Drag buckets?

[Antonsoarer]

Drag buckets?

When I am looking airfoils in profili and I look at the CD-alpha curves I attempt to translate these into how a slope soarer would behave in turns.

Question-1:
Is it true to say that a wide low drag curve will allow me to pull tighter turns (high alpha) without slowing down the plane as much as a narrow CD-alpha curve would? This assumes all other factors are equal in terms of CL, aspect ratio, loading and model configuration (tail moments etc).

Question-2:
A thought on CL/CD curves: If the max CL/CD is at a higher alpha on airfoil B than airfoil A (but equal) and all other factors are equal what will this mean subjectively/inflight and turns?

Tony.

[Ollie]

In tight turns, the wing has to produce the most lift and therefore has to operate at a high lift coefficient and the associated high angle of attack. In level flight or a dive, at high speed, the wing operates at a low angle of attack and low lift coefficient.

How much to weigh the importance of the two contrasting operating conditions depends on how much time the plane spends in eash and how much energy is available from the lift conditions.

One strategy is to select a thin airfoil with a narrow low drag range that encompasses low lift coefficients with low mean line camber and extend the low drag range to higher lift coefficients by increasing mean line camber with flaps of small angular deflections and low drag. Similar requirements apply to thermal ships that must be efficient over a wide speed range to turn tightly in thermals and penetrate rapidly through sink between thermals. The main difference is the higher wing loading of slope than thermal types.

Dr. Drela's AG4- series of airfoils use a new technique that necks down the thickness of the airfoil a little at the hinge line. When operating at a high lift coefficient the flap is drooped a little to make the upper surface contour smooth to meet the critical flow conditions there. When operating at low lift coefficients, the flap is reflexed slightly to make the bottom contour smooth at the hinge line so that the critical flow on the bottom of the wing isn't disturbed. Turbulance and seperation are minimized over the whole range.

What you get for this refinement is two airfoils for the price of one. Every unique position of a flap produces a functionally unique airfoil.

If simplicity is desired so that flaps are ruled out, then a thicker airfoil with higher drag but a wider low drag bucket is the type of airfoil to choose.

If you can deside how tightly you want to turn versus haw fast you want to go at some wing loading then you can calculate the required range of coefficients of lift and choose airfoils the give you the best balance of low drag over that range of lift coefficients. The air speeds and wing chords allow you to calculate the reynolds numbers. Armed with the coefficients of lift and the reynolds numbers, you can search the available airfoil data for the lowest drag combinations in order to select the airfoil(s).

Start the design with light wing loading for low wind conditions and select the airfoils that operate best at the low end of the reynolds number range. When you do this you will get airfoils that also operate well at higher wing loadings and airspeeds. If you start the search at the high speed end of the range you may get airfoils that don't perform well at low speeds under light lift conditions.

[Antonsoarer]

Thanks Ollie, a comprehensive reply as usual. My intuitive visualisations seemed to have been correct. I have noticed that some thin airfoils like RG14 (popular with the led-sled guys) really don't keep there LD curve well at high Reynolds and even low alphas.

I have been developing airfoils from existing ones and have arrived at very low drag (despite being comparatively thick) and also consistent results from about 90k to well beyond 500k and with good inverted curves. I am sure many have dabbled in this area for some time. I have done it visually and by trial and error for the last 25 years, now with Profili and a clear set of requirements I think I can progress.

By blending, adjusting the thickness point and adjusting thickness I have managed to reduce the compromises and establish wide Reynolds windows, wide buckets and very low drag (as good a drag many of today's thin favourites). The closest airfoil I have seen to those I am currently tweaking would be an Epler-201. The intention is to use this new family of foils on large (72-108") low aspect ratio (1:6) slope-soarers in place of my usual E374 style sections.

I have had a lot of fun with these designs, the mass, the high Reynolds (maily due to large chords) and wide speed range translate to manoeuvres that don't bleed much energy and at times look impossible for an un-powered model especially when moving slowly (usually Epler-374-style @ 10-18oz sq-ft).

The foils I am also working on for planks don't seem to have any peers in the extensive database within Profili and share the same wide low drag bucket but with a neutral CM.

Field tests start in two weeks if I can get the test beds finished.

Tony.

[Ollie]

Don't forget to take the induced drag into account. The low aspect ratio wings that you mention will slow down a lot in the turns because the induced drag will dominate the drag budget in the turns no matter what airfoil you use. Most successful slope racers have aspect ratios of about 12 or higher in order to keep the induced drag down.

If the purpose of the model is aerobatics instead of slope racing then the design priorities are different and drag reduction is less important than some other things.

The absolute lowest drag that Prof. Selig and company have measured in the wind tunnel at model reynolds numbers was for the S6063 airfoil. The wind tunnel test model wasn't very accurate because it had a little trailing edge reflex which contributed to the low drag. Have a look at the S6060 through S6063. I think you will find them well suited to your purpose. The S6060 was designed to be an improvement on the E374.

[Antonsoarer]

I see what you mean about the 6060, very like the 374. I have already used the S8025 on a slightly swept tailess wing and it really moves.

The purpose of the model is mainly aerobatics. On the subject of aspect ratio and induced drag, does the tip chord influence induced drag?

If I were to build two wings of the same AR (say 1:10) but one was tapered to 50% and the other equal root and tip chords would there be any difference in the induced drag?

[Ollie]

See:
http://aero.stanford.edu/WingCalc.html
At a lift coefficient of 1.0, an aspect ratio 10 wing with no sweep or twist, comparing a 0.5 tapered wing and an untapered wing. The induced drag coeficient of the tapered wing is 0.032 and the untapered wing 0.034. That's about a 6% higher induced drag for the untapered wing.