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How did WWI Planes Fly?

by fugg
Tags: aerodynamics
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russ_watters
#37
Mar30-09, 04:38 PM
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Quote Quote by Jeff Reid View Post
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
#38
Mar30-09, 05:03 PM
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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
#39
Mar30-09, 05:24 PM
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Quote Quote by Cyrus View Post
Quote Quote by Cantab Morgan View Post
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
#40
Mar30-09, 10:24 PM
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I don't understand what you mean by the phrase "the least forwards". A wing does not accelerate the air forewards.
rcgldr
#41
Mar30-09, 11:09 PM
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Quote Quote by Cyrus View Post
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
#42
Mar30-09, 11:26 PM
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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
#43
Mar30-09, 11:41 PM
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"Forward acceleration of the air" is technically correct, but it just sounds cumbersome.
rcgldr
#44
Mar30-09, 11:46 PM
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Quote Quote by Cyrus View Post
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
#45
Mar30-09, 11:51 PM
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Quote Quote by Jeff Reid View Post
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).

[tex]F=m\dot{V}+\omega x mV[/tex]

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
#46
Mar31-09, 02:06 AM
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Quote Quote by Cyrus View Post
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
#47
Mar31-09, 04:25 AM
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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
#48
Mar31-09, 08:23 AM
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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
#49
Mar31-09, 10:49 AM
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Quote Quote by Jeff Reid View Post
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 eachother 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
#50
Mar31-09, 11:19 AM
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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.
Cyrus
#51
Mar31-09, 11:22 AM
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Quote Quote by DaveC426913 View Post
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.
I know, and I agree!

I'm saying 'forward acceleration' sounds very awkward. It's a local deceleration of the air.
DaveC426913
#52
Mar31-09, 11:29 AM
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Quote Quote by Cyrus View Post
I know, and I agree!
Right. I noted you were the one who got it. Not sure if it's still in contention with others...
rcgldr
#53
Mar31-09, 11:30 AM
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Quote Quote by Cyrus View Post
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.
I'm not sure I got your point about the rotational differences in frame of references in your earlier post. I understand that circulation, vortices, and turbulence play a role in lift, but I was considering the ambient air, unaffected by an aircraft, as the alternate frame of reference, and I don't see a rotational aspect to the ambient air with respect to the aircraft.

F = ma
As far as the F=ma aspect of lift and drag, I thought that turbulence made things a bit more complicated. Some of the "a" (acceleration) isn't linear, but angular, which complicates things. Vortices usually add to the drag of an aircraft, and in the case of a delta wing at high AOA, the leading edge induced vortices can contribute to lift, allowing large AOA around 20 degrees or so.

In the case of solids, the linear accelerations are related to the linear forces / mass, regardless of any torque induced angular accelerations (the classic non-spnning ball sliding from a frictionless surface onto a surface with friction problem), but in the case of fluids, or gases, I'm not sure the same principle holds. For example, is all of the drag on a bus related to linear acceleration of air forwards and none of it due to turbulence (ignoring temperature effects)?
Phrak
#54
Mar31-09, 03:06 PM
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I hadn't considered that. The end result of turbulence is heat energy, once the turbulence has died out. Skin heating is another source of heat. In the context of Newton, drag could be more exactly modeled as a partially elastic collision.

But momentum is still conserved.

[tex]\frac {d(mv)_{air}}{dt} + \frac{d(mv)_{plane}}{dt} = 0[/tex]


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