Uncovering the Secrets of How WWI Planes Took Flight

[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. :grumpy:

 

[Cyrus]

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. :grumpy:

I know, and I agree!

I'm saying 'forward acceleration' sounds very awkward. It's a local deceleration of the air.

 

[DaveC426913]

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]

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]

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.

$$\frac {d(mv)_{air}}{dt} + \frac{d(mv)_{plane}}{dt} = 0$$

 

[Cyrus]

What I said about irrotational flow may not be accurate. I'm going to verify it by asking my friend who is doing his PhD in CFD and see what he says. Irrotational flow does not rotate, but I am not sure if that necessarily has a coordinate system attached to the fluid element or not.

 

[Phrak]

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.

I've been looking for example pictures of bridges for a 1)biplane, 2)externally stessed monoplane, and a 3) internally stessed wing.

I haven't had a lot of luck.

 

[Phrak]

I don't know how rotational flow got into all this, but one can either examine the forces from the inertial frame of the wing, or an inertial frame at rest with the free stream velocity of the air. Accelerations are equal in both.

However, in an idealized nonviscous fluid, lift and induced drag are obtained by superimposing the flow fields of a vortex bound to the wing, together with a linear flow field. This will first get your lift around a cylinder. Using a 'conformal transform' (angle preserving transformation) of the coordinates about the cylinder, one can get the flow field around a wing shape. The cylinder is transformed into the shape of a wing using the right conformal transform.

No wing is infinitely long, so the vortex field leaves the wing toward the tips and trails behind, just as we should expect with real wings in real viscous fluids like air.

 

[nucleus]

Originally Posted by mgb_phys 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.
“I've been looking for example pictures of bridges for a 1)biplane, 2)externally stessed monoplane, and a 3) internally stessed wing.
I haven't had a lot of luck.”

You can’t see them in most pictures but there were many cables joining the struts on the early flyers. All the cables had to be adjusted to “box the wing, fuselage etc.”

Octave Chanute, who was a civil engineer, did a lot of work on airplane stresses and helped the Wright’s with their planes. Several airplanes and bridges used truss type construction. Even large airplanes today use trusses like a bridge, although you have to get inside the fuel tanks to see them. There are many types of trusses, but the Warren was uses on both bridges and airplanes.
http://en.wikipedia.org/wiki/Interplane_strut
http://en.wikipedia.org/wiki/Truss_bridge
If you go to chapter1 at the following site (first 17 pages), it explains the different types of airplane structures, from trusses to monocoque and stress skin.
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/key/AC 65-15A
For a history of early flight, especially in America, the following site has a ton of information:
http://www.centennialofflight.gov/hof/index.htm
For pictures and videos of a 100-year-old replica plane that flew this winter see this site and others on the silver dart:
http://best-breezes.squarespace.com...er-dart-replica-flies-into-history-books.html
One of the most common WW1 era airplanes was the Avro 504. It came equipped with a wind driven fuel pump. More information can be found at this site and other sites. It even has a parts manual for the plane:
http://www.avro504.org/
The engines on the WW1 planes were unique. The crankshaft was bolted to the plane and the cylinders and prop rotated around it. There was no throttle so to slow down they cut the ignition or fuel. Problem was the huge gyroscopic loads, which made it very hard to turn the airplane.

To start the engine they had to remove the spark plugs, then, with an oil can, squirt some gas into each cylinder. After replacing the plugs they would swing the prop. The majority of the time it would not start so they would pull the plugs and start over again. It would typically take a half hour to start.
http://en.wikipedia.org/wiki/Gnome_Monosoupape

 

[mgb_phys]

To start the engine they had to remove the spark plugs, then, with an oil can, squirt some gas into each cylinder. After replacing the plugs they would swing the prop. The majority of the time it would not start so they would pull the plugs and start over again. It would typically take a half hour to start.

Sounds like a motorbike I used to own!

 

[rcgldr]

The engines on the WW1 planes were unique. The crankshaft was bolted to the plane and the cylinders and prop rotated around it. There was no throttle so to slow down they cut the ignition or fuel. Problem was the huge gyroscopic loads, which made it very hard to turn the airplane.

They had to use some rudder inputs to change pitch, and some pitch inputs to change yaw. Helicopters have the same issue, but the pitch and roll controls (vertical axis) are already set 90 degrees out of phase.

 

[ruko]

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 .

OK, if it's wing shape resulting in less pressure on top than on the bottom, then how do they fly upside down? Ailerons do not control up and down movement. Elevators do that.

 

[Doggy Darb]

I was looking for information about biplanes when I can upon this forum.

There are a few reasons biplanes were popular during WWI. As we all know from the drag equation, drag increases with speed. At 100mph, drag is much lower than at 1200mph let alone 200mph, which was the speed being achieved when monoplanes became popular in combat. Lower drag forces resulting from lower speeds meant that it was not unpractical to add the additional drag of a second wing in exchange for the lift and agility it provides. Triplanes fell from favor because they restricted vision. As mentioned before, multiple wings are made into a truss. If you've ever seen any of these aircraft, you will be astonished that they are more like furniture than any vehicle you are familiar with. Monoplanes were better suited to endurance flights as they lacked the structure to withstand radical maneuvers and stunt flying.

Pressure differentials do play a part in lift. A wing is a baffle. Think of how water skis work; that is how a wing works. The air under the wing is exerting a pressure directly to the wing. If there is no angle of attack, there is normally not enough lift to generate flight. Ailerons, elevators, stabilators, elevons, flaps, and air brakes all work off of the same principle. They redirect the flow of air. Flaps generate more drag because they redirect air more radically, but they direct air downwards. This allows for slower speeds to generate sufficient lift. The force vector is more vertical. Stabilizers are horizontal in order to counter the moment that the wing imparts on the aircraft. If the stabilizer has the same angle of attack that the wing has, the aircraft would never leave the runway.

Airplanes fly upside down by "diving" up while inverted. The elevating mechanism is directing air upwards, driving the tail section down. This counters the downward velocity the aircraft would otherwise have. If an aircraft is inverted and no other control is exerted, the aircraft loses altitude.