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Author Topic: help a total novice  (Read 1072 times)
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miloman
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« on: September 07, 2016, 03:14:01 PM »

I thought i was a good builder but......! i have an eight inch cat glider. When I launch it (at about a 30* angle it goes about five feet up (fast) and immediately straight down (fast). I've added nose weight but it still heads down immediately. I guess something major is wrong with the model. The same thing happens when i just hand toss it. Any help you can offer? milo
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glidermaster
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« Reply #1 on: September 07, 2016, 05:23:48 PM »

Well, if it really is only going up about 5 feet, you are not giving it any chance to 'recover'.
When a glider runs out of launch energy, and the nose is still pointing up, it will stall, put the nose down, and dive. If it is at good height, and is set up properly (i.e. centre of gravity in the right place and relative wing to tail incidences set right) the nose will pull up as the speed increases in the dive after the stall. This tendency is what we call 'recovery'.
So, from what you described, it either needs more height, or a lower launch angle.

If it launches to good height, stalls, then arrows straight back down without showing any tendency to recover, then try bending the back of the h.stab up (up elevator). Go easy, though, catapult gliders are sensitive.

John
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calgoddard
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« Reply #2 on: September 07, 2016, 07:52:45 PM »

Catapult launched gliders (CLGs) require special attention to fly correctly.

They are launched at high speeds well above 50 mph but then must glide at far lower speeds, e.g. 5 - 10 mph.

There is much written about trimming CLGs, probably on HPA.  You'll hear about zero-zero incidence. This is the supposed combination of incidence of the wing and the stab that will prevent looping and diving.

In a nutshell, the decalage, i.e. the difference between the angle of attack of the wing and stab, that normally produces a nice smooth glide in a hand launched glider (HLG) is often incompatible with a high speed launch. It will typically produce a loop at high speed.

So first make sure your glider will glide nicely with a horizontal hand toss.  

Then try a low power catapult launch at a low angle.  You might have to adjust the incidence of the stab to prevent looping or diving when you increase the angle of catapult launch.  A good launch requires a precise predetermined combination of azimuth angle, roll angle and degree of pull that varies for each glider. This is found by experimentation.

Indoor gliders are sometimes configured as "flappers."  The aft half of each side of the wing is not connected to the fuselage.  It is typically made of very thin A-grain balsa wood (e.g. 0.020") that deflects to a flat shape at high speed and then returns to its curved shape to produce a low speed glide.  Thus the wing is more or less flat at high speed and the glider does not loop.  However the wing returns to an efficient airfoil shape at low speed to produce a nice glide.

CLGs launched in very high indoor sites, and CLGs launched outdoors, are typically not flappers.  Flappers are very delicate and are difficult to launch more than 30 - 40 feet up.  An expert (e.g. Stan Buddenbohm) can easily launch an outdoor glider higher than 100 feet. Many outdoor gliders have a DT to avoid flying away in a thermal.

Tiny amounts of fin skew (e.g. 1/100" - 1/64") are used on CLGs to help with the transition at apogee. Stab tilt is very important in that it ensures a nice glide circle.

I am not an expert at CLGs by any means and perhaps others can direct you to a nice set of instructions for trimming.  I recall that the first four indoor CLGs that I built never flew correctly.  Finally my fifth CLG flew correctly, but I did not know about right-right versus right-left for indoor gliders.  My fifth indoor glider flew right-left, and it was hard to keep it from hitting the walls of the gym.  Better to go right-right indoors and right-left outdoors.

Good luck to you.

 
« Last Edit: September 07, 2016, 08:08:46 PM by calgoddard » Logged
miloman
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« Reply #3 on: September 07, 2016, 11:00:33 PM »

Thanx guys. I'll forge ahead.....Milo
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« Reply #4 on: September 08, 2016, 05:55:24 AM »

Post a side view photo. Try to fill the frame to show the complete model.  Seems, from your description, that the decalage is way off.  Where does it "balance"?
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calgoddard
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« Reply #5 on: September 08, 2016, 05:52:37 PM »

I forgot to mention there are two types of transition for a CLG at apogee.  In the conventional flight trajectory the CLG will roll about the longitudinal axis of its fuselage into a more or less horizontal attitude and commence its slow speed glide.  In the "bunt" type of trajectory (used by world record holder Bill Gowen) the glider will not roll about the longitudinal axis of its fuselage but will instead pitch downward from a steep inclined angle into a horizontal attitude and commence its slow speed glide.  At least this is my understanding of how some of Bill's CLGs fly. I have not attempted to trim a CLG for a bunt type of flight pattern. I think that CLG flight pattern is best suited for indoor Cat I flying sites. 

The best way to get trimming help via HPA is to post a video of one of your flights on Youtube and then post a link to the video on HPA.

It sounds like the first thing you need to check is to confirm that the CG of your glider is at the location recommended on the plan.      
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miloman
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« Reply #6 on: September 08, 2016, 10:16:14 PM »

Great idea about posting a video. I'll give it a try when it stops blowing and raining.....Milo
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calgoddard
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« Reply #7 on: September 09, 2016, 05:32:04 PM »

I made a mistake in Reply #2.

It should refer to "elevation angle" not - - azimuth angle - -.

Sorry about that.
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miloman
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« Reply #8 on: September 21, 2016, 12:46:47 PM »

Finally go it flying well! Thanx to all for the help.....Milo
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calgoddard
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« Reply #9 on: September 21, 2016, 02:32:50 PM »

Congrats!
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