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Outdoor Free Flight Forum => Catapult Launched Gliders => Topic started by: ScienceGuy on April 27, 2018, 01:59:44 PM



Title: Adventures of Getting a Gllder to Transition Properly
Post by: ScienceGuy on April 27, 2018, 01:59:44 PM
http://scienceguyorg.blogspot.com/2018/04/free-flight-hand-launch-glider-story.html (http://scienceguyorg.blogspot.com/2018/04/free-flight-hand-launch-glider-story.html)

I have had a heck of a time getting a certain glider kit to fly properly, after many changes I had it launching fairly well. On the best launch it climbs in a thermal, flies for awhile and spins into the blacktop. More details in my link.

Bill Kuhl


Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: strat-o on April 27, 2018, 02:26:20 PM
Interesting read.  Actually you may have, in the end, achieved fairly good trim.

I am surprised that there was no mention of stabilizer tilt.  It's common, if you want your glider to transition to a left turn, to set the left side of the stabilizer high by about 1/8".  The stab tilt is especially useful for gliders with wide speed ranges because:
1. It helps maintain the desired left turn during the glide.
2. During high-speed flight right after launch the effect of stab-tilt doesn't seem to intensify like you might get with, say, a bit of left rudder.

Also, a lot of folks like to add a bit of clay to the top of the left wing tip.  This is done so that when the glider stalls at the top, it's supposed to help the left wing drop and this helps establish the left turn quicker.

Marlin


Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: ScienceGuy on April 27, 2018, 02:30:30 PM
I did forget to mention that, it did have stab tilt and I did try clay also.  Someone with way more experience than myself had trouble getting this design to fly.

Last year I built a Stan Buddenbohm Scout, super simple design but it flew so well with almost no adjusting.

Bill


Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: flydean1 on April 27, 2018, 08:51:09 PM
Bill, you did not mention a wash-in wedge.  That's a Buddenbohm development that everyone uses.  That should stop the spinning in.

Everything you wrote on your link sounded like the CG was too far aft.  It doesn't take much, sometimes moving it forward 1/8 inch makes all the difference.  Allows more nose-up trim which helps the transition become more reliable, and helps it to recover from upsets in the turbulence of thermals.  Note that it generally glided well.   That's usually the sign that the CG is close, and a little more forward would fix it.  Lack of nose-up also is indicated by it's resistance to a right launch.  Also, when reducing fin and stab, it is more effective to clip the tips rather than the trailing edge.

Hopefully someone with real insight like Leeper will weigh in with better advice.

You can't go wrong with anything from Stan Buddenbohm.


Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: Rewinged on April 28, 2018, 04:15:48 PM
Bill,

I think the important things have been covered, and you have tried most, if not all, of them.  The final key is the proportion of things.  I hesitate to respond because I think you already know all this, but FWIW, here is my approach, which is pretty robust (I learned much or most of what I know from Stan, plus Leeper and some others):

The initial build should include a little stab tilt and the washin wedge.  The washin wedge goes on the wing that is on the inside of the glide turn.  It is typically placed next to the dihedral joint, usually on the inside side of the joint, but sometimes it seems better on the outside, depending on where the dihedral breaks are.

The initial build should have the rudder straight.  Then, depending on glider size, tweak only the bottom of the rudder next to the fuse from 1/64 to 1/16 to the left.

Start with the decalage a little high, and adjust CG for the best glide.  (If CG is shown on the plan, start with it a little forward of where shown, and adjust the decalage for the best glide.)

Make slightly hard glide tosses, hard enough to confirm that the glider climbs and starts toward the glide turn.  Adjust decalage and nose weight, until you get a slight climb off the slightly hard hand tosses, with the required left turn in the glide.    It should only go slightly to the left on the initial climb.  Adjust the rudder tweak to make certain it goes to the left, but just slightly, on the initial climb.

If you are not using a proven design, or if you have changed anything, even re-scaling from a proven design, at this time you should adjust fin area.  Cut down the top of the fin, little-by-little, until you have a tiny bit of dutch roll in the glide.  Testing this can only be done in very calm conditions.  If the glider is for fun, or will be used in quite turbulent conditions, and it is only a tiny bit of dutch roll, you can leave the fin at this size.  You will get slight better performance if you add a tiny bit of fin area back. 

Proceed to cat launches.  45 degrees up with 45 degrees right bank is usually pretty safe if you have a little excess decalage as suggested.

Adjust the rudder for the best climb.  Again, only use the bottom of the rudder.  If needed, adjust the stab tilt for the glide.  For CLG, rudder is used to trim the climb, and stab tilt to trim the glide.

My preference is for max performance with relatively high risk.  So from this point, I start reducing decalage little-by-little, and also reducing nose weight to re-trim the glide.  This may also mean slight reduction in the rudder tweak for climb.  Launches get more-and-more toward vertical with this approach, and bank angle increases.  My best gliders end up launched at about 80 degrees up and with up to 135 to 170 degrees bank (yes, upside down).

This minimum decalage type of trim increases the possibility of spinning in.  However, the combination of washin wedge and absolute minimum fin area almost always prevents this.  I don't have a great approach for sizing the washin wedge.  I usually start with TLAR, based on previous models, and perhaps a little thicker than necessary.  If I can't get sufficient climb roll with a tiny amount of rudder offset, I'll sand the washin wedge down a little.  But usually it is OK; it seems to be the least sensitive of the things to trim.

--Bill





Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: OZPAF on April 28, 2018, 08:54:35 PM
Bill, you have been given some good advice here and RW's account is comprehensive, and I also hope that I am not .

I read your account and can well understand the frustration.

I take a different approach to rewinged in that I start from close to a 0 deg decalage. Thus after initial check trim glides to establish a bit of turn etc, my gliders will nearly always dive in on the first few launches. I regard this as good as it usually only takes a slight increase in decalage by adding a bit of up elevator. I already have wash in added to the inboard inner panel. I warp or tweak in the wash in( approx. 1/32"), and so it appears about mid way along the inner panel. Wash in wedges are possibly better and may be easier but not as easily adjustable.

After eliminating the dive to the ground - I keep adjusting the decalage till I get a bout a 1/4 to a 1/2 turn to the right before the transition to the left glide. Anything much less than this I find to produce inconsistent transition with out stalls. The rudder is used as well, but only enough to ensure a good transition. Most of my glide turn comes from tail tilt.

The launch angles need to be consistent during the trimming as well as the amount of pull on the catapult. It will then not work well for pull strength below or above this value and for launch angles largely different. I use about 70 deg up and 50-60 deg right bank as a rough guide.

Finally when the transition is god without a stall - I trim the glide by removing small amounts of nose ballast(pin heads of modelling clay) until it starts a mild dutch roll(slight wing rocking). I don't touch the elevator or wash in while trimming the glide. In calm conditions I have a very open glide of around 80- 100' dia.

Looking at the picture of your glider - 2 areas that would seem to possibly make it a little harder to transition are the low dihedral overall and the largish tail. I hope I'm not being offensive to the original designer but these are impressions taken from the photos.

I hope this gives some new insight and is not telling you stuff you already know.

John



Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: ScienceGuy on April 30, 2018, 03:58:27 PM
Thanks for any advice, your observation that the tail was too large must be correct as at least part of what I did to get it to transition was cut down the vertical fin first and then the stabilizer. To me it seemed like it was too stable, in some way but not completely.  I also remember that until I made the tail smaller if it was inverted it would not flip upright.

Also it had a rather long nose moment which I shortened.

Further flights will tell more now that it is closer to where it should be.

Bill Kuhl


Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: OZPAF on April 30, 2018, 06:56:31 PM
As a bit of a guide to what works for me - here is a 3 view of my current CLG. The stab on this is only around 13% of the wing and you can also see that I have a small fin and a fair bit of dihedral. The dihedral gives this model good roll response and good roll stability. The fin is just enough to trim the climb and help with the transition.
Another source of very good info is Tony Mathews - he has written a lot on HPA re CLG's and trimming.

Good luck and happy flying.

John


Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: Rewinged on April 30, 2018, 09:26:51 PM
John,

I am likely having a brain freeze after working too much, but what are the units for the dimensions on your drawing?  They are not making sense for me.  (I'm an engineer!  I must be losing it...)

Here's some pics of my current #1, since I don't have a drawing.  It's basically a combo of good CLGs from others.

--Bill   


Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: OZPAF on May 02, 2018, 09:10:38 PM
Bill it's not a very clear copy unfortunately, but the dimensions are in mm. If you are interested send a Pm with your email address and I can send full size template drawings as well as a better 3 view.

John


Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: ScienceGuy on May 03, 2018, 09:30:47 AM
[email protected] is my email

Thanks for the offer, might be awhile before I can build one as I am trying to finish up other planes.

Bill Kuhl


Title: Re: Adventures of Getting a Gllder to Transition Properly
Post by: Rewinged on May 04, 2018, 08:57:50 PM
John,

I was expecting it to be larger, which is why I had trouble with the dimensions.  I always like seeing plans, so PM has been sent.

Thanks!

Also Bill K