SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old May 23, 2013, 10:10 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.
killerunicorn7 is offline Find More Posts by killerunicorn7
Reply With Quote
Sign up now
to remove ads between posts
Old May 23, 2013, 10:42 PM
Duane
Wazmo's Avatar
No. VA
Joined Nov 2004
1,444 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.
Wazmo is online now Find More Posts by Wazmo
Reply With Quote
Old May 23, 2013, 11:52 PM
Registered User
Joined Jul 2009
566 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.
edfmaniac is offline Find More Posts by edfmaniac
Last edited by edfmaniac; May 24, 2013 at 12:00 AM.
Reply With Quote
Old May 24, 2013, 02:55 AM
Registered User
Cromer,Norfolk, UK
Joined Nov 2006
2,505 Posts
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)
MCarlton is offline Find More Posts by MCarlton
Last edited by MCarlton; May 24, 2013 at 03:04 AM.
Reply With Quote
Old May 24, 2013, 08:00 AM
Registered User
United States, NY, Kingston
Joined Feb 2006
242 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
Bariboy is offline Find More Posts by Bariboy
Reply With Quote
Old May 24, 2013, 02:58 PM
Registered User
Joined Feb 2006
579 Posts
Quote:
Originally Posted by Wazmo View Post
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.
williamson is offline Find More Posts by williamson
Last edited by williamson; May 24, 2013 at 03:30 PM.
Reply With Quote
Old May 24, 2013, 11:15 PM
Registered User
United States, FL, Bradenton
Joined Jan 2012
316 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.
overmyhead is offline Find More Posts by overmyhead
Reply With Quote
Old May 25, 2013, 05:57 AM
Live for speed
GoFaster's Avatar
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.
GoFaster is offline Find More Posts by GoFaster
Last edited by GoFaster; May 25, 2013 at 08:04 AM.
Reply With Quote
Old May 25, 2013, 08:20 AM
Registered User
GliderJim's Avatar
Michigan, USA
Joined Jul 2006
577 Posts
String is cheap and light - rubber is expensive and heavy. Only use as much rubber as you need to get the job done.
GliderJim is offline Find More Posts by GliderJim
Reply With Quote
Old May 25, 2013, 11:58 AM
Registered User
Dangair's Avatar
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
Dangair is offline Find More Posts by Dangair
Reply With Quote
Old May 26, 2013, 07:34 PM
Registered User
United States, NY, Buffalo
Joined May 2013
1,213 Posts
Quote:
Originally Posted by MCarlton View Post
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!
DrewV is online now Find More Posts by DrewV
Last edited by DrewV; May 26, 2013 at 07:49 PM.
Reply With Quote
Old May 26, 2013, 08:19 PM
Registered User
United States, NY, Kingston
Joined Feb 2006
242 Posts
JATO....let me know how it works out!
Bariboy is offline Find More Posts by Bariboy
Reply With Quote
Old May 26, 2013, 11:14 PM
Registered User
xrx1113's Avatar
United States, FL, Lehigh Acres
Joined Aug 2011
852 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
xrx1113 is online now Find More Posts by xrx1113
Reply With Quote
Old May 29, 2013, 05:22 PM
AMA 353531
rdeis's Avatar
United States, CO, Colorado Springs
Joined Aug 2003
6,553 Posts
Quote:
Originally Posted by killerunicorn7 View Post
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.
rdeis is offline Find More Posts by rdeis
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Physics and Engineering questions on Quadcopter clarissa37 Modeling Science 11 May 23, 2013 04:04 PM
Sold Hi-Start, Up-Start Dynaflite $30 shipped to US treehouse99 Aircraft - Sailplanes (FS/W) 3 Mar 22, 2013 07:55 PM
Sold Hi Start Up-Start-large! $35 shipped treehouse99 Aircraft - Sailplanes (FS/W) 1 Mar 21, 2013 08:47 PM
Question Question about Electric Motor Physics and my HC2812 Mathieas Beginner Training Area (Aircraft-Electric) 15 Jul 26, 2008 02:39 AM
Question about Hi-Starts hifiman Sailplane Talk 1 Sep 18, 2004 08:57 PM