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Old Nov 02, 2009, 07:01 AM
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Hey, Ed, thanks for the timely advice. I'm probably going to the field this afternoon for more high-start practice.

Speaking of which, you and several others were right on the money with the advice I was given several weeks ago regarding "pull force" on the high-start.

As you may recall, I was not getting the height I thought I was expecting - even though I was stepping off the "recommended" distance. (I've got a 50' / 150' rubber/twine setup).

You suggested that I get a fish scale and go for about 4x the weight of the plane. As it turns out... "Perfect". Looks like I needed about another 20 or so paces, and what a difference that makes!!

Lauches nearly over the ground spike with little (if any) slack.

My thermal skills are next to zero, but now I have one less excuse.

Again, thanks a lot!

Best... Roger Parrett
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Old Nov 02, 2009, 07:58 AM
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Glad to hear this helped.

Once you get comfortable with your plane and the hi-start you will have two things you will want to do.

1) Start working the CG of your glider back from the recommended start point. This can be done by removing weight from the nose, assuming you have added weight to the nose. However for quick testing, adding weight to the tail can be much easier and give you a quick feel of the effect of moving the CG.

Try taping a dime near the tail. It is only one gram, but it is like removing 5 grams from teh nose on most gliders. See if you feel a difference. If that does not give you trouble, then add a second dime, or, if you feel brave, replace the dime with a quarter. But be prepared for the plane to be much more responsive. Before you launch, always do a hand throw to see how the trim sits. In most cases you will have to add some down trim ( or take out some up trim) as you add weight to the tail.

Also, read this article.
http://forums.flyesl.com/topic.asp?TOPIC_ID=255

If you find you like the way the plane flies with a more rearward CG, you will also find it thermals better. However, now that you have moved the CG back, you may find that you will want to move the hook back. But don't rush it. This is a process that should occur over several hours or several visits to the field. Don't rush it.

Then take a look at this article on maximizing your launch height with a hi-start
http://forums.flyesl.com/topic.asp?TOPIC_ID=257
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Old Nov 02, 2009, 11:23 AM
agony sweetns the victory
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Roger, whatever delicate balances you strive to achieve regarding the CG of the plane while it is in flight, those are ALL thrown out the window when it comes to launching your plane. What physics says is that wherever your towhook is, is also the CG when your plane is on launch. The forces on your plane during launch are orders of magnitude greater than the forces in flight. Think about it, adding or subtracting an ounce or two nose weight or tail weight to shift CG (is great in-flight stuff that needs to be worked out) will not effect how your plane will launch when the towhook in under 10s of lbs. of force from a winch line or hi-start. Changing how the plane will launch can be achieved by three variables (1) changes to your wing's trailing edge via camber inputs if you have this capability (2) changes to your elevator setting (3) towhook placement relative to your wing's mean aerodynamic chord.
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Old Nov 02, 2009, 11:32 AM
agony sweetns the victory
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Roger, another thought if you are repeatedly going to be using a hi-start to launch your plane, and you want to set-up your plane to maximize your launch settings, then you need to have repeatability in the towforces you apply to your plane. Hi-starts are notoriously bad at replicating the forces involved. Some say you can mark a spot on the ground and walk back to it every time and launch from that spot and apply the same forces. This is definitely not the case! I've done that test scores of times and they differ. In fact, hi-starts will change over the course of a day's use as to how much they stretch, and the next day you go to the field that hi-start's output will be different again. Go to wally-world, spend $10 and get a digital fish scale. Then you'll now exactly how much force you apply each and every time to you launch your plane.
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Old Nov 02, 2009, 12:17 PM
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Quote:
Originally Posted by atjurhs View Post
Roger, whatever delicate balances you strive to achieve regarding the CG of the plane while it is in flight, those are ALL thrown out the window when it comes to launching your plane. What physics says is that wherever your towhook is, is also the CG when your plane is on launch. The forces on your plane during launch are orders of magnitude greater than the forces in flight. Think about it, adding or subtracting an ounce or two nose weight or tail weight to shift CG (is great in-flight stuff that needs to be worked out) will not effect how your plane will launch when the towhook in under 10s of lbs. of force from a winch line or hi-start. Changing how the plane will launch can be achieved by three variables (1) changes to your wing's trailing edge via camber inputs if you have this capability (2) changes to your elevator setting (3) towhook placement relative to your wing's mean aerodynamic chord.
I won't argue the physics, I will argue the effects that I have seen over hundreds of launches and I do see that it matters. It matters because it changes the trim of the plane which changes how the plane will handle on the launch.

Based on your statement then it should not matter where I put the hook relative to the CG, but it does. I have seen how the launch will change by moving the tow hook without changing the CG and that tow hook location changes how the plane launches as it changes the piviot point, for lack of a better word, which determines the angle of attack on tow.

Perhaps I am not understanding your post but based on practical experience I don't agree with you.
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Old Nov 02, 2009, 01:03 PM
agony sweetns the victory
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aeajr, hopefully you are also changing your elevator's setting when you change your CG; however, if that new elevator setting has input into your launch mode (I don't know how your TX is set-up or what mixes you have or don't or carry from one flight mode to another) then yes you will see changes in your launch because of what I listed as reason #3. Elevator changes made for "normal flight mode" CG reasons should not coincide with elevator settings for "launch mode". The elevator setting for launch mode should be separate and specific just to launch mode.
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Old Nov 02, 2009, 01:11 PM
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Ahhhh launch mode. You are assuming the pilot has launch modes in use.

As it says in my posts, yes, I am expecting that a change in balance will result in a change in elevator settings.

My launch modes have no offset elevator settings. I launch with neutral elevator. I don't recommend elevator settings on launch unless it is to overcome some bad habit on launch.

As you cited above:

(1) changes to your wing's trailing edge via camber inputs if you have this capability (2) changes to your elevator setting (3) towhook placement relative to your wing's mean aerodynamic chord.

So, the CG setting on your plane will impact your elevator setting. If you override that setting with a launch mix then you may negate the effect of CG settings.

As for 3, tow hook placement relative to your wing's mean aerodynamic cord, frankly I don't know any beginner glider pilot who would be able to set their hook that way. Perhaps that is the more accurate way of doing it but it is beyond me and the scope of this discussion as to how one might find that location much less set to it.

This thread was created for beginners and newbies who are learning to use a hi-start. Often they are working on standard 3 and 4 channel radios or entry level computer radios. Calculatig MAC is typically not something this group will know or use. I know I don't.
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Old Nov 02, 2009, 03:07 PM
agony sweetns the victory
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Once again to review the physics, launch set-up is not effected by CG.

An ounce or so change in nose weight will mean nothing compared to the forces applied from a hi-starts. It's the elevator changes, but the elevator setting doesn't have to be changed just because you add or drop nose weight. If these are newcomers to the hobby then let's be accurate with our words.

Changing how the plane will launch can be achieved by the three variables previously listed. CG is not one of them. Also, the towhook reference point doesn't have to be the wing's MAC. It could just as easily be referenced to the leading edge or the trailing edge of the wing. I could have said the nose of the plane or the rudder hinge-line, but then the fuselage might get changed, so I'll just reference the wing.



Now boys, will you stop fighting and go play with your toy airplanes nicely.
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Old Nov 02, 2009, 04:05 PM
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Sounds good to me.
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Old Nov 02, 2009, 04:31 PM
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Ed, you said...

"This thread was created for beginners and newbies who are learning to use a hi-start. Often they are working on standard 3 and 4 channel radios or entry level computer radios."

I got a good chuckle from that one. As a "pretty decent" nitro pilot and fairly good with computer radios (got a 9CAPsuper for the Spirit Elite, and a Futaba 12FG for the nitro), and the fact that I've got a full house sailplane, let me tell you I programmed the heck out of it.

I had functionality for EVERY switch and knob on that 9CAP - you have one so you know...lol... Oh, that top left momentary switch? Had it programmed for full-up ELE / full left RUD "just in case I got sucked into a thermal" and had to get down. Dual rates galore. Ditto for expo - just in case, of course...(wait, it gets better...)

Modes? I had four. Normal, Launch, Speed, and Reflex. Var-knobs on each one? Of course. Crow? Naturally, and with the timed three-point elevator curve for good measure...

(laughing yet?) Then I had AIL->Rud mix set, and well as full-span ailerons.

So here I am - ready with the high-start, scanning my radio for the proper switch settings like a NASA shuttle pilot. Launched the plane, fiddled slightly with the rudder for straight tracking, and then spent most of the time obeying gravity in search of a thermal using rudder and elevator.

Did I use any of the fancy set-ups? Hell no...lol. I was too busy inching my way around 150 feet up. Oh, I think I did play with Crow prior to landing. Once.

My new radio settings? No dual rates, no expo, no mixing, no flight modes, no crow (but the left stick DOES drop the flaps).

Hope you had a good laugh. A newbie is a newbie, regardless of previous nitro experience. The only thing it gives you is the ability to bring a plane out of a tip stall...lol

Best... Roger
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Old Nov 02, 2009, 04:41 PM
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I have two 9C Supers myself. One on 72 and one on 2.4 Spektrum.

Yes, we can be experienced pilots and know all about our craft, but when we approach something new, like a hi-start, we become newbies again, at least for a little while.

Add to that the learning process of seeking unseen forces that will carry our craft to thousands of feet in altitude without the aid of a motor and we are as children seeing the world as for the first time.

Glad you were able to launch successfully.
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Old Nov 03, 2009, 07:08 PM
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Quote:
Originally Posted by atjurhs View Post
Roger, whatever delicate balances you strive to achieve regarding the CG of the plane while it is in flight, those are ALL thrown out the window when it comes to launching your plane. What physics says is that wherever your towhook is, is also the CG when your plane is on launch. The forces on your plane during launch are orders of magnitude greater than the forces in flight. Think about it, adding or subtracting an ounce or two nose weight or tail weight to shift CG (is great in-flight stuff that needs to be worked out) will not effect how your plane will launch when the towhook in under 10s of lbs. of force from a winch line or hi-start. Changing how the plane will launch can be achieved by three variables (1) changes to your wing's trailing edge via camber inputs if you have this capability (2) changes to your elevator setting (3) towhook placement relative to your wing's mean aerodynamic chord.
I believe the term I would have used - during the launch mode - is that where the towhook is, is where the pivot point is. The CG doesn't change during launch anymore than it would change during a dive or steep climb into the wind. After launch, the pivot point becomes the CG, but not vice-versa prior to launch.

That said, mounting the towhook a bit forward of the plane's natural (static) CG gives a more stable launch absent of any modifications of the plane's surfaces (notably the elevator) to add or subtract force along the plane's pivot point during launch.

Since most newbies (myself, especially), have enough to do just keeping the beast tracking straight at launch, keeping the towhook a bit forward of the plane's CG prevents the newbie from being concerned with elevator positioning, camber settings, etc. needed to counteract undesireable forces due to pivot point being either too close or even behind the CG.
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Old Nov 03, 2009, 07:40 PM
Yep, Naza-controlled Tricopter
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Your way of expressing it is good, Roger. The point Todd is making is that changes made to the CG during flight trimming will have zero effect on launch trim.

However, there can be a second-order effect to changing the flying CG. Suppose you move the CG back, and then add some down elevator trim to compensate. If you then launch using the new trim, your launch will be flatter than before. Similarly, if you add noseweight and then add uptrim to compensate, and launch using this trim, your launch will be steeper and possibly more squirrely.

For full-house planes that have a separate launch mode controlled by a switch, the elevator trim used for launch is unaffected by changes made in the other flight modes, so there's no interaction between flight trim and launch trim. In this case you can move the CG all over the place without having any effect on the launch characteristics.

Notwithstanding all this, the main message is: once you get the plane properly set up for a hi-start launch, you should just have to pull back and toss it, without much pilot interference on the sticks. This is especially true on a nice hi-start day when you have a slight headwind of 5 mph or so.
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Old Nov 03, 2009, 08:29 PM
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I agree 100% based on the limited launch experience I have.

I was basing my ramblings, however, mostly on my engineering background as opposed to what I've experienced on the field.

It is my belief that the plane manufacturers expect most newbies (me) would static balance their planes with the control surfaces in the neutral position and not be versed in launch modes. Therefore stating the "safe" recommendation of mounting the towhook forward the CG.

BTW, I really enjoy the technical challenges of sailplanes verses powered flight. My nitro planes are feeling more "unloved" every day... Little does my pattern plane know that it's exclusive "ownership" of the Futaba 12FG system is numbered... until I go to Toledo next April see what Mr. Kennedy or Skip Miller has on sale...
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Old Nov 03, 2009, 10:42 PM
Yep, Naza-controlled Tricopter
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Just to add a further note:

The CG is a notional point through which the down force may be considered to act. In fact, of course, gravity pulls all parts of the plane down. The towhook, however, is an actual physical point through which all downforce actually does pull. Since the pull on the towline is many times the weight of the plane, the forces of gravity on the various parts of the plane, and their moments around the hook position, have a negligible effect on trim. Your engineering rationale will be confirmed by experience as you fool with launch setups.

BTW if you have a 12FG you have a really dynamite radio!
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