of lift. In some applications, say composite sailplanes produced in CNC formed molds, I'd be concerned about those fine points. For your model, I'd simply choose a section with enough thickness for structural purposes and then build enough wing area to ensure the minimum flying speed you desire. If you can't adjust the wing area, then adjust the weight. The "ClarkY" that was previously mentioned would be a great choice.

Great job on the fuselage!!!
Oct 15, 2008, 12:47 AM
B for Bruce
BMatthews's Avatar
Meatbomber, an airfoil with more camber will help a lot but it isn't going to get you to two fuselage lengths per second. The only way you'll do that is to get the weght down to about what a Gentle Lady glider weighs and even that may not be light enough. Slow speed comes from one simple thing and that is light weight. Flaps and high camber airfoils can aid this but they won't make up for more than a little bit of extra weight.

Your model as spec'ed comes in very close in both wing area and weight to my own Record Hound radio controlled old timer electric model. Now while it flies in a stately manner it's still doing around 4 fuselage lengths per second at best. And this is with a NACA 6409 airfoil so it's like a thin clarkY with flaps already deployed to the best lift angle.

For your needs I'd suggest a little more thickness for spars. Maybe a 6412?

Or if I was a betting man and knowing how well another old timer of mine flies with a rather thick Goetingen 501 I'd try the similar camber value but slightly thinner G500. Why thinner? Free flighters know that thin is in when it comes to slow flying with relatively narrow chords. Even at 10% with the G500 this would be too thick for a true free flight model with 1/2 your chord and a truly slow flying speed. But for your model size and weight it should work superbly.

If you don't have it already download Profili 2. The Gottingen 500 is in the airfoil library that comes with the program. It's freeware for the basic functions and you can pay a pittance of something like $15 to unlock all the extras.
Oct 15, 2008, 03:26 AM
meatbomber's Avatar
i have profili thanks i`ll look up the Profiles!

i`m not fixed in the 2 fuselage lengths / second... that`s the scale speed, which i know is next to impossible to attain unless you fill it with helium
if it flies at 5-6m/s it`s still a stately looking gracefull scale bird. which get`s it right into a more manageable speed range.

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