How much do I round the corners on a flat plate airfoil?

[Mel Duval]

Got into a discussion about rounding corners of a flat plate airfoil. I prefer rounding it to a broad point, while the other opinion is to just break the corner and leave the edge flat. Which one is better? Is it different for tails versus wings? What about trailing edges? Anyone got an opinion? Or even better, some data?

THANKS!
Mel D>

[Tom Harper]

Mel,

On large (6 - 7 ft span) flate plates I have used parabolic leading edges and 2-3" tapered TEs. It sounds like your 'broad point' is close to a parabola. I think there is an advantage to reasonable treatment of the LEs and TEs. However, I have seen flat plate 3D models perform flawlessly with square edges.

As you point out there is no data. So, maybe it is a matter of art. In that case round away. It is your airplane.

[raptor22]

While it is best to do what tom says, most people jsut make them flat. Since most flat plate planes are not really meant to be fast anyway, drag isn't a very major concern.

[fhhuber506771]

Round the LE some.. and you get a bit better airflow over the plate and get a little more lift. (because you are approaching a very thin symetrical airfoil) Depending on the ratio of thickness to chord... you get varying results.

Most of the .40 and under size models use flat plate tailplanes. Rounding thier LE helps make the controls have less "dead band" near neutral. making the tailplanes thicker and giving them an actual airfoil can eliminate the dead band. (makes the controls more crisp) We continue to use the flat plates because.. they are sipler... and most of the time we don't need the extreme precision. (but look at the .6o and larger "Pattern" competition model's tailplanes.. they ALL are airfoiled now)

[Sparky Paul]

Got into a discussion about rounding corners of a flat plate airfoil. I prefer rounding it to a broad point, while the other opinion is to just break the corner and leave the edge flat. Which one is better? Is it different for tails versus wings? What about trailing edges? Anyone got an opinion? Or even better, some data?

THANKS!
Mel D>
"better" depends on useage.
Those of us that fly all types find blunt is perfectly fine for some things, and inadequate for others.
There is no blanket "better"..
Depends entirely on the purpose of the plane.

[Mel Duval]

Hi Paul,
Should have listed that. I am building a Mountain Models Flashback 2 Pattern-type Parkflyer. So it most likely won't be flown real slowly and will be flown towards the top of the speed range most of the time. Any thoughts?

thanks,
Mel D>

[Sparky Paul]

Hi Paul,
Should have listed that. I am building a Mountain Models Flashback 2 Pattern-type Parkflyer. So it most likely won't be flown real slowly and will be flown towards the top of the speed range most of the time. Any thoughts?

thanks,
Mel D>
.
The new Mini-Funtana is something like you want... it uses a fully symmetrical wing. It's not a slow-speed parkflyer, so look at one of those for ideas.
Flat wings don't go fast well, they're better in the low-to-zero airspeed 3D "scare yourself" fun fliers, where the aiflow is turbulent to seperated most of the time.
A built-up wing behaves better thru the wider speed range you want.
The most important thing with these small planes is lightness.. add lots of that.. The wing profile can be most anything, but somewhat fat towards the front for the occasional snap or stall manuver to keep the airflow under control.
Normal pattern manuvers don't need much thickness, but your plane won't all that fast relatively, compared to a pattern plane, so plan to more manuverability than stability.. it'll be more fun. :)

Looking at the Flashback, it appears to have all the features.. what/why are you changing?
It resembles the Electric Hots, moment wise.. the Hots is almost perfect for a small pattern style airplane. It's the only plane I feel comfortable rolling at eyelevel over the runway. (I have an OS 15 in mine, after giving up on e-power for a plane that size.)

[Mel Duval]

Hi Paul,
My main question is on the tail. They are flat plates and in the instructions is says to only just break the edges. But I don't like having it look like I did not finish the plane properly. I DON'T like square edges! The trailing edges of the ailerons are supposed to be left square too.....

thx,
Mel D>

[fhhuber506771]

leaving the trailing edges squared helps prevent or delay the initiation of flutter. It makes the airflow leave the control surface more cleanly (as oppsed to rounding... tapering to a point is actually better but very weak and easilly damaged)

[Sparky Paul]

Hi Paul,
My main question is on the tail. They are flat plates and in the instructions is says to only just break the edges. But I don't like having it look like I did not finish the plane properly. I DON'T like square edges! The trailing edges of the ailerons are supposed to be left square too.....

thx,
Mel D>
The shape of the trailing edges is always a point of contention. For this type of plane it makes NO difference. Sqaured, rounded, tapered.. all will work, as the airflow back there isn't laminar anyway.
When speeds get very high, or the least amount of drag is mandatory, then a tapered trailing edge is best, but anything else will work for your purpose. Rounding won't change anything about the way plane flies, and be a bit easier to finish.