EF

I heard that when using hi-start the correct starting point is when the tension is 4 times the model's weight or 3 times when the wind is strong.
Is this correct or should I tension the cable only the specific distance stretched?

RRW_SOAR

Check this [link=http://www.charlesriverrc.org/articles/design/dickwilliamson_histartphysics.htm]link[/link]

da Rock

The link is certainly very informative, but your question can be answered with fewer words. Unfortunately, I'm not real good at doing "fewer words."

I've launched every size glider known to man off almost every size high start. Every combination works AFTER you've gained enough experience. Some combinations are just riskier than others. Some risk the airplane, others risk a poor launch. That said.......

Your strength is an important aspect of the deal. How much you can hold with your launching arm decides a lot. But none of this is rocket science. The answer to your problem lies right in front of you when you're at the field. And you can easily, quickly, and safely find your own answer. And truth is, we can write a ton of words with all manner of numbers and you won't have any way to apply the numbers, will you. None of us do.

Hook up your glider and start walking back. Keep an eye on the line and rubber. You ought to do that every launch to insure nothing snags. It's easy for the system to snag and break. It's also easy for the system to snag an you still try to launch because it feels tight enough. Watch the line and rubber and as soon as the line lifts off hold the glider up as you would when launching. Does it feel tight enough? If you don't know yet through inexperience, at least you have a perfectly reproducable situation to use as a learning point.

At that first "test position" the bungee probably isn't pulling max is it? So hold the glider up as if you're going to do a test/trim glide. You know those trimming tosses you did earlier to see how the glider glided? Same deal now except you're not going to jog and throw. You're going to let the bungee test glide throw for you. Point the glider at the horizon and let go. Make very sure it's held well above your head before letting go. Expect the bungee to have insufficient energy in it and that it is going to exhaust it almost right away. Be prepared for your glider to come off with not very much airspeed and with a noseup attitude. If any of this seems risky to you, don't do it. Walk back more. How much?

The next "test position" with the bungee is when you've walked back far enough that you're having some degree of difficulty against the pull. You've been watching the line and rubber each step of the walk, right? The rubber should be obviously stretched. You feel it right? Scratch a line on the ground or drop something to mark that extension. (Don't drop the TX to mark the spot even if it is the only other thing you got in your hands.) If you feel like the rubber is going to pull you over when you try to hold the airplane up in launch attitude, you may want to walk a little forward. If at any time you think it's too risky to do, then don't.

I've launched 2M gliders on HEAVY DUTY bungees with my veins popping. They had what I'd consider normal construction in the wings, spruce spars with 100% spar webbing. They took the launch with no protest.

now.................

da Rock

Now..........

The one underlying idea in that longwinded thread is simple. If you feel like what you're about to do is risky, don't do it.

Finding the right tension for your glider on the bungee you're using is really very, very simple. Try something. But try something that seems safe. With almost every glider/bungee combination it will be.

If you haven't done any of this before, then you are going to have to develop a bunch of skills anyway. You need to feel how strong to pull the bungee for the airplane. Best way is to try any feel at all. You need to know where to point the airplane. Read all the advice you can, pick something, and try it. Best way is to try it. You need to learn how to fly the airplane up and over. You're about to get started learning that. How else would you do that otherwise?

Good thing is that most gliders will stand way more heavy handed launching than most beginners would try. Most beginners start out not stretching the bungee enough. Which makes this exercise almost fool proof.

da Rock

BTW, most "measurement" based HowToGuides on launching are pretty funny. Have you ever seen a pull scale being sold as field equipment for newbie glider flyers? Most flyers don't even know for sure how much their glider weighs in pounds. They'd be absolutely lost as to what 3 or 5 times that weight would be. And I don't know another glider flyer who takes a pull scale to the field and does pull tests on his bungee. (I've got one and done it more than once, but I'm not normal.) So even if they MIGHT be able to quote a pull weight, how would they go about doing something with that info.

Don't let this stuff get silly. Go out and try something. If you think while you're doing it, it's amazing what you'll learn just trying stuff.

And consider this: Glider flying ain't smart anyway. We expect to fly airplanes with no means of propulsion, right? And some of us take our powerless planes to big old cliffs and THROW THEM OFF. Hey, if somebody tells you that what you're doing is stupid, don't laugh. It is.

But by golly it sure is fun.

EF

Thanks darock for all this info! will definitely give it a careful try.

I am flying an Airtronics Peregrine which I built from the old kit, found it by coincidence, and I have so far only flown it from the slope.
I have however fitted the provided tow hook and would like to try some flat field soaring.

I did fly from a hi start 20 years ago but only a Graupner Mosquito on a simple and very small hi start. Was OK but not spectacular.

My major activity is in scale models but do enjoy an occasional sailplane build and some "pure" flying. No electrics for me.

Also have a BOT kit I plan to build soon and a Graupner scale LS-4 which will combine scale and soaring for me.

It seems I have much more access to flat fields than slopes, so decided to give it a try.

Fxrs_tim

You don't have to worry about abusing the Peregrine on a high start. I fly an old Sapphire (very similar) and routinely pull a heavy duty highstart back 100 paces of tension, or until I can barely hold on to the plane. Assuming everything is trimmed (elevator compensation for flaps is critical), drop your flaps 15-30 degrees depending on wind (less wind = more flap), and let her rip. As you near the top, go to reflex, come over top, dive *slightly* for speed, then zoom off of the tow. Just before it stalls, level out and return wing to normal configuration. After that, you're on your own to find the lift.

Good luck w/ it - you'll enjoy that plane a bunch!

Tim

daveosoar

Ho Ho (well its near Xmas) All the above is right on. Just give it a try. Too little tension is more of a worry than anything else. Just remember that if you have ailerons and rudder DONT use the ailerons in the early part of the launch, just the rudder to correct the yaw, once straight up the line anything goes. Whilst it aint rocket science it seems, to a newbie, to 'rocket' up. Give yourself a little time (0.005Sec) to think.
It will go.o.oo.o!
Dave.

SoCal GliderGuider

I always back up till the rubber breaks then take two steps forward.

Sneasle

Nice plane EF.

When you start working on your BOT would you start a build thread? I am considering it and would love to see a build thread on it.

RRW_SOAR

There's a very nice BOT Building Log on [link=http://www.rcgroups.com/forums/showthread.php?t=518499]this [/link] thread, although it's a scratch built BOT.

Anyway, have fun

Liberator

We use a fish scale and back up until we find the sweet spot.

Recently I lost my Thermic 2.5M because we had the tension just a tad too strong.

The plane weighed 31 ounces and I had it pulled back to about 22 lbs. The plane can absolutely handle this much tension. Been doing it for months. The problem is that with that big a difference the plane comes out of your hand like a freakin rocket. Getting out of the way of the plane is an art form unto itself. THe tail caught my Brothers hand and the tail feathers blew up. It stuft in under full tension and needless to say, there is not much left. Long story longer, use a fish scale and keep going back until you think it's good, just test it at each point so you don't blow it up.

thx408

thank you very much . you have lightened the load, you just keep going and try again. iam new to rc gliders .fly a easy glider change that i crash the glider not that easy .but iam haveing fun doing it .yes it does fly 2 and a half min .ill work my way up .but at 50 yr young ill learn to fly with grandkids. thanks again.

Bax

My Feedback: (11)

Long time ago, I had an Astro-Flite hi-start that acually used bungee cord instead of rubber tubing. It would launch my 60-ounce AFI ASW-17 like nobody's business...would get very high launches. I don't know the diameter and length of the bungee or the length of the string, but would like to know if anybody has that information. I'd make a new one that was similar...loved how it launched the larger models.

daveosoar

When we had no money(not much has changed!) we used shockcord(bungee rubber) at the same rate - 30M of cord,150-200M of 100lb nylon monofilament fishing line - because it stre-e-etches. The s/cord is much more immediate in pull but with a stretchy line is controllable, initial acceleration is greater than tube, more like a winch so check your wings.
Dave.

aeajr

My Feedback: (2)

Well, we all have an approach and opinion, here comes mine.

I actually use a fish scale to measure pull. I did not start with the scale I used it after I found the comfortable launch to see what it was.

My experience is that you want 3-6X the weight of the model. I have launched my 2M Spirit at 12 pounds many times with no ill effects. Frankly that is stronger than I would recommend.

If I had to peg the "perfect" measure it would run like this:

I want a hi-start to offer a pull equal to 3-5X the weight of my model when it is pulled 3X the rest length of the rubber. That will give me a good pull over a long period which will yield a good launch.

For the Spirit that would be 6-10 pounds pull. I know I like this plane launched at 8 pounds on a full lenght pull. However I often launch it at 12 pounds using my 4M hi-start pulled 150 feet 1.5X its rested length. I can't hold any more than that without crushing the plane. The launch is very fast and I get some wrinkle of the monokote which indicates some stress to the wings.

When I launch my Airtronics Legend, weighing 5 pounds, I pull that 4M hi-start about 250 feet and get about 25 pounds of pull. That is very hard to hold but it launches the plane pretty well.

If you have a head wind you need less power to get a good launch. In my opinion 3-4X work great with a 5-7 mph wind. 5-6X are helpful if there is no real head wind.

Your mileage may vary.

daveosoar

Ed.,
I use the sub-tension on my mistress's (note the plural) bra straps. Twang for buck?
Dave.