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May 05, 2015, 12:03 PM
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Maximum Launch Height


I will publish my DLG design analysis here to share it with others and get feedback. So please feel free to do so
Last edited by l shems; May 05, 2015 at 12:25 PM. Reason: Wrong title
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May 05, 2015, 12:12 PM
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Sink Speed “Classical GA calculation”


These calculations use the classical General Aviation simplifications and assumptions. They are only of limited use for DLG, as I will explain later.
May 05, 2015, 12:17 PM
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Total flight time optimization


Total flight time is determined by simply taking launch height and minimum sink speed, and calculate the resulting air time. We will neglect the launch time, which will be in the orders of several seconds, with respect to flight time, which will be in the order of several 100 of seconds.
May 06, 2015, 10:54 AM
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Neil Stainton's Avatar
This is interesting, but hard to take in one page at a time. Could you publish it as a PDF, or at least with page numbers?

Sorry to make such a pedestrian request, when you have obviously put a lot of time and effort into making this analysis.

Does 1-30% = 0.7 or 70%? Why not say
launch height = A * 0.7B?
(where A and B are as in your equation)
That would be more normal notation I think.

Neil.
May 06, 2015, 11:32 AM
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Neil,

thanks for you comments. Actually, the formula should be read by applying the normal order of preference of arithmatic operators, which is:

multiplication, division, addition and substraction

So that part of the equation should be read as ( 1 - [ 30% * Vlaunch / Vterminal ] ) and then it doesn't make any difference anymore: 30% and 0.3 are exactly the same.

I deliberatly used the 30% to make the connection with the rule of thumb to memorise that "maximum launch height is lowered by thirty percent times the launch speed relative to the terminal speed".

I hope this clarifies things, and I'm still thinking if it helps if I would change the formula. Perhaps just adding the brackets??

Guido
Last edited by l shems; May 06, 2015 at 11:46 AM. Reason: removed spaces
May 06, 2015, 11:47 AM
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PDF attached of the above posts
May 06, 2015, 11:57 AM
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Thanks of your reply. Reviewing the equation, and looking more carefully at the graph, it becomes clearer. I do think an extra set of brackets would help.

I don't think this finding will ever become a rule of thumb, because average modellers use RoTs but don't do complex equations or know their launch velocity or the glider's terminal velocity. And aeronautical engineers may estimate the velocities but aren't keen on RoTs!

Neil.
May 06, 2015, 12:01 PM
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Thanks for making the PDF. I will try to read and comprehend it over the next few days. It is a long time since I went to uni!

Neil.
May 06, 2015, 12:15 PM
launch low, fly high
The assumption of constant launch speed independent of weight may not be accurate. I went with a total energy approach, modeling the kinetic energy of the arm and airplane. The uncertainty lies in the mass distribution of the arm, and the center of rotation. A few assumptions and one can get something that matches experimental results. Of course, when I did my launch analysis I modeled my throw, although data suggests that for different pilots there is a wide variance in the mass and mass distribution, energy available and center of rotation point. The net result is that for me there is significant change in initial velocity with aircraft weight changes, making the zero wind launch height rather flat with weight until the weight exceeds about 260-270g, with additional weight reducing launch height despite the lowered ballistic drag coefficient.
May 06, 2015, 12:24 PM
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Sorry to butt in, but Joe what do you estimate your launch and the Snipe's terminal velocities are, at the Snipes design weight?

Neil.
May 06, 2015, 12:36 PM
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Guido,

Interesting and impressive and beyond the maths associated with my pre-university physics.

As well as aerodynamic drag airframe flexibility must be a factor in launch height. Anecdotally a stiffer frame will launch higher, but I accept the assumption that all the launch force is translated to velocity for you calculations.
May 06, 2015, 12:42 PM
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Quote:
Originally Posted by Cush
Guido,

Interesting and impressive and beyond the maths associated with my pre-university physics.

As well as aerodynamic drag airframe flexibility must be a factor in launch height. Anecdotally a stiffer frame will launch higher, but I accept the assumption that all the launch force is translated to velocity for you calculations.

My other comment has proved controversial in the past is that L/D improves with weight as well as the increased range from speed. This is accepted in full size sailplanes with glide calculators improving L/D with ballast. This seems to be blasphemy in the toy aeroplane world and wait to be burnt at the stake (again).
Cush,

if you read carefully, you will notice that I specifically mentioned that the GA generalisations aren't valid for DLG, and that I willl explain later what the consequences are.

So you have to wait for that to see if you can get some arguments which perhaps support your blasphemy
May 06, 2015, 12:50 PM
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Quote:
Originally Posted by Joe W
The assumption of constant launch speed independent of weight may not be accurate. I went with a total energy approach, modeling the kinetic energy of the arm and airplane. The uncertainty lies in the mass distribution of the arm, and the center of rotation. A few assumptions and one can get something that matches experimental results. Of course, when I did my launch analysis I modeled my throw, although data suggests that for different pilots there is a wide variance in the mass and mass distribution, energy available and center of rotation point. The net result is that for me there is significant change in initial velocity with aircraft weight changes, making the zero wind launch height rather flat with weight until the weight exceeds about 260-270g, with additional weight reducing launch height despite the lowered ballistic drag coefficient.
Joe,

thanks for this very useful comment. I will certainly look into that approach, and see if I can factor it in.

For the moment, I am more concerned with the very funny charateristics of drag for these planes.
In my first approach, based on General Aviation approximations, I tried to get an idea on the drag effects based on the terminal velocity. This would have been a very practical way of estimating zero lift drag in the field, and use that to get some ballpark figures on L/D and drag effects on launch.
It turns out that General Aviation approximations are not usefull, so I need some other approximations to be able to do some simplification on L/D ratios etc.

I will post on that in the coming weeks, and hope to get similar responses as well
Nov 28, 2015, 10:24 PM
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How does this compare to the info Dr Drela posted on launch height and airframe weight?
Dec 04, 2015, 12:29 PM
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Quote:
Originally Posted by GaryO
How does this compare to the info Dr Drela posted on launch height and airframe weight?
Found some on launch angle and wind speed, but not on airframe weight??



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