Hi everyone My first post dont ya know.

I have read the laws that apply to flight and it talks about the air that travels over the wing creates lift because it has to travel farther than the wind under said wing.

Having said this I have been looking to graduate to my second plane and have noticed that the more sporty the planes description reads {"beginner stay away, not for beginner, If youre a beginners and buy this plane theres something wrong with you"} the more the wings are either semi or fully symmetrical.
now to the question. If the air traveling over as well as under a fully symmetrical wing the air is traveling the same distance, would this not neutralize any pressure differences, if so where does your lift come from? Or am I missing something very obvious?

Thanx for your time

I don't think I can use big enough words to answer this one........lol...............John

It's a good question and the simple answer is that in straight and level flight the wing flies at a very slight angle of attack. This gives a slightly longer distance over the top of the wing to produce the lift. When flying inverted it still flies with the same slight (positive) angle of attack. This angle is so small it's not even noticeable (unless you've got a REALLY heavy model )

Hi Ace,

Airfoil shape is only one way a wing makes lift. The other way is thru angle of attack.

It is as you think, in the case of a symetrical airfoil, you have no potential for lift if there is no attack angle.

The flat bottomed wings are mostly for beginners, slow fliers or gliders. because they allow the plane to generate enough lift at lower speeds and angle of attack than the symetrical ones. I think it was bernouly who made some very famous equations regarding relationships of presure and velocity in a fluid. For an airfoil these equations were used based on an assumption that a two particles separated by a leading edge would end up at the trailing edge at the same time so the top one had to travel faster and thus these particles would be father apart so pressure would drop. turns out this assumption was not quite right. Suprizingly the particle on top actually makes it to the trialing edge before the bottom one would, so Bernouly effect is even greater than the equations would predict.

Anyway the point is, the flat bottomed wing is great when you want extra lifting, but it kinda sucks for inverted flight, outside loops.... the semetrical airfoil is great for aerobatics. once you have gotten the hang of a trainer, don't feel that you have to wait a long time to try a symetrical airfoil. For most planes like Sticks and a long list of other great symetrical airfoiled planes the diference in take off and landing and basic cruising is pretty small. All of my planes are symetrical, and i wouldn't have it any other way. You will love it.

I know Im just asking for someone more airfoil learned than I to jump on my face......but dont write off the flat bottomed airfoil (almost said Clark Y) too quickly for performance.My 88" Lazy Ace Bipe will perform all basic aerobatics in a box the size of our field and is neatral when inverted,although I admit it does not tuck under well in outside maneuvers.As far as full size goes.....can you say Bucker Jungmeister?....maybe the best aerobatic bipe of all time?

A symmetrical wing is better for inverted flight but there are other factors which can over-ride this. In full size I usually fly aerobatics in a Robin 2160. This has a symmetrical wing. Sometimes I am lucky enough to fly a friend's Yak52 which has a flat bottomed wing. Yet the Yak is by far the better handling plane when inverted. This is due to balance points, stability etc, the Robin being designed for mass market private fliers whereas the Yak is designed for the strictly controlled and selected world of military fliers.

Although some previous posts have mentioned that the upper surface is longer than the lower surface even for a symmetrical wing with an angle of attack, this has nothing to do with lift. Air molecules which separate at the leading edge do not meet again at the trailing edge, there is no need for them to alter speed so that they meet up again. Unfortunately this story is still often printed in books and model magazines.

There are pressure differences around a wing and these are related to the fact that the flow is turned in a different direction. Changing direction is a velocity change. Bernoulli's law states that the sum of the energies of velocity and pressure must remain the same if no energy is added or removed, so if velocity changes then pressure must change in the opposite way.
NASA's Glenn Research Centre kindly puts all this on the web for you, see www.grc.nasa.gov/WWW/K-12/airplane/right2.html keep following the links and you will also find the pages where NASA explains why a wing is not a venturi. By turning the flow downwards to get downwash, we have a change of velocity which Bernoulli tells us must go hand in hand with a pressure change.

Before anyone jumps on this as a Bernoulli versus Newton thing, you should be aware that Bernoulli's Law is a purely Newtonian law! Newton gave us the general laws about the conservation of mechanical energy, Bernoulli's Law is simply a specific case of this law as it applies to a gas's mechanical energy. Bernoulli versus Newton is therefore impossible! Anyway, the paragraph above tells you that both downwash and Bernoulli go together.

Bernoulli is usually misunderstood to have said that in a venturi, the speed rises and causes a pressure drop. He said no such thing. For the speed to rise, a force must be applied. If no energy is being added or removed, where did the force come from? The pressure drop of course. So the pressure drop caused the speed to rise. But why did the pressure drop? Because the speed rose! And round and round it goes. Neither is the cause of the other, both of them are co-incident effects of a venturi and Bernoulli simply gave us the equation that relates the two effects. It's like engine noise and exhaust smoke, neither is the cause of the other, they are both effects of something else, yet there is a relationship between volume of noise and the amount of smoke. More noise goes hand in hand with more smoke but we don't say that noise causes the smoke.

Harry

In our engines the force comes from the piston rising and causing a pressure drop below the carby. So there's no roundy roundy stuff

This is a subject that I have debated with many people smarter than me (IE physics professors at M.I.T.) and we agree on several points.
1. If Bernoulli's principal has anything to do with creating lift, it is minimal at best. If you disagree with this, then explain the following... The wing shape that gives the greatest amount of lift is the under-cambered wing. In an under-cambered wing, the air has the same distance to travel on both top and bottom. Therefore (if you subscribe to the Bernoulli thing) an under-cambered wing produces no lift!
2. When the wing passes through the air (not vise versa) on a flat bottom, or under cambered wing, the shape of the wing forces air to leave the wing in a downward flow, and as we all know, "for every action, there is an equal and opposite reaction". (So much for not comparing Newton to Bernoulli)

Now getting back to W.A.'s original question, YES! You're right! The pressure IS neutralized! And, as was mentioned before, the lift is generated purely by angle of attack. By tilting the wing slightly you are directing the flow of air downward thereby causing the wing (and anything attached to it) to go up. it is the same thing you experience when you stick your hand out the window of a moving car and twist your wrist, your hand goes up or down.

So why are the "Sporty" models using fully symmetrical airfoils? Mainly for speed. If, for example, you want to do a "Split 'S'" (From straight and level flight, roll halfway to inverted, then add "up" elevator causing the plane to come down in a half loop and pulling out right side up going 180 degrees from your start heading) on the downside of the Split S, you pick up airspeed, now, when you try to pull out, a flat bottom wing going so fast will have a tendency to "Balloon" at the bottom causing a straight and level pull-out to be very difficult to maintain. With a fully symmetrical wing, the extra speed results in very little extra lift.

While downwash is the creator of lift, it needs a mechanism by which it transmits its reaction to the wing surface. This is done by pressure of air molecules. Since the change of velocity incurred by turning the free stream downwards will alter the pressure, the Bernoulli equation can be used at any point to calculate the pressure distribution. Bernoulli's law is absolutely correct at any point on the surface, where it goes wrong is when people use it as an explanation of lift with the wing somehow being one side of a venturi tube!

It is not true that flat bottom wings zoom with extra speed. The YAk52 full size that I fly has a flat bottom wing yet is considerably better at aeros than the symmetrical wing Robin 2160. The Robin zooms immediately on an increase in speed whereas the Yak is much more neutrally stable. And that is the crucial word - stability. Stability is all about tail volume ratios and static margins, the wing section has no variable in the equations used. You are used to flat bottom models zooming because it is used mostly on trainers and they have a lot of stability. It is the stability that causes the zoom, not the section. Symmetrical foils tend to have blunter noses so have less sharp stalls, and they perform as well either way up, that is why they are popular.

Harry

I would like to see a cross section of that Yak wing.

Sorry can't help, I have no close up photos, just me posing at the cockpit. However there are quite a few of this ex-USSR aircraft in the West. There may be one at your nearest airfield you can look at.

The formula for calculating a CG position and hence the degree of pitch stability is

0.25-staticmargin+(0.25*(aspectratio^0.25)*((tailarea*t ailmoment)/(wingarea*MAC)))

where static margin varies between 0.1 and 0.2 and describes the amount of pitch stability desired expressed as a relationship of the CG to the Neutral Point.

Pitch stability is what determines if a plane pulls the nose up as speed increases. Note that there is no term in the equation for the wing section or its moment pitch co-efficient. The wing section has no effect on stability. What matters is the tail volume ratio, the position of the neutral point and the CG in relation to the neutral point. The further forward the CG is then the greater the longitudinal dihedral required, and the greater the speed stability of the aircraft.

Harry

WOW
The longer I am in this sport the more I have come to realize that thers a lot of very interesting things to know and apply to the hobby, as well as other circumstances you run into in a daily travel.

thanx again and may the hair on your toes never fall off

The ace

As minnflyer suggested, just stick your hand out the car window and tilt it up and down and you'll feel exactly what's happening with a wing. The angle of attack simply creates a difference in pressure on the bottom and top of the wing. The same thing that allows a kite to fly. The airfoil makes a big difference in fine tuning the effect, but a straight board will fly if given enough speed and held at a slight positive attack angle.