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Author Topic: E-36: a power race to the top!  (Read 757 times)
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PaulR
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« on: December 25, 2017, 06:11:05 AM »

I was wondering how the extra weight due to more powerful motor/ESC/battery combinations might affect the total flight time of an E-36 and found the answer interesting.

The reasoning is fairly simple. Given that there is a limited motor run duration (5 or 10 seconds), the height achieved will be roughly linearly proportional to the power. Once there, the sink rate will depend upon the square root of the total weight and this in turn can be broken down into the airframe weight plus the weight of the power system (motor/ESC/battery).  By comparing data for a number of models, it appears that the airframe weight is around 80-90g while the power system weight in grams is roughly 0.6 x P, where P is the power in watts (the exact factor is not important). Thus the sink rate will vary as something less than the square root of the power. Based upon this simple reasoning, the total flight time will thus always increase with power (the linear dependence of the climb height dominating the weaker dependence of the sink rate on power).

An exact calculation would need to take account of other factors such as the increased drag at higher climb and glide rates etc, but for the sorts of powers we are interested in I think that the general message would be the same, though I guess that there must ultimately be some limit.

Anyone for 200, 300, 400,... watts??

Regards,

Paul
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danberry
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« Reply #1 on: December 25, 2017, 11:40:52 AM »


The reasoning is fairly simple. Given that there is a limited motor run duration (5 or 10 seconds), the height achieved will be roughly linearly proportional to the power.


No.
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airplay
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« Reply #2 on: December 25, 2017, 01:25:18 PM »

There is no limit with a class without restrictions on materials or energy mass. But there are practical restrictions. You can't spot these tiny models very far...
I played a bit with Ecalc and 30m/s is very possible with loading around 17-20 g/dm2.  But of course you could then deploy a big dt parachute and hope for few thermals on the descent... 

Lovely class!

Jens
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PaulR
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« Reply #3 on: December 25, 2017, 07:09:30 PM »


The reasoning is fairly simple. Given that there is a limited motor run duration (5 or 10 seconds), the height achieved  will be roughly linearly proportional to the power.


No.

Anything to add Danberry?

My reasoning is this: the potential energy gained (i.e. work done) in reaching a height h is m x g x h, where m is the mass and g the gravitational constant. The rate at which this work is done is the power, and as the work has to be done in a fixed time interval (5 or 10 seconds) ) the height achieved will be linearly dependent upon power (increases in drag etc aside). Stated another way, a higher power means a higher rate of doing work and so, over a fixed time interval, a higher amount of work done (potential energy) and hence greater height.

Of course, drag cannot be neglected and so the model cannot reach that theoretical height and, since the work done against drag increases as the square of the velocity, the higher climb rate will result in proportionally higher energy dissipation. Consequently, the height climbed will fall somewhat below the linear proportionality upon power,  but will still be greater than the square root dependence of the sink rate. This is verified by the observed climb heights and total durations of current models: both increase with increased power (other things being equal, such as the same airframe).


Paul.
« Last Edit: December 25, 2017, 08:00:34 PM by PaulR » Logged
danberry
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« Reply #4 on: December 26, 2017, 12:15:35 AM »

The increase in speed isn't gonna be linear.
The extra weight of more motor plus the extra weight of more structure so as to handle the extra speed won't allow it.
Drag increase is not linear with speed.
The same airframe will not handle the added speed. --- I wrote that twice because it is REAL important.
There is a point of no return with these things.

I cannot believe how poorly so many E36 planes perform.
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Tapio Linkosalo
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« Reply #5 on: December 26, 2017, 02:48:02 AM »

  But of course you could then deploy a big dt parachute and hope for few thermals on the descent... 

That would count as changing model geometry, methinks. But still I think the current model design is strongly influenced by the tradition of open-structure, spiral-climbing power models. You could build a sleek, all-composite model that could handle 40m/s airspeed with decent weight, allow for a possibly poor transition and fast glide due to higher weight, and maybe still be competitive?

Except that the becoming EASA (European Aviation Safety Agency) rules for model airplanes (and drones) are about to set the maximum allowed altitude for any model airplanes to 120 meters throughout the Europe. So within a few years, even our current E-36 models will be illegal!
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USch
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« Reply #6 on: December 26, 2017, 02:01:06 PM »

...the height achieved will be roughly linearly proportional to the power....
Anyone for 200, 300, 400,... watts??
There is a point of no return with these things.

Of course Paul's 300, 400W are a provocation to stimulate the discussion. Today even 200W are fantasy, let's alone 300-400W. The point of no return will be further up on the scale and not been touched for some time. So the trend indicated by Paul is OK for todays (and probably also tomorrows) state of art in F1S.

Airframe weight can be below 55g if the same design principle are used as on the modern FAI models, i.e. carbon D-box, trailing edge and rib caps, and still have sufficient rigidity for the climb speed obtained at about 25m/sec.

Urs
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