[Bill Cummings]

By the way, here are the equations in more legible format (I'd like to add a mathematics capability to this forum some time!!):

equations.gif

If you look at that last equation, it tells you what velocity you need in various conditions. You can see that the required velocity goes up as you need more lift (L) ... but only as the square root of L. So to quadruple your lifting ability, you only need to double your speed. You can also see that your needed velocity goes down as the surface area of the wing (Sw) gets larger. However, remember that there's a similar equation for drag, so increasing your wing's area makes it harder to go faster!!

These equations are nice, and they let you know what's going on. But the complexity of aerodynamic forces on a real wing defy such simple analysis. That's why wind tunnels (and numerical methods) were invented (in fact, these equations sweep all of the really complex stuff into CL).That's also why there have been so many deaths during the development of aviation. Designing a safe, sound, and stable aircraft is not an easy task. Even with my degree in aeronautical engineering, it's not something that I would attempt without a lot of help from people who had done it successfully before.

Hello Bob and Tom,
While looking over Bob’s equations’ for “p” I got off on a tangent.
To what degree or percentage of the equations is the effect of relative humidity?

Let me ask again using different words-- Is the effect of relative humidity large or small when figuring Density Altitude? Or is Altitude and temperature the two biggest considerations?

[Bob Kuczewski]

Hello Bob and Tom,
While looking over Bob’s equations’ for “p” I got off on a tangent.
To what degree or percentage of the equations is the effect of relative humidity?

Let me ask again using different words-- Is the effect of relative humidity large or small when figuring Density Altitude? Or is Altitude and temperature the two biggest considerations?

It's all wrapped up in "p" (the Greek symbol pronounced as "rho"). I don't recall the exact answer off the top of my head, but there appears to be a pretty good explanation on Wikipedia at: http://en.wikipedia.org/wiki/Density_of_air.

By the way, I've always felt that it was confusing to use the Greek "p" for density since it looks so much like "p" which is used for "pressure" in many of the same expressions where "p" is used.

This is a great side topic, and I think some of this should go in the Training Manual that we (mostly Bill ) have been working on.

[Bob Kuczewski]

So, I repeat, what is a good low takeoff speed, high lift wing, and where can I find a detailed diagram?

Hi Tom,

I may have misunderstood what you were asking. I thought you were asking for a takeoff speed, but with more careful reading (sorry), it looks like you're asking for a wing and not a speed.

If you're asking for a generic "wing" (as in a particular cross-section), you might get started with the NACA series (see http://en.wikipedia.org/wiki/NACA_airfoil). They appear to be mostly (or exclusively?) double-surface, but it might get you headed in the right direction if you're looking for single surface as well.

If you're asking about production wings, the slowest wing I know of is the Wills Wing Condor 330. It was designed to allow a student to fly at a speed where an instructor can actually run along side the student to help control the glider. They do this all the time at Dockweiler Beach in Los Angeles. The Condor 330 was designed to be lightweight for students, but that means that it's not meant for actual flight beyond ground skimming. I do know of at least one Condor 330 that was "beefed up" for normal flight and it was intended to be used to tow paragliders (although I don't recall how that went). I actually flew that particular Condor 330 at Torrey Pines and I was able to fly at paraglider speeds.

If you're looking for slow speeds, you might also check out tandem wings. I believe that both Wills Wing and NorthWing produce tandem gliders. But remember that they have a weight range that must be followed for them to be controllable through a range of flying conditions.

Does that help?

[tomsines27]

Hi Bob, My aeronautical vernacular is very limited, but there goes. What do you think the best upper and lower camber is for a hangglider wing to get high lift? Meaning a cut away of the cord, and a diagram.
Tom

[Bill Cummings]

Hey Tom,
Some information to help our understanding of camber.
French cambrer, from Middle French cambre curved, from Latin camur
intransitive verb
to curve upward in the middle
see also
http://www.grc.nasa.gov/WWW/K-12/airplane/geom.html
The Space Shuttle has a low aspect ratio because of high speed
effects, and therefore is a very poor glider. The F-14 and F-111 have the best of both worlds.
They can change the aspect ratio in flight by pivoting the wings--large span for low speed, small span for high speed.
PS
Be sure to scroll down to the interactive Java applet that allows the user to change the geometry of a wing.

[tomsines27]

Hi Bill, Interesting, but that doesn't answer my question. Round and round we go.
Tom

[tomsines27]

Hi Bob and Bill, I guess I should have said, what do you think the best airfoil is for a hangglider for the best lift?
Tom

[Bill Cummings]

Hi Bob, My aeronautical vernacular is very limited, but there goes. What do you think the best upper and lower camber is for a hangglider wing to get high lift? Meaning a cut away of the cord, and a diagram.
Tom

Sorry but I don't know. Too much camber will have me flying slower with more lift but not enough penetration (against a quartering head wind) to make it to my LZ which is 3.4 miles from launch.
I was hoping that the NASA link I posted would allow you to experiment with different camber to get your answer.

[tomsines27]

Hi Bill, No problem, I think Bob can answer my question.
Tom

[Bob Kuczewski]

Hi Bill, No problem, I think Bob can answer my question.
Tom

Hi Tom,

I don't really have an answer either. That's like asking "What's the best shape for a car?". There are too many tradeoffs involved to really give a good answer. That's what the design process is all about. That's what makes it a challenge.

I once saw a great cartoon about what the ideal airplane would look like to a structures guy and an aerodynamics guy. You can imagine that they were quite different!!

I also remember a story one of my professors told about aircraft design. He said than an aircraft company did a survey to find out why pilots preferred high wing or low wing airplanes. The survey found that there were clearly two large camps - those who preferred high wings and those who preferred low wings. But surprisingly, their reasons were both the same. In both cases (whether they liked high wing or low wing) the pilots justified their choice with "better visibility".   

The conclusion is that it depends on what you want to see.