are thin wings always less drag and faster?

[Joel K. Scholz]

I am in the process of designing a wing and want to utilize a low drag airfoil for speed. Any recomendations?estimate 90+ mph. Is there a reason not to use a reflexed airfoil?

[Cory]

Do you just want straight line speed, or do you want it to be fast and retain speed in turns? If the latter is the case, the PW51 would be a good choice.

[biber]

For energy retention in turns, AR is nothing less important, than a particular airfoil.
However, thin is less draggy than thick, in a nutshell.
But too thin doesn't lift well and doesn't support tight turns well.
Reflex does not hurt speed performance.

biber

[BMatthews]

For a flying wing you must have some reflex. Even with a symetrical section you get the reflex from the click or two of up trim you end up needing to make it fly in a stable manner. On airfoils with camber this becomes even more important. The more camber and the further back the camber the more reflex you need in the camber line near the trailing edge.

For any sort of high speed model a symetrical section or something with up to perhaps 1.2 to 1.5 % camber is fine. Thickness would be determined on something this wide as much by how strong it needs to be as the aerodynamic issues.

[macboffin]

There is an alternative to reflex ; washout at the tips, ie the wings have a twist in so that the tips are at less angle of attack than the roots. The greater the sweepback, the less washout needed.If you use washout, the actual section can be Clark "Y" or virtually any other section, symmetrical or cambered. The example below has two dgrees washout at tips.

[MCarlton]

I can see a small hypothetical problem with using washout instead of reflex, but I don't know if it is a problem in practice. Given that the angle of attack is less at the tips, if you fly the model in a straight line at full throttle (given the original poster was talking about a speed model) the AOA of the tips may actually become negative, thus imparting a big twisting force on the wing, as the tips may reach the "zero lift" angle whilst the rest of the wing is still lifting. I don't know if this is the case in practice, but I once had a small flying wing slope soarer which I built too light and in a dive, the wing began to flutter at about mid span, the movement being a rapid vibration of the trailing edge at the tips. I put this down to this phenomenon and my poor structural design not being capable of handling the forces generated.

On the thin section being faster, it isn't always the case, as has been stated, a thin section will start to get very draggy in tight turns, meaning that even if you fly "speed runs" across the field, you will be scrubbing off a lot of your speed at each end, meaning the model has to use energy accelerating again once the turn is complete. Using a slightly thicker more cambered section will allow you to retain that energy in a turn better, meaning your next speed dash across the patch will start from a faster beginning. Think of it like a racing driver, if you drive hell for leather into a turn and then have to slam on the brakes at the apex, by the time you get out of the turn again, you'll be going slower than the guy who came into the corner slower but retained his speed through it.



I stand by to be corrected on this though :)

Matt

[BMatthews]

No, you're quite right Matt. I recall seeing some info about washout used on full scale aircraft. In the case of commercial aircraft the washout is typically set such that the tips are almost flying at a Cl = 0 for drag reduction issues. However this only works at one particular flying speed. Or more accurately a very limited speed range.

And when you see the extremely small angles of attack that a truly high speed model flys at you'll see that almost any washout will see the tips working at a negative angle of attack.

EDIT- just hit up Foilsim to try it out. Granted it's a rather crude tool since it is somewhat limited but it seems accurate for examples such as this. A 5x1 foot wing flying at 85 mph with an airfoil that is 6% thick and 0% camber (symetrical) develops 5lbs of lift (to match the weight of our imaginary model) with an angle of attack of only 0.48 degrees and a Cl of only 0.055. Obviously if the wing was washed out even by only 1 degree in this case the tips would be flying with a negative 0.52 angle of attack.

So I'm going to agree that reflex in the form of some uptrim in the elevons is all that this model will need to fly in a stable manner over the speed range.

For other designs where the speed range is more limited and the sweep angle more noticable then yes, I can sure see using washout twist but I don't think it's the best choice in this case.

[vintage1]

Just a thought..if you make MOST of the reflex on the elevons, you will naturally reduce that as it gets to top speed..

I have a picojet, and that turns on a fime and has greatt speed retention. Thick wing.Not great on top speed tho.

[MCarlton]

Here's an evolution of that thought V1, how about a variable camber reflex section? Build a wing with a reflex section which is hinged around the 50% chord line, but which also has elevons hinged on the 80% chord line. Now, mix the "camber" function to the elevons so that as the elevons go up, the rear portion of the wing goes down, thus, at neutral you have a thin, low camber section for straight line speed, but when cranked into a turn, you have more lift to keep the speed through the turn.

[Cory]

Like I hinted at back in post #2, use a Dynamic Soaring airfoil. It will be capable of higher speeds than you will probably fly this plane, and will not bleed of too much speed in the turns.

[vintage1]

Here's an evolution of that thought V1, how about a variable camber reflex section? Build a wing with a reflex section which is hinged around the 50% chord line, but which also has elevons hinged on the 80% chord line. Now, mix the "camber" function to the elevons so that as the elevons go up, the rear portion of the wing goes down, thus, at neutral you have a thin, low camber section for straight line speed, but when cranked into a turn, you have more lift to keep the speed through the turn.

I think my brain just stalled on that one, and fell out of the intellectual race. :D

It SOUNDS bloody clever..a bit like using flaps to make a sharp turn..on a conventional aircraft..or the way the F1 cars 'bend' their wings a bit flatter at speed, to reduce drag..

[BMatthews]

MC, you've got a decent idea but it doesn't need to be that fancy.

Look at jet fighters. They typically use variable leading edge segments in conjunction with the ailerons. For example the F18 droops the flaps and ailerons as well as the leading edge to create more camber when the pilot pulls back on the stick sharply for a high G pitching maneuver.

In the case of a flying wing you want the camber adding leading edge to be further forward. Like perhaps the first 10 to 15% of the wing. When you add up elevon the leading edge would droop down to add camber.

But in actual flying the complications of structure and possible flutter outweigh the lift advantages, if any, and it's easier to just stick to something simple.

[Brandano]

There's also the old automatic slats, like on the ME 262. They are a tad complex to design, but if made properly they will open at high AOA thanks to air pressure differences between the top and the bottom of the wing, and delay the stall. Pretty much like having a higher camber on the wing, though at the price of increased drag in turns.
Incidentally, the shape of the wing reminds me of the Avro Vulcan, which had severely drooped leading edges on the outer wing portions, a modification from the original pure delta design for improved handling.

[BMatthews]

Very fussy to fabricate and extremely fussy to keep working evenly. In WW2 many of the German squadrons flying ME109's had their mechanics lock them closed because it was near impossible to make them both react at the exact same moment. Having one snap open before the other led to snap rolling out of high G turns while chasing the enemy.

I seem to recall that F-86 sabers in Korea had much the same problem but I don't remember reading what the outcome of that one was.