Airfoil for Scale WW2 Fighter?

[hul]

I've heard of scale warbirds having nasty flying habits due to their undersized horizontal stabilizer.

What is the effect of having too small a horizontal stab?
Can this situation be improved by picking the right airfoil?
Would a reflexed airfoil (such as MH45, MH60 or similar) help?
Is the correct CG for a reflexed airfoil more forward than for a standard airfoil on an otherwise identical plane?

This is for a 1/8 scale (52") FW190A8 I'm trying to build from scratch.

Thanks, Hans

[vintage1]

Keep it light with a forward CG and you will be allright, some people add washout. I find tapering the thickness towards the tips helps a lot.

The key is to have a wing that stalls inboard first. And puts its nose down when it does.

[hul]

thanks vintage

what airfoil would you use? Something like a NACA2415?

I was planning on using the original's washout (3deg) and incidences (+3deg for the wing and +2deg for the stabilizer; might have to tweak that a little depending on the airfoil). Forward CG might be a bit difficult with the plane's short nose though.
Need thickness in the wing for the retracts, but can probably taper the thickness from about 15% (root) to about 13% (tip).

Any idea how a too small stabilizer manifests itself?

Hans

[vintage1]

In order.

Airfoil? No idea. Use edge of wellington boot for top, and mainly flat underneath? :D

Too small stab? Forward CG is mandatory for decent stability with small tailplane. You want downforce on it (tailplane), that increases with speed, to make sure that it is stable. The center of gravity must be in front of the wing center of lift at all times up to and including the stall, or you will have a plane that stays stalled! ZAGI death spirals are classic examples of small (nonexistent) tailplanes and too rearward CG. Most warbirds I have seen fly at 20-25% chord for CG, nowhere near the 33-40% of other models.

Lead up the nose and take out slowly, rather than fly light and have to rebuild ...

The key thing is as you approach the stall, espacially if the stab is 'blanked' unless you have inherent stability in the wing,its going to go into a deep stall and stay there, and either mush out, or worse still go into a flat spin or equivalent. Major power applications may help if the slipstream can reach the stab, but maybe not either.

The key is noeweight, noseweight and noseweight. Regard them as flying wings, with a little bit of a tail shoved out the back.

If in doubt bult balsa chuck glider and fly till its stable, then duplicate CG position, won't be perfect on flat plate wing, but won't be far off.

Washout, from my rather linited experience, sounds OK.

[hul]

is this valid?
small tailplane gives small useable CG range (20...25%)?
big tailplane gives more useable CG range (15...40%)?

Hans

[Sparky Paul]

is this valid?
small tailplane gives small useable CG range (20...25%)?
big tailplane gives more useable CG range (15...40%)?

Hans
.
It depends on the location of the horizontal more than the area.
A small tail way far back can be as stable as a large horizontal up close.
It's just more of the compromises that are typical of aerodynamics.

[Sail 'n Soar]

Hey, Sparky, why don't you expand on your comment by takling about tail volume and static stability.

In terms of washout, I would put in at least scale wash-out for any full scale aircraft with tapered wings.

[DICKEYBIRD]

is this valid?
small tailplane gives small useable CG range (20...25%)?
big tailplane gives more useable CG range (15...40%)?Take a look at http://www.aalmps.com/freestuff.htm 2nd row from the bottom, 2nd across: "Tail Volume" It's a simple method for us non-engineer types to establish how effective the tail is at controlling that nasty 'ol wing and where to put the little CG symbol. I take a whole sheet of sticky-back CG symbols with me on maiden-flight days so I can easily adjust the CG. ;) :)

The Tail Volume Coefficient tool mentioned above is on a FF rubber model site but has worked well for me in the past on home-designed r/c (and FF) planes. Bolting on a chunk of lead in the nose works OK but a little bit of math can go a long way towards making a happy model.

[Sparky Paul]

Hey, Sparky, why don't you expand on your comment by takling about tail volume and static stability.

In terms of washout, I would put in at least scale wash-out for any full scale aircraft with tapered wings.
.
Tail volume on a scale plane is a moot point.
Unless you cheat.
Unless the plane is a bipe or a Cub style, with a lot of wing area, a scale plane tends to be much heavier for the size than anyone would make a sporty plane.
Therefore finessing things like washout is a secondary feature, with keeping the speed up at all times, due to the wing loading being primary.
And choosing the plane carefully.
P-38s, DH-88s, Mosquitos... anything with a lot of wing taper... NO!
Most planes of this type have poorly designed wings in the first place, and rarely survive long.
Wing loading is the most important feature for a scale plane.
It must be as low as practical, AND the plane can't be flown like a Kadet.
There's a guy in the club that builds good looking scale planes.. a LOT of planes, because he's such a poor flier, they seldom last 5 flights.

[DICKEYBIRD]

.
Tail volume on a scale plane is a moot point.I think computing the the tail volume is important when trying to figger out where to put the CG, heavy scale plane or not. The simple formula given on the site uses the T.V.C. to help establish a reasonably safe CG point.

[hul]

Take a look at http://www.aalmps.com/freestuff.htmIt's a simple method for us non-engineer types to establish how effective the tail is at controlling that nasty 'ol wing and where to put the little CG symbol.

this method gives me about 37% for that FW190A8. Sounds pretty far back to me. I assume that is percentage of MAC for tapered wings?

Choice of airfoil doesn't change this?

Some of those old free flight models had lifting (non-symmetrical) tails. I thought that was why their CG was way back.

really appreciate your input

Hans

[Sparky Paul]

37% is unflyable!
Try 25%.
Then work it back, cautiously.

[Sail 'n Soar]

.
Tail volume on a scale plane is a moot point.
Unless you cheat.
Unless the plane is a bipe or a Cub style, with a lot of wing area, a scale plane tends to be much heavier for the size than anyone would make a sporty plane.
Therefore finessing things like washout is a secondary feature, with keeping the speed up at all times, due to the wing loading being primary.
And choosing the plane carefully.
P-38s, DH-88s, Mosquitos... anything with a lot of wing taper... NO!
Most planes of this type have poorly designed wings in the first place, and rarely survive long.
Wing loading is the most important feature for a scale plane.
It must be as low as practical, AND the plane can't be flown like a Kadet.
There's a guy in the club that builds good looking scale planes.. a LOT of planes, because he's such a poor flier, they seldom last 5 flights.

But Sparky, you were writing earlier about the length of the tail being more important than the area. Now you know that isn't really the case. It is the product of tail length and tail area. And that leads directly to tail volume. If you calculate the tail volume divided by the product of mean wing cord and wing area of full scale planes, that static stability measure is very often something like .3, while I at least have found values of .4 and higher resulting in more comfortably stable models. Other posts have also suggested sticking with values between .4 and .5. And that leads to the necessity of either increasing the tail length and/or area, or moving the CG ahead of the quarter cord to achieve the same effect.

As to wash-out, unless you are planning to do significant lower speed flying inverted - now there is where you need to keep your speed up - if you run the numbers or use some of the free wing design sites on the web, you'll see very little, if any, negative impact of the washout in terms of induced drag or decreased lift (may actuall improve things), while you will see very positive changes in where the wing stalls first. I repeat, stick with the full scale wash-out at least, or even increase it from there and you will have fewer surprises when you accidently get a little slow. I guess another way to look at it is that keeping your speed up to avoid the down side of a poorly designed wing means you are carrying aroung more wing than should be necessary.

[Sail 'n Soar]

On the other hand, there are loads of full scale aircraft, including the WWII examples Sparky mentioned, that have highly tapered wings. The amount of washout required to resolve tip stall and spin problems with these aircraft is quite extreme. As Sparky indicated, keeping the speed up and the wing loading down is the best and most reliable approach for such models. Of course, there are many other aircraft to model with more modeling friendly proportions, as reflected in the abundance of models of these aircraft kitted and flying.

[Sparky Paul]

"Tail volume on a scale plane is a moot point."
If the plane is SCALE, you can't change that!
It's fixed by the design.
If you want to "standoff" the thing, then changes in moments and areas are done, and should be done all the time.
The Dynaflight P-51 is one of the best flying examples of a P-51 around.
Looks scale, but has reasonable changes to make it fly quite well.
Wing is different, horizontal is larger...

[DICKEYBIRD]

this method gives me about 37% for that FW190A8. Sounds pretty far back to me.Yup, that sounds too far back to me as well....maybe re-check your sums?? I've never had any trouble with the formulas but who knows, maybe it's wrong in this case. These other fellers are more experienced than I so maybe you should go with what they say.

There's also a thread about an Excel file floating around this site somewhere that does this kind of work.

[hul]

....maybe re-check your sums??

good idea....

the correct number is 32% :o

Hans

[Salto]

....If you calculate the tail volume divided by the product of mean wing cord and wing area of full scale planes, that static stability measure is very often something like .3, while I at least have found values of .4 and higher resulting in more comfortably stable models. Other posts have also suggested sticking with values between .4 and .5. And that leads to the necessity of either increasing the tail length and/or area, or moving the CG ahead of the quarter cord to achieve the same effect.....


Sail 'n Soar,
Slightly off topic, but reading your comments here got me thinking about another factor to consider when determining how a model flies apart from it's pitch stability.

My experience is that simply moving the C/G ahead of the quarter chord does not as you suggest above achieve the same effect on flyability as having a generous tail volume co-eficient. The pitch damping also has an impact on how a model handles in the air, and tail volume co-efficient seems to be a stronger measure of pitch damping than it is of static stability (which is largely determined by relative position of C/G and neutral point).

I've built plank style flying wings that never got comfortable to fly, even with the C/G well ahead of the neutral point.

I wonder if this means that a canard design with equal wing and canard areas has stronger pitch damping than other layouts, because this design would have a very high tail volume co-efficient.


Graham.