Reading a bad hi-start launch

[Hytram]

Firstly... total newbie here, so be gentle :D

before today I have only had about 1/2 a dozen Hi-start launches and for the last 12 months I have really only flown slope.

I headed up to my parents place who live on a farm and took my windrider Bee (chevron wing sloper) and my 2nd hand but unflown my me, Ricochet 2.5mt full house glider
http://members.optusnet.com.au/~rlearmont/SportRico.html (http://members.optusnet.com.au/%7Erlearmont/SportRico.html)

I had never flown the Bee off a Hi-start and had a bit of fun with it, more trying to catch it on landing than stay in the air, but by the end of the day I could keep it in the air for a couple of minutes.

The problem I had was with the Ricochet . The first 6 or so flights it hi-started great. there wasn't much in the way of thermals but I was having a ball landing it without breaking it, something I cant seem to do so well on the slope :o But the thing I noticed was that it was tail heavy so I decided to ad a little more nose weight, the cg moved about 5mm forward, this is where in when pair shaped.

I let go and the Ricochet rotated sideways about 30 degress on the hi-start and it popped off and head straight for the ground, it was only about 10mt (30ft) above the ground, I was lucky enough to get it almost level but still put the wing tip in. A couple of broke clevages was the only damage.

I had the exact same thing happen to another 2.5mt 2ch glider when I first started fly a year ago.

after doing some research in here, moving the tow hook forward seem to be one solution but there is other arguments that the CG has nothing to do with it, the aerodynamic centre is more important. could it be as simple as putting some down elevator on during launch? was the slew to one side caused by the the tow hook being to far back or simple operator error in regards to the control surfaces? I don't really want to move the hook forward if I can help it as it has been done once before and some strengthening will be need, but if I have to I will.

also, would adding flaps to the launch help or hinder things

and while you are at it, how long is a piece of string :rolleyes:

thanks

Marty

[Resurgam]

Well, behavior on tow depends partly on the relationship between the CG and the tow hook position. The further back the hook is relative to the CG, the steeper the climb. If it's too far back, things become first twitchy, then unstable. When you moved the CG forward, it had the same effect as moving the hook back ...

If you like the CG where it is now, I'd move the hook forward 5mm. Alternatively, dial in some down trim for launch (then take it out for the rest of the flight!) This is easy if you have a computer radio, more difficult otherwise.

[Robglover]

Marty -

The CG is pretty much irrelevant to towhook position, the towhook will become the cg when it's pulling hard on the plane. They are related, but not the same.

The simple answer is that a little down elevator will cure your problem. Moving the towhook forward will also cure it.

Dropping the wing trailing edge 15-20 degrees will help gain more altitude on launch, and will result in yet another "happy" position for the towhook/elevator.

Have fun, don't hurt your plane.

[mocgp]

Broke the clevage, huh. Well, if you were talking about bosoms, I think you meant cleavage, but I'm guessing that you meant clevises. At any rate, thanks for the malaprop, it was a good belly laugh. :)

[BMatthews]

A touch of down will help. Remember that when you added that nose weight you would also have had to add a touch of up trim to restore the glide trim. That touch of up may have been all it took to make the model rotate too far and result in too high an apparent angle of attach and a stall of the wing. When models suddenly pivot for no reason on the line it's (almost) always due to a stall or near stall condition of the wing.

Throwing it up and along the line of flight like your house mortage is on the line can help a lot as well. I've seen a lot of models destroyed from whimpy wuss releases or throws but I've never, ever seen a model come to greif because the flyer threw it too hard at the launch. The line pull induces a HUGE apparent wing loading on the model and that means it needs to be at or over the suddenly much higher stall speed for this apparent wing loading. That means you toss it like it's the Olympic javelin competition and the gold medal is at stake.....

[kablair]

A touch of down elevator won't hurt, but I suspect the rotation is due to the plane not starting straight down the line. This can be due to a combination of the toss (javelin throw) and the wind.

I almost lost my 'moldie' due to a wimpy toss yesterday. After several winch launches getting the club trainer (a Tower Vista) trimmed out - which required a very light touch on the pedal; I stepped up with my Hera. Too little tension on the line and a weak toss sent it left about 50' and only 10' off the ground. I stayed on the pedal and finished up with a respectable zoom to about 450', but it was a bit scary there.

Toss it straight and firm!
-Keith

[glidagida]

Hi Marty

What you are describing is a tip stall on the early part of the launch. This is caused by the wing trying to fly at an angle of attack which is too high and the wing stalls, and usually the inaccuracies in construction will make one wing stall before the other and the result is usually a violent lurch to that side. Keep trying and it will keep on lurching to the same side. Your stall was quite violent as you hit the wing tip and to continue in that configuration may spell disaster.

So how do you cure it? If my learned colleagues will excuse me for being overly cautious, as you say you are an absolute newbie there is only one solution that is safe for you and that is to move the towhook forward of the CofG [Centre of Gravity].

While it is possble, and sometimes desirable to have your contest ship so finely tuned for maximum launch height that you need to feed in a little down to kill the [expected] tip stall, this is most definitely not what you need as a newbie. The slow reactions/clumsy fingers of a beginner mixed with side/rear gusts on launch mean that you need a safety margin.

From what you said the only change from a model that launched well 6 times to an uncontrollable stall was moving the CofG. This means that the Aerodynamic centre/tow hook relationship was not changed, yet your model stalled, and the conclusion is that it is not this relationship that is governing your problem.

The relationship which changed was the towhook/CofG. The 'rule of thumb' is to have your towhook 3-6mm in front of your CofG. To locate the CofG make a 1metre loop of string/twine and insert it under your wing before bolting it on. Suspend the plane upside down with and move the string until the fuselage balances level with the string 90 degrees across the fuselage. Mark this point with a small texta.

Transfer that position vertically to the bottom of the fuselage and check the towhook position. If it is on or behind the CofG then that is the problem and simply move the hook forward so that it is at least 3mm in front of the CofG.

It helps as a newbie to have some help from knowledgable flyers, so why not join a glider club?

Cheers
GG

[Bryan Quick]

To obtain an easily flyable plane on tow, you want the towhook further ahead of the center_of_lift when the plane is on tow. Starting with the plane trimmed to fly faster, i.e. down elevator, will help. As stated earlier, your towhook was probably just ahead of the center of lift of your wing planform causing a tip stall.

To answer your other question,to optimize for height you will probably drop your trailing edge accross the span about 20 degrees and move the tow position back to just ahead of the center of lift.

I suggest you get experience with the plane on tow before you experiment too much.

Bryan

[ejett]

Marty, Your post above says "I let go and..". You must always throw the glider forward when high start launching, not just letting go. If the glider did not "pop off" the line, then you just may not be getting a good enough throw on the launch. Making sure the plane is flying when it leaves your hand is critical.

Another thing is that if the plane was flying good at the slope, it may not have actually been tail heavy when you were flying on the thermal field. Many times when flying your plane, neutrally balanced, at the thermal field, the tail will drop which looks like "tail heavy". It is when you fly through sink because the falling air is pushing down on the tail. So, you may have been flying through some sink that caused you to think the plane was tail heavy. When you fly this same, neutrally balanced, plane through lift, the tail will come up because the rising air is now lifting the tail.

EJ

[BMatthews]

Ejett makes a good point. If you're just holding it level and letting it go and rely on the model nosing up on it's own then it may just be overrotating too quickly and that's causing you grief when it overrotates, slows to the accelerated stall speed and suddenly pivots.

This is the sort of thing you can get away with on polyhedral models for the most part but when you start working with models that are a bit faster flying and heavier loaded and especially having ailerons you need to do what is needed to ensure that it has airspeed.

It would be nice if someone was to post up some videos of a proper javelin launch technique sometime for folks to look at.

[Hytram]

thanks guys, I will reply in greater depth tonight, when the boss inst looking over my shoulder :)

Marty

[flyonline]

Having been given a friends f3J plane and told "knock yourself out" I know how it feels to stand at the end of the winch with shaking boots!

If you don't want to do any surgery to move the tow hook forward, I'd suggest that you bend one up and tape it on. I've hi-started a 2M with a taped on hook no worries, so I reckon it would work fine for you as well. Having someone else to launch for you can help as well, though this may not be possible always. Also launch forwards like a normal slope throw rather than straight up for starters, this gets the wing lifting first, then you can rotate back for height.

Steve

[aeajr]

A good strong throw is critical to successful launches.

Adding the nose weigth may have resulted in the hook being behing the CG which will make the launch hard to control, especially if you don't throw the plane. If you want to try and visualize this, imagine if you put the hook on the tail. As the tow starts the tendancy would be for the tail to come forward.

Well, to a small extent, having the hook behind the CG has a similar effect.

[Hytram]

After reading all this again..

Yes, a girlie throw is half to blame and I think a little added up on the elevator might have not helped in things either. I was basically just throwing it the same speed as the hi-start was taking it.(the vid below is from an earlier launch, my 10yo was behind the camera) I will have to check were the to hook is located in relation to the CG.

I might order a movable tow hook and install it, have it a little forward until I get it right.

This plane is more a sloper than a soarer but I when there is no wind, guy got to do what a guys got to do, and it was quite enjoyable and relaxing, sloping sometimes is far from relaxing :)

http://youtube.com/watch?v=qirrVNF_10c

also some of my other stuff
http://youtube.com/user/Hytram1


thanks all again

Marty

[aeajr]

Good video.

I sometimes hi-start or winch my slope planes too. In fact I have one slope site where I have to use a hi-start or a motor in order to get to the lift.

[BMatthews]

I checked You Tube and oddly enough out of the few launch videos I could find there wasn't one that I'd consider safe on a hotter sort of model.

Let's see if I can describe now I do it and I've seen other folks with heavier and higher performance models do it.

First, since you're right handed your left foot goes forward and points along the line. Your right foot is positioned to the rear directly along the towline and at 90 degrees to the line. Space your feet quite a bit wider than your shoulders but not uncomfortably wide. You're after the classic Judo or Kung Fu stance. Seriously. Knees should be bent slightly. If you're using a winch instead of a histart then your left foot is on the switch but with toes only. Try to keep most of your foot on the ground.

Now you hold the model under the wing or just behind it if there isn't enough grip under the wing. Hold the model back behind you with the nose pointed up at 20 to 30 degrees. Your chest is sideways to the line and the line should be crossing the FRONT of your chest. Like the shaft of an arrow in archery. Your left arm with the transmitter should be pointed low and forward with the antenna off to your left to avoid snagging the line. Your right arm should be back and pretty much straight or even slightly angled down so that the tail of the model is perhaps 12 to 18 inches off the ground with that nose pointed up. You now prepare for the javelin throw.

Try to use a bit of Zen to ignore the line's tension. You need to THROW this puppy as though there's no line pulling on it and as if you're trying to handlaunch it up to a good 35 to 40 feet. This sort of effort requires a coordinated lunge from your right leg, chest, shoulders and right arm all working together. When it's time to go lean back so more of your weight is on that back foot and tense your body and then THROW your glider like a javelin along the line up and forward. Your arm should arc up and over as your leg pushes and your chest pulls and rotates your shoulders around and your arm comes up and over and forward like a pro baseball player but instead of a round house follow through you are pushing the glider up and out. Release the fuselage partway through the arms forward extension before your arm reaches it's limt and let the line's pull take over. A healthy Kung Fu grunt or yell doesn't hurt either.

When done right the model will climb right away at almost a vertical path. Almost no line tension will be lost to bringing the model up to speed since you've provided what is required. Also those stalls caused by premature rotation to too high an angle of attack too soon will be a thing of the past.

I once saw a picture of Joe Wurts launching his F3B sailplane at some world champs or team trial. Someone else was standing on the winch pedal so Joe could concentrate on the launch timing. The picture of him and his glider showed it about 3 feet from his outstretched hand and going straight up. Meanwhile Joe's feet were about 2 feet in the air. I think that should give you a good idea of what the hotter gliders require for a high power launch. But rest easy. Your model won't need THAT sort of commitment... :D But actually speeding it on it's way with a good throw rather than merely releasing it will go a huge way to avoiding the launch moment stalls and pivots into the ground.

Good luck!