Why do C-shaped airfoils, which are more parasail-like produce more lift? When a 747 drops its flaps (and the "flaps" on the front of the wing, I forget what they're called), it's doing this to increase lift so it can fly slower (i.e. for takeoff and landing). These flaps make the wing more c-shaped, like a parachute.

Is this added lift due to increased pressure underneath the wing, or further decreased pressure over the wing?

Daniel - the flat plate kite at low angles of attack (what ever that means, its all relative) is still acting like an airfoil and not so much like the particle reaction you are thinking about. I remember reading somewhere that the particle reaction would only account for a very smalll portion of the lift.

At angle of attack there is a forward stagnation point on the leading edge, higher velocities on the top and full lift vortex formation stuff like regular airfoils. Of course it merges into the full flat plate aerodynamics when approaching 90 degrees. It depends how hard the wind is blowing.

For a first approximation you can start looking at a single surface cambered airfoil design by following the patterns of the upper air flow of the flat plate.

I used to make what was called a French War Kite which would set on its tail on the ground. In a gentle breeze a light yank on the string would cause a VTOL and it would rise to fly almost overhead. Pretty neat to watch but not as neat as airplanes.

NebullaDDS - I need to get a little detailed about the pressures on the wing because you are right that the pressure on the bottom of the wing is pushing the wing up......... But there is more to it.

The lift on anything is the integral sum of the forces acting on it. Whether the forces are greater on the bottom of the wing or the top depends on the angle of attack of the wing.

At positive angles of attack the pressures are lower on the top of the wing than the bottom. The intergal of the the pressures over the upper and lower surfaces indicates the direction that the wing will go. In the sense that vacuum cannot suck (a small pun but important in the final analysis) but just removes some force over part of the wing so that the pressures over the other part of the wing can push the wing then there is a force on the bottom of the wing pushing up.

However as aerodynamics types we tend to look at the pressures on the wing relative to the freestream value. A typical wing airfoil that is working nicely will have the pressures under the wing close to the freestream value while the negative pressures above the wing will be significantly lower than the freestream value. Since those negative pressures are so dominating we tend to say that they are lifting the wing but in actuallity since vacuum can't exert a force the wing is responding to the differential pressure over it.

Yes all lift that is not reaction lift is caused by pressures under the wing but only because the pressures over the wing are removed by the wing. If you could increase the pressure under the wing without having to decrease the pressure over the wing it would indeed lift. Nature has made the phenomena work the other way.

Slats are the ones in front of the wing, flaps are in the back. The aero mechanism that are being emphasized are different. Slats change a standard airfoil into a cambered airfoil at the front. You get higher lift at lower angles of attack.

Flaps are a different horse in some ways depending on how much deflection is being used (kinda, but useful to thing about it that way) Small deflections are basically making a mild undercambered airfoil. They are good for maneuvering and some airplanes are designed with this in mind.

The result of the flap deflection is to cause a significant increase in lift (at the same speeds and angles of attack) because of the increase in downwash from the wing/flap, lowered pressures over the wing/flap and pressures under the wing/flap also go higher. A plot of pressures and velocities around a flap are very interesting. There are some big pressure bumps and dips aroun them. Along with the lift you get a bucket full of drag. Good for sticking a landing but not much else.

Of couse again, this phenomena can only create lift if it results in someting that in the final analysis creates a positive net integral of the pressures.

Propellers make us think that they are something special because we feel the big wind behind them. That is just the downwash from the airfoil of the blade, wings do the same thing but tend to spread it over the surface over which they are flying. Stand under a low flying large airplane (not a model) and you can feel the downwash as he goes over. With the propeller the downwash forms the propwash. However the propeller creates thrust on the motor shaft because of the integral of the pressures over the prop blade.

Again to restate the obvious the pressures under the wing may be higher than ambient on the bottom and lower than ambient on the top, the reverse of those or the same on both sides - the angle of attack determines this. Useful pressures are a function of finely tuned shapes but they all work the same.

Indeed, lift is generated by a net difference in pressures above and below the wing. Pressure under the wing must be greater than pressure above the wing. Let us call this "upward pressure", since it lifts the wing up. There's probably a technical term for it, but......My point is the following:

Whether you have "upward pressure" due to highly decreased pressure over the wing or due to highly increased pressure under the wing, if I understand correctly varies with airfoil and indeed angle of attack. (For this discussion, I will consider parachutes/parasails, kites, and wings all to be "airfoils".) With its slats and flaps down, the wing adopts a C-shape, which is a little more parasail-like. I guess what I'm saying is that the flaps and slats help to cup air under the wing. With the plane being thrust foreward, air is essentially being compressed under the wing (much the same way as air gets crammed into a parasail)thus increasing pressure below. Does the pressure above the wing change much with the extension of slats and flaps? I'm curious.

Let me ask you this:

Let us say that you know that an engine/prop combination has "X-lbs of thrust". If you were to put a scale right behind the engine/prop that has the same surface area as the prop, would you get a force reading near X-lbs? (assuming that air isn't being dispersed outside the borders of the scale)? I'm probably missing some minor points here like varition in thrust from the base of the prop blades to the tips, but try to ignore these if possible. I think you get my point.

If the answer to the above question is "no", then the wind you get behind the prop is purely downwash, which contributes little to foreward thrust.

If that is the case, then answer this for me: why do turbine-powered airplanes suffer from control lag when traveling slowly, whereas prop planes do not? I always thought that what kept prop planes maneuverable at low speeds was the fact that the prop was always blowing air forcefully over the control surfaces. With a turbine engine, the control surfaces don't have this luxury. Instead, the plane must be going fast in order for their controls to be significantly effective.

Ben- the example of the kite was for when it would by flying at somewhere around 60-80 degrees to the wind. At that high of AoA, there shouldn't be any real flow over the top surface, and any lift should be from particle reaction. Don't know why I said 'flat plate kite at low angle of attack', it was late.

Neb- As far as the propwash goes, that's a really good question. I'm not sure. Has anyone ever tried to measure the force of the propwash? But you are right, the propwash does act as an apparent wind over the control surfaces, just watch a stunt plane manuever while hovering.

Low speed handling is really determined by the wing loading. A turbine powered Learjet with small wings that weighs alot is going to be very sluggish at low speeds, whearas a light piston powered Cessna will have no problems. Of course, low speed is relative here; the bizjet's stall speed is probably right around the Cessna's max speed. But for full size aircraft in general, the propwash really doesn't help control.

------------ Whether you have "upward pressure" due to highly decreased pressure over the wing or due to highly increased pressure under the wing, if I understand correctly varies with airfoil and indeed angle of attack. (For this discussion, I will consider parachutes/parasails, kites, and wings all to be "airfoils".) With its slats and flaps down, the wing adopts a C-shape, which is a little more parasail-like. I guess what I'm saying is that the flaps and slats help to cup air under the wing. With the plane being thrust foreward, air is essentially being compressed under the wing (much the same way as air gets crammed into a parasail)thus increasing pressure below. Does the pressure above the wing change much with the extension of slats and flaps? I'm curious. -----------

Dear curious -- sorry about that couldn't help it.

It is a matter of emphasis. Long ago aero types discovered that the wings worked best if they tried to decrease the pressure on top rather than increase pressure on the bottom. Decreased pressure results from camber or angle of attack - easy to do. Increasing the pressure on the bottom is hard to do. Not impossible but hard. This is what the birds discovered some time ago. So since the pressure differential is coming from the greatly lowered pressure on the top of the wing it tends to get talked about the most.

Let's call the C shape, camber, which is the right term. With flaps and slats down a little the camber of the wing is increased. They don't cup air as much as they redirect the flow and cause a greatly increased negative pressure above the wing. So yes the wing pressure is lowered on the top. This gets more complex when the flaps are extremely deflected or when there are multilple slots in the flaps. The pressure distributions become complex and will greatly change but the whole idea is to create less pressure on the upper surfaces of the wing and greater pressure under the wing.

In the process of creating lift and actually occurring simultaneously with it is the creatiion of downwash. The more lift, whether coming from angle of attack increases or slats and flaps coming down (it doesn't matter), the more downwash. Lift and downwash are metric (that is items that can be measured) forces.

------------ Let me ask you this:

Let us say that you know that an engine/prop combination has "X-lbs of thrust". If you were to put a scale right behind the engine/prop that has the same surface area as the prop, would you get a force reading near X-lbs? (assuming that air isn't being dispersed outside the borders of the scale)? I'm probably missing some minor points here like varition in thrust from the base of the prop blades to the tips, but try to ignore these if possible. I think you get my point. --------------

There are losses in all processes but in general yes. You can think lift = downwash and be pretty accurate for most cases. The prop wash conditions are strongest just aft of the prop and decrease as they travel aft and the pressures and velocities tend to return to freestream conditions.

-------------- If the answer to the above question is "no", then the wind you get behind the prop is purely downwash, which contributes little to foreward thrust. --------------

As we noted lift and downwash are totally screwed together. With a wing you can't have one without the other. Propwash as a part of the act of creating lift with the prop blades and has a lot to do with the thrust.

--------------- If that is the case, then answer this for me: why do turbine-powered airplanes suffer from control lag when traveling slowly, whereas prop planes do not? I always thought that what kept prop planes maneuverable at low speeds was the fact that the prop was always blowing air forcefully over the control surfaces. With a turbine engine, the control surfaces don't have this luxury. Instead, the plane must be going fast in order for their controls to be significantly effective. ---------------

As Daniel noted it is not a function of how they are powered. Take the elevator for example. The non dimensionallzed control surface effectiveness of the elevator which is pitching moment as a function of elevator deflection is a function of only geometry and is the same whether it is a small wind tunnel model or a full size airplane. This is good for designers.

With that elevator effectiveness number, knowing the size of the airplane and the flight conditions desired then the actual moment, in foot pounds, on the airplane from the elevator deflection can be calculated.

For that size of airplane being considered there is a pitch moment of inertia. With pitch moment of inertia and moment input you can calculate pitching accelerations, then integrate for velocities and positions. Cange of angular position converts to lift, acceleration, etc. It is pretty standard stuff for all control surfaces and airplanes.

The amount of control response that the designers desire is based on specifications given to them in terms of maneuverability in load factor desired or landing speeds, etc. For example a passenger airliner does not need to fly like an Extra.

The forces out of the controls is then a function of the control and airplane geometry, size, and the flight conditions. The responses to those forces are a result of the moments of inertia of the airplane. It does not matter what the airplane is powered by --- Props, rockets, jets, or witch doctorery.

You are thinking about little sporty aerobatic airplanes and big not too sporty airliners. They fly the way they do because the designers were satisfying a set of specifications given them. That is all

Along the way you can do some neat maneuvers if you have enough prop or jet wash over the control surfaces. All this does is to increase the local dynamic pressure over the surface. Then it is working like a normal elevator with some pressure distribution changes.

You can think of the surface as deflecting the propwash but only if the propwash is hitting just one side of the surface and not the other. It rarely happens that way. The only airplanes that have true flow deflection are those like the Harrier that can redirect the jet exhaust.

When I'm typing in that tiny gray box these post don't look that long. I'm watching StarTrek on TV at the same time so there may be an oopsey or Warp 3 thrown in along the way.

"They don't cup air as much as they redirect the flow and cause a greatly increased negative pressure above the wing. So yes the wing pressure is lowered on the top. This gets more complex when the flaps are extremely deflected or when there are multilple slots in the flaps. The pressure distributions become complex and will greatly change but the whole idea is to create less pressure on the upper surfaces of the wing and greater pressure under the wing."

Which is it? Redirecting flow leading to negative pressure? Or creating less pressure over AND greater pressure under?

So tell me, if lift=downwash in a propeller, what's the difference between a propeller and a wing? Isn't a propeller blade a wing that's traveling in a circle? I think it is, but I could be wrong.

As far as turbine engine planes having control lag, I'm not speaking of lag due to tubbine planes having smaller control surfaces relative to the plane. You could take two P-51 Mustangs, one with a prop in the front, another with a turbine thrusting out the back, and the one with the prop would have better maneuverability at slow speeds because the prop is blowing air over the control surfaces. The control surfaces of the turbine-powered airplane, however, are dependent on the speed of the airplane to provide air passing over them at adequate speed for deflection. An F-18, whose elevators and rudders are well in front of the rear "nozzles", doesn't have jet wash going over its control surfaces. Does it?

If everything I've said above is correct, then we can come to the conclusion that a propeller, which is essentially a cambered wing, produces foreward motion by 1. increased pressure "underneath" the blade, and 2. wash.

Consider this as well: A propeller with a greater pitch is harder for the engine to spin at a given RPM than the same size propeller with a lesser pitch. But, when spun at the same RPMs, the higher-pitched prop will provide more thrust, regardless of the shape of the prop. Agreed? Similarly, an airfoil with a greater angle of attack is harder to push through the air than one with a lesser angle of attack, yet, if pushed through the air at the same speed, the greater angle of attack will produce more lift (until stall occurs, of course), REGARDLESS of the shape of the airfoil. Agreed?

So, would you say that I've successfully and accurately compared a propeller to a wing?

------------- Which is it? Redirecting flow leading to negative pressure? Or creating less pressure over AND greater pressure under? --------------

Both. All of the characteristics of the airfoil that we think about happen at the same time.

Think of the 3 blind men telling about the elephant by just touching one part of it. One touched the trunk and said the elephant was like a large snake, one the tail and said it was like a rope, one the leg and said it was like a tree. Don't think that way.

The rate of increase of negative pressure is probably greater than the rate of increase of positive pressures but that depends on a host of configuration variables and there are probably lots of exceptions.

---------- So tell me, if lift=downwash in a propeller, what's the difference between a propeller and a wing? Isn't a propeller blade a wing that's traveling in a circle? I think it is, but I could be wrong. -----------

Did I leave that out? There is no difference except that a particular airfoil is optimized for the intended purpose. A prop is a just a wing going in little circles, a lot like some of us. It becomes most obvious when you consider the ultra light indoor microfilm covered models. The rpm is about 30-40-50 or so and the props can easily be thought of as lifting the airplane forward rather than pushing. We tend to feel the propwash and think of that as pushing forward.

--------------- As far as turbine engine planes having control lag, I'm not speaking of lag due to tubbine planes having smaller control surfaces relative to the plane. You could take two P-51 Mustangs, one with a prop in the front, another with a turbine thrusting out the back, and the one with the prop would have better maneuverability at slow speeds because the prop is blowing air over the control surfaces. The control surfaces of the turbine-powered airplane, however, are dependent on the speed of the airplane to provide air passing over them at adequate speed for deflection. An F-18, whose elevators and rudders are well in front of the rear "nozzles", doesn't have jet wash going over its control surfaces. Does it? --------------

Turbine planes don't necessarily have smaller control surfaces than prop planes. The F-15, -16, -18 etc. all have all moving tails. It is hard to get bigger than that.

Toward the end of the last writing I mentioned that with jetwash or propwash the dynamic pressure over the surfaces increase and so does the control effectiveness. Blowing air over a surface is doing that, the effectiveness does increase. A prudent designer does not depend on that because the motor might quit and you would still have to control the airplane. With extreme control throws it becomes more like a true deflection. It is a gradual variation from one mode to another with deflection angle increases.

There is jet induced flow around the F-18 tail surfaces but the magnitude is pretty small. It is however strong enough that when we did low speed wind tunnel tests (I worked at McDonnell Douglas St. Louis) that duplicating the jet with a piped in flow of air scaled to give the proper mass flow characteristics of the engines is given a lot of attention and test time. When trying to put the airplane on a deck you take all the help you can get.

-------------- If everything I've said above is correct, then we can come to the conclusion that a propeller, which is essentially a cambered wing, produces foreward motion by 1. increased pressure "underneath" the blade, and 2. wash. -------------------

No, you are telling about that part of the elephant that you want to see.

The blades (which are airfoils) rotate, the pressure in front of the blade drops and at the same time the pressure underneath increases and at the same time propwash (downwash) is generated. All at the same time, all 3. Incidently the work put into the prop by the motor should be close to the acceleration given the air by the prop when including friction loses. The forces from the pressures are carried through to the prop hub which accelerates the airplane etc. But the main thing is that the action of negative pressure, positive pressure and downwash are all simultaneously happening. It is just the way a wing works. There are some nice math that points this out and says things like vortex and circulation and bound things but at the end of the math it says that it all happens at the same time.

-------------Consider this as well: A propeller with a greater pitch is harder for the engine to spin at a given RPM than the same size propeller with a lesser pitch. But, when spun at the same RPMs, the higher-pitched prop will provide more thrust, regardless of the shape of the prop. Agreed? Similarly, an airfoil with a greater angle of attack is harder to push through the air than one with a lesser angle of attack, yet, if pushed through the air at the same speed, the greater angle of attack will produce more lift (until stall occurs, of course), REGARDLESS of the shape of the airfoil. Agreed? ------------

Yes, this is a fairly reasonably fundamental fact of aerodynamics though. Be careful of the use of the statement, "REGARDLESS of the shape of the airfoil." If you stall the airfoil with too much angle of attack it will lose lift.

That reminds me, all the discussions have been about an airfoil/wing at an unstalled, in the linear range of lift curve slope, condition. Stalling is a good indication of the relative importance of upper vs lower pressures in wing design. When a wing stalls the lower pressures don't change (for most practical purposes), the upper negative pressures just die and down comes the airplane.

--------------So, would you say that I've successfully and accurately compared a propeller to a wing?-------------

Yes for the most part but no prizes. This has been known since the Wright Brothers (very smart fellows) put props on their airplanes.

A lot of good ideas thrown around, but I must comment on a couple.

(quote) For a stable airplane the center of lift, or more precisely the Neutral Point, is located at some place forward of the CG. It is indeed a function of the wing, body, tail, etc aerodynamics.

This in not correct! The center of lift must be behind the center of gravity.


(quote) Upwards pitch during flight comes from the fact that the CG is placed slightly back because of the stabilizer, which produces enough lift to prevent the plane from pointing straight up.

The horizontal stabilizer acts opposite the wing. With the c of l behind the cg the wing will lift and attempt to rotate around the cg (which is in from of the c of l) therefore w/o any counteraction would cause the trailing edge to pitch up. The Horizontal stabilizer counteracts this by acting in the opposite direction therefore “stabilizing” the airframe. It is a beautiful balancing act!!

I don't know how a 3 year old post got to the top of the list, but in case anyone else is still reading it, Model Airplane News publishes a book titled "Basics of R/C Model Aircraft Design" that is a must read for anyone interested.

I was an aero student a while back and what you've postulated just turned my degree to utter crap. Maybe now I could be a dentist by using pliers to pull teeth and full strength peroxide to whiten them.

b

Ye gads...I should have read the dates on this thread. It's old! Still through, after reading all of this, I really applaud Ben for his patience. When Nebula said the formulas were easy, I was gonna find an easy one for him but thought, why?

In any case, I think the good doctor should also look into the Coanda effect on airfoils. It's a different way of looking at aerodynamics and such. It's easier for non aero-types to understand are can really make a good stir.

B