Thickening airfoil to reduce stall speed?

[astroboy]

Is there anything close to a rule of thumb to estimate how much stall speed is reduced by thickening the airfoil? I played around with it a bit in Motocalc, but their results tend to be rounded off, so it was a little hard to tell.

I'd like to lower the stall speed of the model I'm about to start; below is a section of the wing, and a version where the vertical dimension has been scaled up 25%(which would change the maximum thickness from 10% to 12.5% of the chord). It would also facilitate getting the aileron linkage inside the wing. If it wouldn't have a significant effect--say, at least 5% reduction in stall speed--or if it's likely to introduce other aerodynamic problems, then I won't do it. Forgive my laziness, but I really don't want to get into a whole wing-design thing either, although it wouldn't surprise me if that's what it took to get a meaningful answer.

BTW, for most of its span, the wing is constant chord and constant thickness; the tips are more or less elliptical(it's a P-26).

Thanks,
Jeff

[Ollie]

It's my guess that a 25% increase in the thickness of the airfoil shown will probably give you the desired reduction in stall speed.

There are at least three other things you could do to reduce the stall speed by 5%.

1. You could reduce the wing loading by 10.25% by either increasing the wing area or reducing the gross weight of the model or a combination of the two.
2. You could increase the airfoil mean line camber by about 1%.
3. You could install flaps.

[Viper Pilot]

There are four factors determining the stall speed of our airplane:

1) Weight
2) Maximum Coefficient of Lift
3) Wing Area
4) Air Density

1) As weight increases so does the stall speed. We probably already could have guessed that, but now we can see that the relationship is between stall speed and the square root of the Weight. Thus, if the airplane weighs twice as much the stall speed will increase by the square root of two (1.41).

2) A higher Maximum Coefficient of Lift will result in a Lower stall speed. We can see why designers like wings with high Max. Lift Coefficients.

The more camber the airfoil has the lower the zero lift angle of attack will be. Symmetric airfoils will have a zero lift angle of attack of zero.

Increasing camber increases the Coefficient of Maximum Lift.

The curve then rises in a straight line, at a slope of approximately 0.1 per degree regardless of the airfoils shape.

At some angle of attack the linear rise in Coefficient of Lift begins to break down. This angle is greater for thick airfoils than for thin airfoils.

3) A larger wing is one of the easiest ways to give an airplane a lower stall speed.

4) The stall speed will increase as the Air Density decreases. In other words, stall speed will increase as altitude increases.


Hope this helps

[Dick Huang]

Jeff,
Both Ollie and Viper Pilot are giving you good advice and options for reducing stall speed. The problem is that there are very little aerodynamic data available at the Reynolds number of your model for Clmax. Don't use the old variable density tunnel data (circa 1936) for Clmax at 100k Rn; the data is not very good below 1,000,000 Rn. I know this from working at Chance Vought for 38 years; Back in 1962, I used the 21% thick NACA 2421 airfoil and the published Clmax-it did not reduce the landing speed but it did decrease the max speed. There may be data available now that will help solve your problem; if not try reducing weight and adding flaps that will increase the wing area or items that will decrease the wing loading.
Dick Huang:D

[Vette]

Ollie

What effect do you think a lifting stab has on Stall speed if any or what does it affect anyway?
I am thinking of a light electric photo plane with a fairly large main
wing area. One of the main objectives of this design will be to be
able to land in a small area so as to open up more photo flight
opportunities. The model will have spoilers and flaps operated together.

Vette

[Ollie]

If the stab is big enough and the CG is far enough aft, the stab will produce positive rather than negative lift at the trimmed airspeed. The coefficient of lift produced by the stab times the stab area divided by the wing area is added to the coefficient of lift of the wing to determine the total coefficient of lift. The airspeed, in level flight without acceleration, is inversely proportional to the square root of the total lift coefficient. If the airplane is trimmed to fly just slightly below stall speed. The wing will be operating near its maximum lift coefficient.The airfoil selected for the stab will determine whether it is stalled ot not at its angle of attack as determined by the pitch attitude of the plane and the decalage. If the stab isn't stalled then steady flight can be maintained with the wing near its maximum lift coefficient. How much lift is produced by the stab is determined by its area, airspeed and angle of attack. Drag considerations aside, any stab airfoil will do as long as it isn't stalled and the decalage is adjusted to allow for its zero lift angle of attack to meet the trim conditions for the airspeed. The necessary decalage also depends on the CG location. This also assumes that there is no vertical component to the thrust vector, the thrust vector goes through the CG, the thrust vector is along the same line as the drag vector and the CG is ahead of the neutral point.

[green66]

I'll have to take exception to the general position here; maybe I'm not seeing everything in proper context.

Increasing airfoil thickness increases minimum Cd, and should have no effect on minimum stall speed. Similarly, the drag of an aircraft or any part of the aircraft also has no bearing on min. stall speed.

Consider the basic lift equation:

At equilibrium, Weight = Lift = 1/2 X air density X air speed squared X lift coefficient X wing area

An aircraft must fly at whatever speed is required to produce lift equal to its weight, regardless of the drag present - anything slower, and it falls out of the sky. Rearranging the lift equation, and assuming constant atmospheric conditions, the only things that affect min. stall speed are lift coefficient and wing loading or, practically speaking: camber, weight, and wing area. Drag is absent from the picture.

Higher drag has effect only at speeds well above stall, in the form of reduced power margin at a given speed or, for sailplanes, higher drag steepens the best glide angle (due to reduced L/D), and only reduces the speed of an out-of-trim (shallow-dive) "penetrating" glide.

For the general case (no boundary layer mod's), it seems that a thickened airfoil (assuming camber unchanged) would tend to increase min. stall speed due to the tendency for earlier flow separation and reduced Cl max, especially at low Reynolds numbers.

[Ollie]

Thickening an airfoil can, in some cases, increase the maximum lift coefficient, even at model reynolds numbers.

Case in point:
Summary of Low-Speed Airfoil Data, Vol. 2 by M. Selig et al. gives measurements of two thinned versions of the M6 airfoil. One was 65% of the thickness of the M6 and the other was 85% of the thickness of the M6. At a reynolds number of 100,000 the thinner of the two stalled at a lift coefficient of 0.9 and the thicker airfoil stalled at a lift coefficient of 1.0.

Even so, I retract my guess about the affect of thickening the airfoil in question. The type construction shown in the diagram results in covering sag between the ribs. With the thicker airfoil, the covering sag will be greater because of the increased curvature of the thicker airfoil. This covering sag will reduce the mean line camber of the thicker airfoils averaged spanwise compared to the thinner airfoils' average camber. The reduction in camber will offset any increase in maximum coefficient of lift due to thickness.

This sugests another way of lowering the stall speed. If the original airfoil is sheeted back to the spar on top, the average camber will be increases a little and the stall may be delayed a little by moving the bump at the leading edge, caused by covering sag, aft and reducing it. The weight of the leading edge sheeting should be offset by weight savings elsewhere.

[Arizona Chuck]

I don't know about all the fancy stuff, I only know what works for me.
If you want a low stall speed you need a low wing load. All things being equal, a thinner wing will fly faster and still slow up with a light wing load.
A thicker wing will lift a little better but will slow the top speed most.
An under camber is just built in flaps, lot of lift, slow top end.
If the wing is not sheeted, I like to put a stringer about 1/2" back from the leading edge. If I'm really working on the weight I use a string at the 1/2" and another one half way to the spar.
The bottom line is, if you want lift at slow speed, build light. The smaller the plane, the lighter the wing load has to be.
Here is a 3D plane that will slow up great.

[Viper Pilot]

Originally posted by Arizona Chuck
[B . . . .The bottom line is, if you want lift at slow speed, build light. . . .[/B]

or increase wing area.

Either, or both, will solve the problem.

VP

[Sparky Paul]

NACA disagrees with most of the above! :)
A representative family, the 24xx series... loses max Cl and alpha as it thickens..
Profiles between these extreme show the trend..

[Ollie]

Browsing through Abbott and Von Doenhoff tells me that the conclusion that thickness affects maximum lift coefficient one way or the other depends on which family of airfoils and range of thicknesses you choose as representative. For low (zero or one percent) cambered NACA sections an increase in thickness up to about 12% are acompanied by increases in maximum lift coefficient. For families of NACA airfoils with greater camber and thickness, increases in thickness are acompanied by decreases in maximum lift coefficient.

The examples originally pictured appear to be similar to NACA2410 and NACA2414. Therefore, Paul's conclusion may apply to this case.

[astroboy]

Originally posted by Ollie
Thickening an airfoil can, in some cases, increase the maximum lift coefficient, even at model reynolds numbers.

Even so, I retract my guess about the affect of thickening the airfoil in question. The type construction shown in the diagram results in covering sag between the ribs. With the thicker airfoil, the covering sag will be greater because of the increased curvature of the thicker airfoil. This covering sag will reduce the mean line camber of the thicker airfoils averaged spanwise compared to the thinner airfoils' average camber. The reduction in camber will offset any increase in maximum coefficient of lift due to thickness.

This sugests another way of lowering the stall speed. If the original airfoil is sheeted back to the spar on top....

Actually, I was planning to sheet the bottom surface back to the spar, and the top surface another 20% or so past the spar, but mainly for appearance. I don't like seeing sagging covering and ribs. If it helps the stall speed, so much the better.

It's sounding like thickening the airfoil may not be such a good idea, which is too bad because I'll likely have to move the aileron linkages to the exterior of the wing. Ollie, as to your other original recommendations: Since it's a scale model, I'm reluctant to fiddle much with the wing area, though I am trying to keep the weight down(see my new thread, on wing structure). Flaps would mean too many servos for the BEC--meaning more weight gain. Is it possible, though, to "increase the airfoil mean line camber by about 1%"(whatever THAT means) without completely redesigning the wing?

Thanks,
Jeff

[green66]

Browsing through Abbott and Von Doenhoff tells me that the conclusion that thickness affects maximum lift coefficient one way or the other depends on which family of airfoils and range of thicknesses you choose as representative. Yes, agreed - It appears that a broad-brush statement regarding the effect of a thickened airfoil on max Cl can't be made. From "Radio Control Soaring," 2nd Ed, 1975: "There is a common belief that thick sections "give more lift." This is not supported by the masses of full-scale evidence, though it is true if one considers the range of low thicknesses from 6% to 12% [my emphasis]. Going to 15% offers no consistent advantage, and above this thickness the stall occurs at a lower Cl."

Astroboy - You should obtain a copy of the Profili - click here (http://www.profili2.com) airfoil analysis program. At just $10, the pricetag of this software is not at all representative of its capability. It is essential software for any modeler facing the task of airfoil selection or modification, and has received much praise from the r/c community and press alike.

If you're already using a recognized airfoil, chances are good it's in Profili's database of 2200+ pre-analyzed airfoils. If not, you could enter coordinates, if known, or simply pick a similar airfoil from the database and let Profili show the effect of thickening it on max Cl.

[vintage1]

My limited impression is that thick wings stall a little more viciously than thin ones, too.

I am tending towrads thinner wings now - they seem to have better speed range and more forgiving caharcteristics.

[Sparky Paul]

There's little information in NACA 824 on 1% cambered wings, but the data does support the idea that thicker is better for such shapes.
For 2% and thicker though, lift and alpha drop as thickness increases.
.
Fat wings are preferred for low-speed manuveable aiprlanes.. both fun flies and full-scale aerobats.