May 23, 2013, 11:10 PM Registered User Texas Joined Dec 2009 101 Posts Discussion Question about the physics of a hi-start I used to fly RC a lot and lurk around these forums a bunch, and a project I'm working on now involves similar characteristics of a hi-start, so I figured you guys would be the ones to ask. What is the function of the non-elastic string in a hi-start system? How/why does the system benefit from the string rather than being completely rubber? Does it allow for more power? Ultimately, I'm looking for ways to extract more power out of a small amount of elastic materials, most commonly rubber. Are there easy ways to do this? Any input is appreciated. Thank you.
 May 23, 2013, 11:42 PM Duane No. VA Joined Nov 2004 1,474 Posts Total energy expended is determined by the amount you can store by stretching the rubber (typically 300% elongation, or 4 times its relaxed length). You can get higher force for a shorter time by doubling up the rubber (and thereby halving the length), but total energy only goes up if you have more rubber - either by using a longer piece, or one with a larger cross section. See Dick Williamson's article on high-starts for details.
 May 24, 2013, 12:52 AM Registered User Joined Jul 2009 567 Posts Imagine launching a kite in little to no air. A hi-start is basically like a kite string with the rubber section acting as a runner. You want the string to be as light as possible with as little drag as possible, with only enough rubber to keep the line under tension during the climb phase of your launch. Less rubber equals less weight but gives you a shorter pulse of energy. More rubber is more weight but you get longer or stronger pulls depending on diameter and stretching properties of the power source you are using. The wing loading, head wind, desired launch height and spar strength are all factors that you should think about when choosing the rubber and line length ratios. Hand towing is also very effective but your wing better be up to the task. If what you are doing has nothing to do with flying and you just want to get more power from a given source of energy, the only way to do that without violating the laws of physics is to convert the energy stored in the rubber or elastic into a shorter pull through the use of pulleys or some other system of reduction like a lever and fulcrum. Compound bows may be of interest as well. Last edited by edfmaniac; May 24, 2013 at 01:00 AM.
May 24, 2013, 03:55 AM
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If you used a 100% rubber high start, the total energy in the system would be greater - ie 30m of rubber vs 100m of rubber. In order to "climb" rather than be "catapulted", the glider wing has to be able to pull against the rubber, to maintain the stretch in the rubber, and thus keep it "pulling" against the glider, to keep it moving.

If there is too much "energy" in the rubber for the glider to pull against, then it won't be able to counter the stretch, and will, in effect, be simply catapulted towards the horizon.

This is where some people make an error in going for a powerful high start, and using it on a smaller model, which won't be able to generate sufficient lift to hold the stretch in the rubber, so they get short, low launches.

Thats why some of the EDF guys use a very short, full rubber "bungee" to launch a model, as it very quickly gets the model to flying speed in a short distance.

As above, the energy potential of a particular length of rubber is the same, regardless of whether a line is attached or not, the difference is in the rate at which the energy in the rubber is released.

Note, there should always be some stretch left in the rubber at the top of the launch, and in fact, with a bit of a breeze and a strong model, you can stretch the rubber further during the launch than it was stretched when you let go of the model. A high start isn't a catapult

 Glider High Start Tutorial.mp4 (11 min 45 sec)
Last edited by MCarlton; May 24, 2013 at 04:04 AM.
 May 24, 2013, 09:00 AM Registered User United States, NY, Kingston Joined Feb 2006 257 Posts I'm not hi starting, but to me, and take this with a grain of salt...If you stretch a 25 foot band to a hundred feet and attach the plane and let it go, the vector will be towards the point of ground where the rubber is attached and the glider must convert that all to lift, the lift must fight completely against the contraction of the rubber and thus your glider is going to go forward and ?up...but only to the height of the non stretched rubber plus what ever lift overcoming strength of the rubber can be produced. So if you need 15 pounds of pull to stretch the rubber to full length, your plane will need to generate that lift to maximize elevation. Now if we add a hundred feet of 20 pound power pro fishing line to the end, no stretch, the plane will lift, the rubber will contract but at the apex you will have the length of the rubber in neutral, plus the lift generated by the plane, plus the length of the power pro leader. So now a hundred and 3 feet? Again, not a hi-start specialist, but I did stay at a holiday in last month PS, I like power pro for fishing, it has zero stretch with very good feedback, 20 pound power pro has like a 6-8 pound mono diameter. I'd actually consider going to 50 pound PP just to have a larger feel in the hand. The stuff will cut you to the bone if you wrap it around and your hand and pull tight. treat it with respect! Now a slightly different question...could you hi start in series??? A double hi start? Scott
May 24, 2013, 03:58 PM
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Quote:
 Originally Posted by Wazmo Total energy expended is determined by the amount you can store by stretching the rubber (typically 300% elongation, or 4 times its relaxed length). You can get higher force for a shorter time by doubling up the rubber (and thereby halving the length), but total energy only goes up if you have more rubber - either by using a longer piece, or one with a larger cross section. See Dick Williamson's article on high-starts for details.
Thanks for pointing out my article. I wrote it more than a decade ago, but the basic physics has not changed.

Gary Learned put together a nice summary of various brands of high starts and how they match to the weights of sailplanes.

# Files

Last edited by williamson; May 24, 2013 at 04:30 PM.
 May 25, 2013, 12:15 AM Registered User United States, FL, Bradenton Joined Jan 2012 329 Posts Physics aside don't forget about safety when using strong bungee cords......the longer length of string also separates the user from the rubber and protects you in case the bungee breaks and keeps you at a safer distance from the end of the whip.
 May 25, 2013, 06:57 AM Live for speed USA, CT, Bethany Joined Mar 2004 1,371 Posts The string is also there to minimize the amount (weight) of rubber the plane has to lift + the drag. Last edited by GoFaster; May 25, 2013 at 09:04 AM.
 May 25, 2013, 09:20 AM Registered User Michigan, USA Joined Jul 2006 578 Posts String is cheap and light - rubber is expensive and heavy. Only use as much rubber as you need to get the job done.
 May 25, 2013, 12:58 PM Registered User Canada, BC, Anglemont Joined Mar 2013 319 Posts A high start uses another unique force that has not been considered here and that is apparent wind speed. The exact math for which I will not go into detail about. A brief explanation, if any of you have any sailing or kiteboarding experience you will know that an airfoil passing through a plane that rotates around an axis perpendicular to wind will generate additional speed, this effect is known as apparent wind and can depending on the angle of attack generate incredible force! A 15 sq. meter wing in a 10 mile per hour wind can lift a 200 lb. man 30 feet off the water. The only difference is that the anchor point is fixed and not moving in relation to the wing, however, if there is wind it will add force to the stored energy in the elastic. If there is a significant gust that energy is multiplied and can result in a failure in either the system or the aircraft. Also the amount of force is in relative the the angle of attack. To some extent that could be controlled by elevator. Stephen
May 26, 2013, 08:34 PM
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Quote:
 Originally Posted by MCarlton This is where some people make an error in going for a powerful high start, and using it on a smaller model, which won't be able to generate sufficient lift to hold the stretch in the rubber, so they get short, low launches.

Wow, this is the first time I've ever heard this, but it makes PERFECT sense. If your rubber is too strong, it will just "drag" your model along while it contracts. A smaller model will not have enough wing area/lift to counteract the load, so it will just get dragged through the air without climbing.

That really is an awesome little detail. Thanks for pointing this out. I was just about to grab a heavy-duty Hi-start for my 2M and 3M gliders, because I figured more power has to be a good thing, but after looking at Dick Williamson's breakdown of the rubber cords, it doesn't look like the "heavy duty" is really any different than the 2M version. My 3M model is around 40 oz., so it's apparently too heavy even for the "heavy duty" version. But man, those Aerofoam launchers are expensive, and even the 3M rubber at Hobby Lobby is \$80 (when it's in stock).

Hmmmm, I need to find a cheaper alternative to launching my old-timers. It's almost cheaper just to go electric!
Last edited by DrewV; May 26, 2013 at 08:49 PM.
 May 26, 2013, 09:19 PM Registered User United States, NY, Kingston Joined Feb 2006 257 Posts JATO....let me know how it works out!
 May 27, 2013, 12:14 AM Registered User United States, FL, Lehigh Acres Joined Aug 2011 864 Posts Dont forget the less pull needed when windy point.... I learned the hard way this weekend. I diddn't need thoughs 5 extra steps
May 29, 2013, 06:22 PM
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Quote:
 Originally Posted by killerunicorn7 What is the function of the non-elastic string in a hi-start system?
When you use string for extra length, you trade away potential energy that you do not need for a weight reduction that is desirable.

The string allows the glider to remain tethered to the hi-start at a greater distance from the pin. The more distance you have on the ground, the more altitude you can have when you cross over the top.

Quote:
 How/why does the system benefit from the string rather than being completely rubber?
The string is far lighter than the rubber, and the excess energy that could be stored in extra rubber in unnecessary to launch to an altitude that equals the unstretched length of the rubber+string

Quote:
 Does it allow for more power?
No. It allows the power you have to be used more effectively.

Quote:
 Ultimately, I'm looking for ways to extract more power out of a small amount of elastic materials, most commonly rubber. Are there easy ways to do this?
Extract more power? No, I expect that's pretty well fixed by the spring constant of the rubber and the allowable stretch length before it breaks.

What you can do easily is either vary the rubber thickness and length or add other mechanisms to change the power release into a form that is most useful for your application-- tune it to release a lot of force over a short distance/time, or a small amount of force over a long distance/time, or someplace in between.