Uncovering the Secrets of How WWI Planes Took Flight

[fugg]

Hello. I've just been curious as to the answer to this question. Modern aerofoils are fine due to the geometry that allows for a pressure difference between the upper/lower surface. But as I remember WWI planes, they all have 4-5 levels of straight wings. Does that serve the same purpose? How? I tried googling it but couldn't sort through the irrelevant topics. I'd appreciate anyone who can feed my curiosity!

 

[Razzor7]

Pilots were much stronger back then.

 

[Cyrus]

The airplanes had airfoils.

 

[derek e]

Hi, interesting question. I don't know the answer but I googled some pictures of WWI planes. A lot of them had two layers of wings that look like they have some curvature. This combined shape looks like a silhouette of sections of an airfoil, (from Cyrus) probably because each wing is an airfoil.
http://www.grahams.com.au/glennsgraphics/aircraftww1-1,1.gif

 

[Cyrus]

http://books.google.com/books?id=pW...gbHxCQ&sa=X&oi=book_result&resnum=1&ct=result

 

[russ_watters]

Just an fyi, even a flat plate will fly if it has a positive angle of attack...but I don't think anyone has ever tried to fly one. Even the Wright Flyer had a real airfoil.

 

[Vanadium 50]

A plane works by pushing air down. That's exactly what the WW1 vintage planes did - and as pointed out, they do have an airfoil.

 

[DaveC426913]

But as I remember WWI planes, they all have 4-5 levels of straight wings. Does that serve the same purpose? How?

4-5 levels? The most was 3 really (Fokker Triplane) the large majority had 2 wings. I think the reason they were bi-wings rather than the later mono-wings has more to do with the strength of materials and how much lift a wing of a given length could support.

 

[fugg]

mersi kurosh
Thanks, good to know!

 

[Cyrus]

mersi kurosh
Thanks, good to know!

 

[Phrak]

4-5 levels? The most was 3 really (Fokker Triplane) the large majority had 2 wings. I think the reason they were bi-wings rather than the later mono-wings has more to do with the strength of materials and how much lift a wing of a given length could support.

Adding vertically stacked layers improves lift, but with diminishing returns with each added layer This is due to interference. There's no improvement in aspect ratio by stacking wings; the drag increases with each layer, as the lift advantage slows.

You're right, the box construction was for strength. The Fokker triplane has an additional small foil between the wheel. Any structural members such as the carriage axle produce drag. Wrapping an aerodynamic shape around it reduced drag. So presumably, it's a source of free lift if you give it an angle of attack.

Even the cross wires where teardrop shaped, eventually. A teardrop shape half an inch across has about the same aerodynamic drag as a wire of about 100 mils diameter.

 

[DaveC426913]

The Fokker triplane has an additional small foil between the wheel. Any structural members such as the carriage axle produce drag. Wrapping an aerodynamic shape around it reduced drag. So presumably, it's a source of free lift if you give it an angle of attack.

OK so, 3 1/2 wings.

 

[Phrak]

I did find one example of a stack of 4 wings as pictured http://www.htmy.org/hattrick/stacked-wing-attacks/".

 

[Cantab Morgan]

A plane works by pushing air down. That's exactly what the WW1 vintage planes did - and as pointed out, they do have an airfoil.

Is that really so or are you speaking whimsically? I'm sure a helicopter works by pushing air down, but a plane?

I thought planes worked because the shape of the wing decreases the air pressure above the wing's surface, giving it a net lift.

 

[cragar]

A plane works because of Bernoulli's principle the air flows faster on top
because of the shape of the wing thus creating a low pressure on top
and the high pressure on the bottom of the wing pushes the plane up , I mean yes it can climb by moving the aileron's .

 

[Cyrus]

A plane works because of Bernoulli's principle the air flows faster on top
because of the shape of the wing thus creating a low pressure on top
and the high pressure on the bottom of the wing pushes the plane up , I mean yes it can climb by moving the aileron's .

<Raises my eyebrow> ...um, no.

 

[rcgldr]

A flat plate will fly if it has a positive angle of attack.

The old "dime store" type balsa gliders have flat wings and glide just fine. Rubber powered balsa planes with flat wings also fly well.

http://www.retroplanet.com/PROD/24887
http://www.retroplanet.com/PROD/24886

Is that really so or are you speaking whimsically? I'm sure a helicopter works by pushing air down, but a plane? I thought planes worked because the shape of the wing decreases the air pressure above the wing's surface, giving it a net lift.

The point is to accelerate the air downwards. The air is drawn downwards towards a low pressure zone above a wing, and/or pushed downwards away from a high pressure zone below. Technically the air accelerates away from higher pressure zones to lower pressure zones in all directions, except that air can't flow through a solid wing, so the net result of a wing moving forwards with an effective angle of attack is to accelerate the air downwards (corresponding to lift), and somewhat forwards (corresponding to drag).

 

[cragar]

ok then how does it work cyrus , learn me

 

[Cyrus]

ok then how does it work cyrus , learn me

I provided you a link to an online book. I would recommend reading it. Ailerons cause pure rolling moment (ideally). They do not make the aircraft climb, that would be the elevator.

 

[cragar]

ailerons can do both roll and elevate , when you pull back
on the stick the elevators and the ailerons move down so I wasn't completely wrong but you wouldn't have known based on your comment and the B-2 bomber doesn't even have elevators

 

[Cyrus]

ailerons can do both roll and elevate , when you pull back
on the stick the elevators and the ailerons move down so I wasn't completely wrong but you wouldn't have known based on your comment and the B-2 bomber doesn't even have elevators

No, they cannot. When you pull back on the stick the ailerons don't do anything. That's not how they work. What you just wrote is fundamentally wrong.

Again, please read the book I linked earlier.

(The B-2 is a flying wing. It has what are called elevons).

 

[Phrak]

This is actually a good question. Why did the WWI era biplanes have thin foil sections?

The Write brothers' wind tunnel tests lead them to believe that a very thin, section with a concave bottom was best--at least as far as their initial applications went. This may have been their conclusion because the Reynold number, within the tunnel where the tests were conducted, was comparatively low. Then again, there were so many possible shapes to test.

Which leads us to biplanes. With a thin wing section, using the materials of the time, there no chance of containing the structual members internal to the wing.

 

[cragar]

ok sorry , you are right , i stand corrected

 

[Cyrus]

ok sorry , you are right , i stand corrected

What you wrote about the wing though, is correct .

 

[rcgldr]

ailerons can do both roll and elevate

When you pull back on the stick the ailerons don't do anything.

Depends on the aircraft. Most aircraft don't use the ailerons as spoilerons or flaps, but some fighter aircraft adjust wing camber with respect to elevator inputs, rotating both leading edge "flaps" and the entire trailing edge of the wing (flaps and alilerons) downwards in addition to moving the elevator upwards when pulling back on the stick. It's also very common to adjust camber on radio control gliders by moving the entire trailing edge of the wing (flaps and ailerons) upwards (reflex), or downwards (more camber), either via a separate control and/or tied into elevator inputs.

In the first part of this video, the ailerons are raised and the flaps lowered ("crow" mode) to allow for slow flight. (It's better to raise the ailerons to reduce adverse yaw, and it also reduces lift):

http://www.youtube.com/watch?v=LUVRUMc7DP8&fmt=22

B-2 bomber doesn't even have elevators

Or ailerons. Fying wings use "elevons", which act as both ailerons and elevators.

On an aircraft where the entire stabilizer acts as an elevator, it's called a "stabilator". Terms for radio control models: "wingeron" - the entire wing acts as an aileron, these models have conventional elevators. "pitcheron" - the entire wing act as both aileron and has an adjustable AOA, the elevator is fixed.

 

[Phrak]

Then there are hang gliders where the entire wing does everything in relation to the pilot's center of mass.

 

[Cantab Morgan]

The point is to accelerate the air downwards. The air is drawn downwards towards a low pressure zone above a wing, and/or pushed downwards away from a high pressure zone below. Technically the air accelerates away from higher pressure zones to lower pressure zones in all directions, except that air can't flow through a solid wing, so the net result of a wing moving forwards with an effective angle of attack is to accelerate the air downwards (corresponding to lift), and somewhat forwards (corresponding to drag).

Ahhh. Then, could it be said that a well-designed wing shape accelerates the most air downwards but the least forwards?

It should have been obvious to me that airplanes have to accelerate air downwards to stay aloft. TANSTAAFL.

 

[Cyrus]

Ahhh. Then, could it be said that a well-designed wing shape accelerates the most air downwards but the least forwards?

It should have been obvious to me that airplanes have to accelerate air downwards to stay aloft. TANSTAAFL.

This doesn't even make any sense. A well designed wing has a high L/D ratio.

As for your second sentence, not really. A wing pushes the air down to a certain degree. But look at the streamlines of an airfoil and you will find the air pretty much leaves at the same angle it came in for laminar flow. The wing isn't shooting air downwards as it trails behind the airfoil section.

 

[rcgldr]

A wing pushes the air down to a certain degree. But look at the streamlines of an airfoil and you will find the air pretty much leaves at the same angle it came in for laminar flow. The wing isn't shooting air downwards as it trails behind the airfoil section.

an observer on the ground would see the air going almost straight down behind the plane.
From this website (similar text at other web sites):
http://home.comcast.net/~clipper-108/lift.htm

Not that I or others agree with everything on that website or anyone website about aerodynamics, but downwash of air in order to produce lift (the Newton part) is a common aspect of aerodynamic related web sites.

For a plane in level flight, or at least not accelerating vertically, gravity exerts a downforce on the plane, which exerts a downforce on the air, and the air ultimately exerts a downforce onto the surface of the earth.

 

[Lok]

Back then airplanes were made out of wood strings and canvas. Not too strong materials for holding 2 big wings on either side. Stacking the wings was stronger and used less material necessary for a good power to weight ratio. The wingspan had to be big as they didn't have enough power to fly on the small wings we see today. Less power means less speed so less lift, so the solution was bigger wings that generate enough lift at small speeds.

As i heard there was a time when nobody believed a normal wing plane would ever fly.

 

[DaveC426913]

As i heard there was a time when nobody believed a normal wing plane would ever fly.

Engineering advancements always seem to come down to the same thing: stronger, lighter materials.

 

[DaveC426913]

The old "dime store" type balsa gliders have flat wings and glide just fine. Rubber powered balsa planes with flat wings also fly well.

http://www.retroplanet.com/PROD/24887
http://www.retroplanet.com/PROD/24886

IIRC, don't the gliders have to have a curve manually applied to their wings?

And as far as the rubber-band-powered ones go, frankly they're way over-powered to need wings. They'll pretty much go where you point em even if you forget to attach the wings.

The larger, balsa-framed models OTOH - which are heavier and therefore not so over-powered - do have classic wing cross-sections.

 

[mgb_phys]

As i heard there was a time when nobody believed a normal wing plane would ever fly.

Not really, Bleriot flew across the channel (France-England) in a monoplane in 1909 well before most WWI biplanes were designed. It's just that the biplanes with a much shorter stiffer wing were more maneuverable and a lot more robust.
It was only really metal monocoque wings that made monoplanes popular in the 20s.

 

[Phrak]

How is a biplane/monoplane like a bridge?

 

[mgb_phys]

How is a biplane/monoplane like a bridge?

The wing is pushed up at the ends (by the lift) and has a load in the centre (weight of the fuselage) = exactly the same engineering problem.

 

[rcgldr]

The old "dime store" type balsa gliders have flat wings and glide just fine. Rubber powered balsa planes with flat wings also fly well.
http://www.retroplanet.com/PROD/24887
http://www.retroplanet.com/PROD/24886

IIRC, the gliders have to have a curve manually applied to their wings?

Not the small ones. This one only has a mild taper at the trailing edge of the upper surface, just a rounded leading edge:

http://www.4p8.com/eric.brasseur/glider2.html

Some small indoor models also have flat wings:

http://jeffareid.net/misc/balsagldrs.jpg

balsa built up - standard airfoils

Balsa framed models use standard airfoils. For the aerobatic models, just as with real aerobatic models, symmetrical airfoils are used. The point here is that flat or nearly flat air foils work just fine, especially with smaller, low Reynolds number models.

 

[russ_watters]

The old "dime store" type balsa gliders have flat wings and glide just fine. Rubber powered balsa planes with flat wings also fly well.

Hmm... I see. I was sure I had a couple with cambered airfoils, but googling around, I can't find any.

All you need to do to get a cambered airfoil in a balsa glider is cut a curved slot in the fuselage. It would also help keep the wing in place. I do remember adding/increasing(?) the camber on mine by wetting down the wings and warping them, plus sanding the leading and trailing edges.

 

[rcgldr]

Straight slot on most of these:

Do a web search for free flight glider, or free flight indoors, and you find a few hits. The model aircraft equivalent of watching grass grow or paint dry. If the wing is shaped, it's usually a flat bottom with some camber on the top. The thrown or launched models have very little camber if any, as too much camber and the pitching down moment becomes an issue because of the high launch speed (some times a rubber band catapult) compared to the gliding speed. The rubber band powered film over wire frame models do use camber, but fly at very slow speeds.

F1D (very slow) model at 1:15 into this video:
http://www.youtube.com/watch?v=MAmVFfnEdBY&fmt=18

F1D model at start of video:
http://www.youtube.com/watch?v=5pOhbJPtPXM&fmt=18

 

[Cantab Morgan]

Ahhh. Then, could it be said that a well-designed wing shape accelerates the most air downwards but the least forwards?

This doesn't even make any sense. A well designed wing has a high L/D ratio.

You say it doesn't make sense, but then you repeat it.

 

[Cyrus]

I don't understand what you mean by the phrase "the least forwards". A wing does not accelerate the air forewards.

 

[rcgldr]

I don't understand what you mean by the phrase "the least forwards". A wing does not accelerate the air forwards.

Drag is related to forwards accleration of air (plus turbulence related angular torques, the vortices that occur at the tips and across the wing chord). For example, if a car drives thorugh a pile of leaves, the leaves are blown forwards by the air that has been accelerated forwards by the car.

 

[Cyrus]

Drag is related to shear stresses and pressure forces, not "forwards acceleration of the air". Just look at any video of an airfoil section in a wind tunnel, at no point is the air moving forwards.

Perhaps I take issue with your use of the word 'forward acceleration', I would call it 'deceleration of the air in the streamwise direction'. The air is being slowed down, not sped up.

 

[russ_watters]

"Forward acceleration of the air" is technically correct, but it just sounds cumbersome.

 

[rcgldr]

Perhaps I take issue with your use of the word 'forward acceleration', I would call it 'deceleration of the air in the streamwise direction'. The air is being slowed down, not sped up.

Using the air as a frame of reference, the air is originally stationary, afterwards it's moving or sped up. Velocity is dependent on the frame of reference, but acceleration isn't. Regardless of the frame of reference, the direction of acceleration of the air by a wing producing lift is downwards and a bit forwards.

 

[Cyrus]

Using the air as a frame of reference, the air is originally stationary, afterwards it's moving or sped up. Velocity is dependent on the frame of reference, but acceleration isn't. Regardless of the frame of reference, the direction of acceleration of the air by a wing producing lift is downwards and a bit forwards.

Acceleration does depend on the frame of reference, this is why you have a transport term in the equations of motion. It's due exactly to the fact that one reference frame is rotating relative to another frame. (Unless I am misreading what your saying).

F=m\dot{V}+\omega x mV

Anyways, that's an odd frame of reference you choose to pick. I would stick to the wing of the airplane as your FOR from now on. Its the conventional way.


scroll down to: " Carrying out the differentiations and re-arranging some terms yields the acceleration in the rotating reference frame"

http://en.wikipedia.org/wiki/Rotating_frame

I agree with what you said for the acceleration directions. It's just very awkward because in steady state flight you don't talk in terms of accelerations but velocity. I would have preferred that you said the air has a component of velocity down and aft, with the aft component reduced from that of the freestream.

 

[rcgldr]

It's due exactly to the fact that one reference frame is rotating relative to another frame.

I was using the ambient air or the aircraft itself as the two main frame of references. These don't rotate with respect to each other, unless you consider the planes path as great circle around the earth, in which case the air also forms a spherical shell around the earth.

I agree with what you said for the acceleration directions. It's just very awkward because in steady state flight you don't talk in terms of accelerations but velocity.

The aerodynamic forces ultimately correspond to aerodynamic accelerations, lift corresponds with downwards acceleration of air, drag with forwards acceleration of air (ignoring the turbulent related changes in angular velocity of air (vortices)).

thin wing

Most of the airplane designers during the early WWI era (1914) assumed that thick air foils would increase drag. From what I read Hugo Junkers started considering thick airfoils in 1915, with the all metal Junkers CL.I being made in 1918. The switch to thicker air foils occurred around 1917 and later.

 

[Phrak]

A few years prior to WWI, Gottingen was experimenting with thick foils. He had a number of very thick and highly cambered teardrops. Some so radical they appear comical to modern eyes.

 

[Cantab Morgan]

Many thanks to Jeff Reid and Cyrus and everybody for exploring this interesting topic. I feel that I am learning quite a bit from this exchange.

 

[Cyrus]

I was using the ambient air or the aircraft itself as the two main frame of references. These don't rotate with respect to each other, unless you consider the planes path as great circle around the earth, in which case the air also forms a spherical shell around the earth.

If your two reference frames are the air and the aircraft, then they don't rotate relative to each other if you consider the differential element of air to be irrotaional.

The aerodynamic forces ultimately correspond to aerodynamic accelerations, lift corresponds with downwards acceleration of air, drag with forwards acceleration of air (ignoring the turbulent related changes in angular velocity of air (vortices)).

Well, duh. F=ma.

 

[DaveC426913]

Look, all Jeff is trying to say is that wings impart forward acceleration on the air mass. Forward acceleration does not have to mean forward velocity.