how do airplanes fly?

Great Debates
41

I am not sure I even understand what you are saying. The air does not follow the inside of the sail? what does it do then? Go out for a beer?

It is not deflected? Of course it is!

How can a boat move forward when the air is blowing across the beam? I cannot even see the problem here. The force generated on the sail is not in the direction of the wind, it is at an angle between the bow and the beam. the forward component drive the boat and the perpendicular is counteracted by the keel. No keel = plenty leeway.

Not only can a boat sail across the wind, it can sail at an angle of less than 90 to the wind so it can effectively sail upwind.

I do not mean to pick on you but I think you have some homework to do to catch up with the general level of this thread.

One much more interesting question is why can a boat sail faster across the wind than dead downwind. Did you ever think of that? If that is obvious to you then you are beginning to understand what all this is about. Now it is so obvious to me and yet I remember when I thought it must be impossible…

42

I thought that was my question, so, yeah, I guess I did think of that. (I’m not sure where the “It is not deflected” comes from, though.)

43

I guess I did not understand your posting correctly. Anyway, let’s see if I can clarify. The air does not push directly and perpendicularly the surface of the sail as some may think. If you want to sail downwind and just put a barn door perpendicular to the direction of the wind, the effect would be dismal because it is stalled. For a sail to work the air has to flow along its surface on both sides, just like the wing of an airplane. The air is deflected creating a force against the sail.

Now… if I am sailing directly downwind, the fastest I could hope to go would be with the wind (I hope this is obvious)and this would be a theoretical, unreachable, limit.

Yet, If I am sailing across the wind, there is no reason why the 10 knot wind cannot give me a 15 knot speed across it. Think of a (triangular) narrow wedge of ice resting on its side on the kitchen counter against the wall. Now you push with a chopstick the ice against the wall and because of the wedge shape the ice will slide sideways. If the angle of the wedge is 45 then the ice will slide sideways the same distance you pushed the stick. If the angle is less than 45 then the ice will slide sideways more distance than you pushed the stick in. See? A sailboat is very similar.

Of course the conservation of energy and momentum still apply. The sail deflects a mass of air backwards (or sideways from the view of the wind) and generates a force in the opposite direction.

If you think of the above example but now point the chopstick at an angle you can see how a boat can sail at an angle of less than 90 to the wind and therefore sail upwind by tacking.

While sailing looks pretty complicated (and it is)the basic thing you are doing is trimming the sails to maintain a constant angle of attack no matter where the wind is coming from and other variables.

All this got me thinking:

The wing of an airplane, the rotor of a helicopter, the sail of a boat, the keel of a boat, the rudder of a boat, the blade of a propeller, the blade of a fan, the blade of a turbine… they all work under exactly the same principle: the fluid is displaced and the reaction creates a force on the solid object called “lift”. (Yes, even the force generated by the keel of a boat is called lift.) This force is always perpendicular to the surface and at an angle to the direction of travel of the fluid.

BTW, the keel of a boat is another good example of a symetrical object with equal lengths on both sides, which generates lift. The different length of travel cannot explain it either.

Any sailor can tell you a sail affects the air as it passes. A boat to windward will rob you of wind and you always want to be the one to windward.

The air speeds up as it goes by the sail but leaves the sail upwind of where it would be if there was no sail: It has been slowed down and deflected sideways (from the view from the wind).

Another thing many people have trouble understanding: When you are sailing, a freewheling propeller creates more drag than a locked one because the locked one is stalled while the turning one is creating drag.

44
      • Hmmmm . . . . .
      • A few days back there was a thread about “NPR Discovers That Bumblebees Can Fly!”. Some got a chuckle about how “scientists couldn’t figure out how bumblebees can fly”, and promtly stepped right in their own ignorance. Bumblebees can fly, but do not utilize aerodynamic lift to do so. That is the reason that they always must flap their wings to remain airborne. The original question (of the scientists) was not “How do they fly?”, but “If they fly, why haven’t they evolved wings that utilize aerodynamic lift?” — Dragonflies and Damselflies (smaller cousins) do create aerodynamic lift. The result is that they can fly farther, turn faster and stay airborne with much less effort than any other insect - they can even glide for considerable distances, something no other insect can do.
      • So you can achieve flight without aerodynamic lift, classic diamond kites do it all the time, but it’s not the most efficient way to do things. Sometimes that matters, sometimes not. Apparently with most insects, not.
      • And by the by, most ultralights do use battens to maintain airfoil shapes on the main wingspan: most every hang-glider I’ve seen, too. Ultralights commonly only use one surface, but that surface is the “upper” curved profile of the classic airfoil. The wings are not simply flat fabric, though the tail & rudder surfaces (if present) commonly are. - MC
45

MC, I know nothing about bees and I am not sure what your point is. I am not sure if what I am going to say supports or contradicts your point.

At this point of the proceedings I am quite certain that any object sustained in flight (be it airplane, helicopter, parachute, glider, insect, bat or pig)is held up by the reaction of air moving down. And furthermore the force (lift) pushing the body up is equal to the mass of air times the acceleration it was given. I think anything else would contradict the laws of physics.

I have come across some of those citations about not understanding how bees fly, that it would be impossible etc. I have no idea how bees fly but I doubt it has been proven their flight contradicts any of the laws of physics we know.

given that it is difficult enough to analyze how an airplane wing or propeller blade works, I think analyzing how a bee flies is way out of my league but I believe there is nothing to make us doubt the basic laws of conservation of energy, momentum etc.

Yes?

46

Sailboats? Sailboats? When did they start flying? While I’m sure there may be some aerodynamic similartities between wings and sails, so what? A sail’s pretty much useless for doing anything except going directly downwind without a keel. A sailboat sails because the action of the water against the keel and the wind against the sail produces the same kind of pressure and effect that you get when you squeeze a watermelon seed between your fingers. Pop! THe boat shoots out in a compromise direction.

Last I checked this thread was about airplanes and wings. If you wanna sail a boat and talk about it why not start a thread on the subject? Show Bantmof the same courtesy he showed when he started this thread so as not to hijack another.

47

Scylla, you are obviously missing my point which is exactly the opposite of what you say. My point is that the sail of a boat works in exactly the same way as the wing of an airplane. It displaces air to create lift. It is not off-topic in the least. It is an example I am using to explain my view of how a wing works. If you read my posts with a minimum of attention you would have realized this.

The forces created by a sail are exactly equivalent to those created by an airplane wing and yes, a boat needs a keel to sail upwind, so what? That changes nothing. An airplane needs an engine to take fly. So?

48

This is really silly-everyone knows that airplanes fly because tinkerbell uses her fairy dust just before takeoff, and that makes more sense than anything else I have ever heard.

Of course, I never got around to taking physics 101, and I am not sure that I beleve they DO fly. Maybe the belief that they fly is just mass hypnosis?

Sorry-just trying to inject a little levity into the debate

49

>> trying to inject a little levity …

Aha! levity! that is what makes airplanes fly! :slight_smile:

50

Sailor said:

" My point is that the sail
of a boat works in exactly the same way as the wing of an airplane. It displaces air to create lift."

Oh yes, lift. That must be why all those sailboats hover above the water and fly over my house all the time.

I maintain that it’s different. It’s not “lift” unless it’s pushing up. The boat is being pushed between the wind on the sail and the keel, not being lifted. Planes, and gliders can fly one over the other. If you put a sailboat directly downwind of another, the former is becalmed. This would suggest that the primary force at work is the pressure against the sail. Airflow over the sail is secondary and only meaningful at certain angles of attack.

I didn’t miss your point at all, I think you’re reaching.

I still think it oughtta be another thread out of courtesy to Bantmof who was kind enough to start this one so as not to hijack another. WOuld that such courtesy were contagious.

51

Scylla, you obviously don’t know sqwit about sailboats or you would know that the forces created by the air on the sails and by the water on the keel are called lift and are similar to those generated by the wing of an airplane or by the blade of a prop. Yes, the keel of a boat creates LIFT! It is not pointed upwards and it is called LIFT! How about that? huh? maybe you want to take that up with boat designers? The sails and keels also stall when fluid flow is not maintained close to the “upper” side. How about that? You may want to take it up with the engineering community if you do not like it.

You obviously do not have a clue. When you posted the first time I tried to inform you of what I know but you come back insisting. If you would have taken the least trouble to search the net you would have saved yourself from making a fool of yourself.

In the following pages you can see the term lift applied to keels and sails, and/or discussions on the forces they generate and how they are comparable to airplane wings. Any engineer can tell you the forces created by fluids on sails and wings are of the same nature.

http://www.madsci.org/posts/archives/may98/893173220.Ph.r.html
http://www.footeprint.com/sailingweb/martin.htm
http://www.hanleyinnovations.com/windsurf.html
In page 5 of http://www.pacificseacraft.com/Brochures/Pacific%20Seacraft%2037%20Brochure.pdf they discuss the design of the keel and say “this approach increases keel lift and its resistance to leeway”

Anyway, the thread is yours and you can have the last word. Thanks to you it is obviously no longer about how wings generate lift.

52

I sense much hostility, but I still don’t see any flying boats.

53

Well, I looked here because someone is confusing lift with
downward “Weight”. IE it was stated more or less that a plane in flight exerts it’s weight on the planet surface via the atmosphere. My contention is that the plane is supported by local pressure differences and some reaction from downward displacement but that this is local and doesn’t exert “Weight” on the ground below. Yes you will certainly feel air movement and possibly shock waves but the support is actually provided by air’s tendency to maintain equal pressure and it’s inertia. (We’re talking high enough to avoid gorund-effect by the way. It is actually the energy from the plane’s fuel holding it up.

Anyone care to comment?

Apologies if there has been a posting which answers this, haven’t time to read the entire history.

54

I can’t prove it mathematically, but I thought that the downward pressure an aircraft exerted would push on the ground … just like an object on the ground. Now, that weight isn’t detectable on the ground, because there’s so much air around the weight of the aircraft is “diluted”, for want of a better word.

In other words, if you had a scale under the state of Montana, and a bunch of planes took off from Montana airfields and flew around in the middle of Montana the scale would register no differently on average. I say “on average” because, if all the planes went into a dive, the scale would register less, and if all planes were climbing, it would register more. The same is true of a person jumping up and down on the scale. Their average weight over the pogo session would be their true weight.

Another way to think of it: you’ve got a bucket full of water (the bucket, being a contained mass of water, is a much better example than reality, in which no airshed is isolated from other airsheds). In the bucket you’ve got 100 kilograms of water and a 1 kg toy waterskier. The waterskiers float like planes fly, i.e., they are heavier than their medium, and will sink while stopped (I was going to create this example using boats, but then I realized the analogy isn’t great, boats being lighter (less dense) than water). So, at first, the waterskier is lying on the bottom, and the bucket weighs 101 kg net. Then, you drag the waterskier on his toy tow-rope, what does the bucket weigh now?

If you guess “100 kg”, well, I hope you’re wrong, or I’m going to have to adjust my world-view. The waterskier should have to exert 1 kg down in order to “fly” above the water’s surface. He can bounce, jump, sink his skies, whatever, but if he averages less than 1 kg in downwash (lift), he’ll sink. Poor blighter.

55

Darn it, four months ago I deliberately passed this by so as not to hijack the thread, but if this thread is coming back, I can no longer resist:

**

**

Hughes HK-1 “Spruce Goose”
Boeing 314 “Clipper”
Martin 130 “China Clipper”
Kawanishi H8K2 “Emily”
Kawanishi H6K5 “Mavis”
Martin PB2M & JRM “Mars”
Short C Class
Saunders-Roe “Princess”
Macchi M24
Dornier Do X

Flying boats, one and all.

Pan Am Flying Boats

56

You know what I meant.

57

Which is why I resisted that shot four months ago.
I have a hard time passing up straight lines twice.

58

Actually, good ol’ Dennis Connor in the years between America Cup’s had a catamaran that had a rigid sail. Quite like an Airplane-wing.

IIRC, the Kiwi’s challanged Connor to a race without the AC restrcitions on hull design and sail construction. So he made the cat with a wing, and wiped up the Pacific with the Kiwi’s, wasn’t even a challenge…

Of course the next AC, Connor failed miserably…

I remember seeing the cat cruise in and out of San Diego Harbor, swift little thing with a funny wing shaped ‘sail’.

HTH

59
      • And I can’t find it now, but there was a custom boat builder that was selling hydrofoil (sail-powered) catamarans. -Or, maybe not selling . . . They were about 40’ long, only two seats, no internal storage space and damned expensive.
  • If there’s anything I can do to confuse the issue further, just let me know. - MC
60

>> You know what I meant.

Yeah, you were just being annoying and a major league asshole (Hey! you’re in good company!) because what you said in no way answered anything and was just meant to be a nuisance and to obstruct the discussion. You had no other intent other than that.