Why arent airplanes designed more like hang gliders?

Seems to me that the greatest lift and buoyancy is attained with concave shapes.

You see this in everything from frisbees to parachutes to hang gliders...they all rely on concave surfaces to capture the air on which they glide or slowly descend to earth.

So, why dont we see the use of concave forms on aircraft???

 

Log in or Sign up to hide all adverts.

Because the primary reason for building and using aircraft is their speed.

And at those desired speeds, the shapes you're suggesting would be torn to shreds by the turbulence they would create.

 

Log in or Sign up to hide all adverts.

Here's a design that uses something like you are talking about a Mc Donell Douglas BWB.

Please Register or Log in to view the hidden image!

 

Log in or Sign up to hide all adverts.

A simple cardboard tube has excellent buoyancy due to its internal concave shape...and yet has little air resistance.

 

Does it have any concave shape to the underbelly?

 

I don't see anything that looks even remotely concave about that design.:shrug:

It's pretty much standard airfoils with more of the body incorporated into them. Much closer to the B-series of steath aircraft than anything else which also uses no concave surfaces for gliding purposes.

 

I'm not quite sure in which orientation relative to the aircraft as a whole you're referring to in considering concavity, Carcano- whether you are referring to spanwise (or lateral or side-to-side) curve, or chordwise (or longitudinal or fore-aft or front-to-back) curve. Both are interesting to think about.

The lateral curve on flexible wings such as parachutes and early "rogallo" hang gliders follows from their structural properties, and not a benefit in terms of lift/drag. As you probably know, a wing designer's task is always to maximize lift and minimize drag. Later evolutions of flexible wings (like parachutes and hang gliders) eliminate lateral curving as much as possible, in the endless pursuit of higher performance.

If you consider the lift vector of particular sections of any wing that is significantly curved along the span, you will realize that sections with significantly different spanwise angle have differing (and opposing) lift vectors. Opposing lift vectors cancel each other out through the shared airframe structure, which produces an increase in drag without a net contribution to "airplaning" or lift.

Another issue with a laterally-curving wing is lateral stability. Curving the wing upward (as elastic sailplane and airliner wings visibly do under load) or designing in what is called dihedral or polyhedral (curving the wing upward in segments) contributes to a beneficial coupling of roll (bank) with yaw (nose left or right of the flight path). That is, provided an upward-curved wing and appropriately-sized vertical stabilizer(s) momentarily self-righting properties are achieved. If you fold or curve a piece of paper, look at it edgewise, and rotate it slightly looking along the line of the curve or fold (looking from what represents the front of the wing) you will see that the angle of attack (the angle that a particular section meets the wind) is unequal whenever the aircraft is yawed. That's what produces the rolling motion, coupled with yaw. Keeping that inherent roll coupling in the same direction as yaw (left or right) is vital in avoiding divergent (unstable) flight conditions when yaw inevitably comes into play. Because wing sweep produces dihedral effect (because in yaw the trailing wing cuts a smaller swath) we see anhedral (downward spanwise wing orientation) to bring yaw-roll coupling within the desired stability parameters: Designers of swept-wing aircraft mount the wings slanting low to counteract what they call Dutch Roll, or excessive dihedral effect. Take away the sweep, and an airplane with anhedral in the wings would tend to tuck under in any yawing turn, in a motion like a car overturning at high speed in a corner.

Early hang gliders mostly overcame the instability of spanwise undercamber by depending on the pendulum effect. These aircraft can become dangerous in a dive or in ballistic (low or no-lift) flight because of their potential for an uncontrollable, divergent rolling motion as a product of their wing curvature.

Frisbees are another special case. Being round wings that do a lot of yawing (spinning in fact) their lift distribution along the span remains constant, and gyroscopic momentum/precession primarily control flight attitude.

Back to airplane wings: There have been many examples of "gull" wings, but stable unswept wing design has always necessitated a net dihedral effect. Because wingtips have the greater rolling leverage, gull wings require some exaggeration of total wing dihedral. Considering the heavier structure required, the opposing lifting forces, and the stability challenges, downward-curving wings have never been advantageous. I would even say that they're for the birds- The complexity of bird wings is best left to nature for now. If we were to try (and in aviation's infancy we have tried) mimicking the folding and active properties of bird wings we would run into all sorts of controllability and reliability issues.

There have been experiments, and some limited commercial successes with downward-curving wingtips. They tend to involve more compromise than benefit but you still see them around occasionally. I bought one of my planes with drooped wingtips, and when I took them off there was a noticeable improvement in stability, speed, and short-field performance. In other words, "Madras" and drooped "Hoerner" wingtips are crap.

If instead of spanwise curving of a wing, you were meaning to take a look at undercamber in an airfoil, Carcano- that's another subject, and there are many interesting potentials there.

 

Carcano: "A simple cardboard tube has excellent buoyancy due to its internal concave shape...and yet has little air resistance."

The apparent bouyancy is not necessarily due to the internal concave, or external convex shape. A tube is one of the most rigid structures you can make from a sheet (a flexible sheet being even more bouyant, but also more unstable). Rolling a sheet into a tube makes it much more rigid, which eliminates the drag of extreme aeroelasticity- but a flat, rigid sheet of the same thickness, area and mass as your flying tube can generate far more lift, with far less drag penalty.

For lots more about the physics of flight, visit one of my favorite references on the subject: See How It Flies

 

In fighter jets...
The more stable the plane, the harder it is to change its orientation, so they make it pretty unstable, and use computers and gyroscopes to keep it as it's meant to be.

The shape most probably plays into that.

 

I was refering to a "hang glider" more than the "concave" design. Sorry that I didn't express that but I thought just looking at the design would lead you to see the similarities of them both.

Please Register or Log in to view the hidden image!

 

OK, that's an early Rogallo wing (1970s vintage) and they are elegantly simple in curve and structure- but far less efficient than modern hang gliders. Later developments eliminate that pronounced spanwise curve as much as possible:

Please Register or Log in to view the hidden image!

Please Register or Log in to view the hidden image!

Please Register or Log in to view the hidden image!

​

 

Informative posts!

I had a longitudinal concave shape in mind, of which a simple tube is an example.

Let me ask: Supposing you had two long cylinders of different materials, same size and weight except one was hollow and the other was solid.

It would be interesting to see the difference in air buoyancy. I believe the hollow tube would fly further and faster with the same momentum.

 

Carcano: "Supposing you had two long cylinders of different materials, same size and weight except one was hollow and the other was solid.

It would be interesting to see the difference in air buoyancy. I believe the hollow tube would fly further and faster with the same momentum."

That's right, the open hollow tube would have less drag, and more lifting surface, compared with a closed cylinder of equal dimensions and mass- that is, unless the tube and cylinder dimensions are long enough to induce stagnation within the open version.

This somewhat counter-intuitive effect is related to how we use cowl flaps to reduce engine cooling drag in many airplanes with reciprocating engines. Cruising at higher speed, and with a surplus of airflow cooling engines, we close cowl flaps and enjoy a significant increase in speed. By partially closing the downstream end of our rather long and convoluted cooling air "tube", drag is reduced significantly- in some airplanes by more than 5% of the total drag of the aircraft- you can actually close cowl flaps on planes with big piston engines (my favorite kind) and feel a slight "push" from behind.

In other words, once the energy required to force air through a tube becomes greater than that required for it to pile up in front of the cylinder and flow around, endplates actually reduce drag in an airstream-aligned cylinder.

Considering lift production, our "test tube" could not become very long before turbulent flow and sudden deceleration would commence within, destroying most of the lifting force there, and rapidly dissipating the kinetic energy of the airflow into the tube walls as drag.

One of the most important and sought-after tricks in aerodynamics is shaping things so that the airflow around them remains laminar instead of turbulent. Natural laminar flow is impossible within a straight tube having a length of much more than a diameter.

Another aerodynamic axiom (a bit less scientific) among pilots is that aeroplanes that look good tend to fly good, and variations on the open cylinder haven't turned out all that sexy:

Please Register or Log in to view the hidden image!

Caproni Stipa​

And just a minor point of terminology- "lift" commonly denotes aerodynamic forces, while "buoyancy" is associated with lighter-than-air flight (balloons, airships, etc).

 

See, I knew there would be at least one person^^^on sciforums with a detailed knowledge of aerodynamics.

Please Register or Log in to view the hidden image!

 

There are probably some here that know more than I. I've just had a lot of airflow past my cheeks (both sets even) having been obsessed with aviation all my life.

 

Do you believe we have already found the ultimate ideal aerodynamic shape?

Maximum lift with minimum drag.

 

"Ideal" is a tricky word when we ply the air in so many various ways, weights, and speeds. Like selecting an ideal pair of shoes, choosing your wings depends what you're setting out to do, and in what conditions you intend to do it.

But yes: With respect to specific specializations of the multiple thousands of airfoils that have been developed, I'll admit we've come very close to the ideal. We came very close within aviation's first half century, and there have only been miniscule improvements in airfoils ever since.

That's part of the reason that a 70-year-old Cub can perform comparably with the latest light aircraft designs. As for bigger and faster aircraft, it's been engine development (not revolutions in airfoils) that have most propelled our advances.

Please Register or Log in to view the hidden image!

Clark Y - 1922

Please Register or Log in to view the hidden image!

PSU-90-125 ~ 1990​

 

I've built plenty of small-scale foam gliders and RC planes with concave wing surfaces (like just the top part of the airfoil). It works well for the little floaters but I wouldn't use it for fast-moving planes. I have made plenty of rubber-powered planes with concave-undersided wings and they would always nose up rapidly and stall when the prop was turning and would glide on a downward angle when flown at a slower speed. I've made some Concorde-type and fighter jet gliders with completely flat wings; they flew fast, very smoothly and a considerable distance.

 

One more question:

Ive noticed that ALL birds have wings above their bodies, but this is not always the case with airplanes. Why would having the wings higher than the bulk of the plane's weight NOT always be the most stable design?

Seagulls btw do have concave wing shapes...just noticed today.
They sweep forward and up and then back and down at the elbow.

 

"Ive noticed that ALL birds have wings above their bodies"

It's the natural way to configure for muscles pulling mostly downward. But as a high-wing airplane owner, my favorite retort when the dispute arises over the best way to build an airplane is "Have you ever seen a low-wing bird?"

Please Register or Log in to view the hidden image!

"Seagulls btw do have concave wing shapes"

Some seagulls are highly creative with wing shape.

I've been in ever-increasing awe of what birds can do, for all my life.