[Bill Cummings]

Inspect your right rear flying wire near the thimble and NiCu.
What is causing that print on the the right back side of the crossbar about a foot right from the hinge point?

[tomsines27]

Hi Bob, Thanks, excellent intormation and pictures. I will study this for a while, and have a few questions later.
Tom

[Bob Kuczewski]

Inspect your right rear flying wire near the thimble and NiCu.
What is causing that print on the the right back side of the crossbar about a foot right from the hinge point?

You've got an "Eagle Eye" Bill!!

I'll check them both out the next time I set up the glider. Thanks.   

[Bob Kuczewski]

Hi Bob, Thanks, excellent intormation and pictures. I will study this for a while, and have a few questions later.
Tom

You're welcome Tom. Let me also apologize that it's taken me so long. Thanks for your patience.

[SamKellner]

. What is causing that print on the crossbar ?

Probably the control frame, when it's in the bag and in transit.

Good place for a carpet pad.

[Bob Kuczewski]

Good place for a carpet pad.

I think you're right.

Some of my glider pads have disappeared over the years, and I should make an effort to replace them.

Thanks for the reminder!!!     

[tomsines27]

Hi Bob, I have a two part question I'm certain you can answer. What is a good low takeoff speed, high lift wing, and where can I find a detailed diagram?
Tom

[Bob Kuczewski]

Hi Bob, I have a two part question I'm certain you can answer. What is a good low takeoff speed, high lift wing, and where can I find a detailed diagram?
Tom

Hi Tom,

That depends on a lot of things.

Here's the general equation for lift:

  Lift   =   q   x   CL   x   Sw

    "q"  is dynamic pressure and it is 1/2 of the air density times the velocity squared ( 1/2 rho v^2 )
    CL  is the coefficient of Lift
    Sw  is the surface area of the wing

Of course, CL also depends (very highly!!) on your angle of attack.

You obviously want to generate enough lift to carry the weight of the pilot and the glider. So that gives you one number. You can measure the area of your wing and get the local air density (be sure to compensate for temperature and altitude). That gives you two other numbers fairly easily. But CL is the tricky rascal. In fact, when I worked in a wind tunnel, one of our most important "products" was a graph of CL as a function of alpha (angle of attack). That graph included the stall point which of course limits your angle of attack.

So your "takeoff speed" must be larger than the speed required to generate enough lift (via that equation) to overcome the weight of your pilot and glider. In fact, it should be considerably higher than that to ensure that you have a safe margin above stall to handle any changes due to gusts and exiting ground effect.

Note, by the way, that your lift (and other aerodynamic forces) goes up with the square of your velocity. This is very important for all pilots to understand. In other words, your glider may be nicely manageable on launch in a 12 mph breeze. But if the wind speed doubles (to 24), then all of the aerodynamic forces will be quadrupled. Unfortunately, neither your muscles nor bones quadruple their strength due to wind speed. This is something that many new pilots don't understand and it can get them in trouble very quickly.

[Bob Kuczewski]

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 (6.75 KiB) Viewed 4838 times

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.

[tomsines27]

Hi Bob, I've clearly come to the right place. That was a very good explanation of aerodynamics. Let me explain what I'm doing in more detail. I have been going to hangglider events, and building models for over a decade. lately, I have had a brake through. I have built a structure that is extremely light , and strong using carbon fiber tube, and aircraft cable, also tuning the cable to certain musical notes to increase strength. I will be building a sit down hangglider, that will be powered by a PMDC electric motor with lithium batteries. Body weight will not shift from side to side the way hanggliders currently do. Although batteries will move fore and aft to help balance the hangglider. It will also have joystick controls,and should weigh between 70 and 80 pounds with a wing span of about 30 ft. I have no plans to fly more than a few feet off the runway to take full advantage of ground effect. Also, there will be a lot of taxiing to make sure the craft is stable. So, I repeat, what is a good low takeoff speed, high lift wing, and where can I find a detailed diagram?
Tom