Hello Miko,
some more hints.
The short version is:
What you actually need to be more efficient is probably just more power and maybe another prop.
Or even shorter: Voll Gas = Yes!!
The long version is as follows:
( I am using a lot of rules of thumb here because there are many variables in the equation.)
Let me first elaborate a bit more on climbing and role of the climb angle with efficiency.
Let's look at glider efficiency in two extreme situations and then in a third more normal situation:
1) level flight (no climbing)
2) vertical climbing
3) 30° climbing
1)
At level flight, you need to compensate the glider drag with the propeller,
but climb efficiency is zero, as all the power is lost with compensating and the climb path is therefore infinitely long.
The power needed to compensate glider drag can be calculated from the glider weight and the glide ratio.
Let's e.g. assume a weight of 8kg, a ratio of 20 (20 should be pretty good for a model glider), a glide rate of 10m/s and a sink rate of 0,5 m/s.
Ratio of 20 means that the drag forces are 1/20 of the weight forces.
That would be 4N of drag at 10m/s, requiring 40W of power.
You could also calculate the power needed to lift 8kg at 0,5m/s - it is the same 40W.
With an optimum efficiency of the electrical drive system including motor and prop of ~55% (will write more on this later), that would be 73W of input power.
2)
At vertical climb with 10m/s, the 4N of drag add on top of the 80N of weight and therefore climb efficiency is 95%.
However you will need much more power - with the same calculation as above, it is 1527W input power.
3)
At a climb angle of 30°, you need 2m of climb path to reach 1m of altitude.
Therefore, the efficiency will be 0,95*0,95 which is roughly 0,9.
Smaller climb angles will result in much longer climb paths and therefore lower efficiency.
However, you also need only half of the force to overcome the weight.
Therefore, you would need 480W of effective power or 872W of input power for a 30° climb of 5m/s.
Now let's look at the magic 55% of drive and prop efficiency - where are they coming from (this is all rules of thumb):
- Battery efficiency 95%
- Losses in cables, plugs and ESC 5% = 95% Efficiency
- Motor/Gearbox efficiency 80% at operating point
- Propeller efficiency 75%
Then let's have a look a the kind of prop you need to get 4800g of thrust at 10m/s at an efficiency of 75%.
This is the actual problem, because you would need a very large prop spinning at a comparatively low speed.
Let's assume a 23x12 operating at 1000W mechanical power (~1300W of input power at the ESC) - it will deliver 8kg of static thrust with a pitch speed of 90km/h which is equivalent to 25m/s. At a speed of half of the pitch speed - 12,5m/s - it will deliver roughly half of the thrust at an efficiency of roundabout 70%. At 10m/s, the thrust of 4800g is probably right, but the efficiency drops to 65%.
Alternatively, you can go for more power and a higher climbing speed.
E.g. at a mechanical power of 1500W (~2000W of input power at the ESC),
the pitch speed of a 23x12 is 103 km/h - equivalent to 28m/s.
The static thrust is 10,6kg. 4800g of thrust are probably reached at 16m/s with an efficiency of roundabout 75%.
Therefore, at higher power and climb speed, the efficiency goal can be reached with this prop.
What do we learn from this ?
- Go for the largest prop possible
- More power means more efficiency
Look at hotliners - they have to produce maximum (and fast) climbing from limited energy:
- Prop is as large as possible, reaching from the nose to the wing.
- Prop is "quadratic" (e.g. 17x18) which increases efficiency.
- Power is > 1kW / 1kg.
Of course this cannot be realized for a normal glider.
However, the same principle applies.
Therefore I am used to "overpowering" of my gliders for better efficiency -
using a drive train that theoretically would allow vertical climbing for something like a 30° - 60° climb angle.
The result is quite a few more climbs from the same battery (which of course must allow for a high C rate).
The efficiency is much dependent from the actual climb angle chosen -
therefore it probably pays off making measurements at different angles.
As a substitute for measurements, I use to watch my glider when climbing.
The glider should not climb at maximum angle and comparatively low speed
but rather give some "dynamic" feeling when climbing.
With best regards,
Georg
