Energy sufficient for thermaling in the Midwest?


#1

Will the OpenPPG have enough energy to get up into the 2500-3000’ altitude range that is required in the midwest in the US to start reliably thermal gliding in the summertime? I’m about 155lbs and I would bring just enough battery to get me up to that altitude and the safely back home if I couldn’t find thermals.


#2

Some quick math…

At 220lbs flying weight… that is 100kg.

100kg * 9.8 m/s^2 = 980 newtons force

2500 ft is 762 meters.

Energy is force X distance, so 980 X 762 = 747kJ

747kJ is 207 Wh of energy.

That is not much! Only 200 Wh worth of energy to move your mass to that altitude.

Lets say you climb to 2500 feet at a rate of 300ft per minute, so 8.3 minutes, or 0.1388 hours.

So 207Wh / 0.1388 = 1.49kW additional power required to climb.

Now the tough part is estimating prop efficiency. We know that motor efficiency should be 80-90%… Lets say prop efficiency brings that down to 60% total efficiency (just guessing).

So then you need 2.5kW extra power to climb.

We know from casual observation that some electric ppgs can maintain altitude using 3-4kW of power to overcome drag. Let’s assume the high end, 4kW, for the OpenPPG (less efficient 4 prop design).

So the average power during climbing will be (4kW +2.5kW) = 6.5kW

After 8.3 minutes, 0.1388 hours, you would have used only 0.840kWh of energy.

Most of the setups for OpenPPG will have between 2-4 kWh of capacity.

This sounds too good. Someone check my math! The big unknown here in my estimation is prop efficiency. Maybe it is really bad. Even so, even if it takes over a kWh to get to altitude, this seems totally within the ability of the OpenPPG!


#3

about prop efficiency:
It is generally agreed that one large prop is more efficient than four small props.

However one argument here in this forum was that one large prop would be spinning behind the back of a pilot, and the air needs to flow around the pilot, whereas with the OpenPPG the four props are more or less spinning in free air without anything obstructing the airflow except for the protection net and the frame parts.

sounds like an interesting theory to me.


#4

Just a quick thought: Its my understanding that in traditional ppgs, the most of pilot shape induced “aerodynamic noise” occurs where the prop is less powerfull, around the center of it where angular velocity is not that high. So, in a single prop ppg if you are able to hide your fuel tank (or batts) and fly with the legs stretched forward, you will have a very “unperturbed” airflow.

In this 4 props configuration you are still having some part of your 4 thrust discs spinning behind the “shadow” of the pilot and in this case wont be the center of each prop but the tips that will be spinning in “perturbed” air. (Theorically you can even say that the blade stress wont be the same around de thrust disc since one third part of it will generate always less thrust that the other 2 thirds parts of. But this can be negligible…)

In the real world I have no clue how much this really affects the overall thrust and pilot happines. Look at Polini engined slalom ppgs, they even put a radiator just in the prop front. And those machines are Saturn 5 rockets…

…But they dont folds nicely to fit in your everyday street car. As they say, there is no free lunch…


#5

In the world of fixed wing RC, pusher planes are MUCH louder than tractor designs. The extreme is a flying wing, because the pusher prop is dropping in and out of disturbed air. All my designs are tractors as a result to fly in clear air, and they are very quiet.

I thought about a design that had two larger props that stuck out on either side by a lot, but I suspect that imbalances in the RPMs would cause yaw, and a failure would cause uncontrollable spinning.

Another option for this design would be to cant the motors at a slight angle to reflect the fact that the air is bending around the body. But it probably won’t affect the noise very much. Once the air is disturbed it is hard to get around that…

OpenPPG seems like a lot of really good design optimizations. I can’t wait to see the specs on the motors so we can use eCalc to try different prop sizes and pitches.


#6

About this: If Im brave enough I will try to go back to my phisics textbooks to check that out. But I dont think (probably Im wrong) you can use this math approach mostly because your lift (that is making you go up) is vertical (and depends also on your wing) and the engine thurst is horizontal. (mostly). Also you just dont climb like in an elevator, you move forward a lot and this also will use energy

Again I can be 100% wrong and wont be the first time…or the last…
,


#7

That is why I take the 4kW estimate for the baseline power to maintain altitude. That accounts for drag losses and all the other factors involved to maintain altitude. Additional power beyond that will result in gains in altitude. We know that increased throttle (thrust) doesn’t have much effect on velocity in a PPG, so little effect on drag losses, thus that energy goes into climbing.

Of course, lots of assumptions made, so error expected. I think the conclusion is sound, though.


#8

In another thread, PDWhite said that his setup required 100A to maintain altitude. I’m not sure if that is at 22v or not. If so, that is about 2.2kW. If he is putting 2 6s batteries in series, then this is 4.4kW.


#9

Indeed. At 12S it is ~4.4kW.

Comparing to Paracell’s videos, he can cruise at 3 to 3.5kW. But that is with a conventional single large prop.

Of course, it all depends on wing, too.


#10

His rig is $9k, or about 3x OpenPPG. On a dollar-per-kW basis I think OpenPPG wins…
(but man, that Paracell looks sweet)


#11

Excellent calcs jhair!
I was after the cruising power so this thread is just what I need.
So just to confirm, OpenPPG requires a 12S battery, which gives 12 x 3.7 = 44.4V @100A for cruise which is 4.4kW?
Do we know what weight the pilot was and what size and type (EN-A/B/C…) of wing?


#12

The question of tombusey is: if it is possible to get up into the 2500-3000’ altitude range with his openppg motor.
I like the technical analysis of jhair a lot, but I reason according to a more practical angle.

First some facts:
I fly a fresh breeze super thorix 200 cc polini motor with 75kgr trust (which is approximately the same trust of a Fresh Breeze Monster motor and thus equals the trust of an openppg motor)
My weight is 100 kgs ( 220 lbs) ; my complete flight weight incl. motor, wing and petrol approximately 140 kgr (308 lbs).
I fly an inefficient ’speedwing’ paramania GTX 24 meter. ( inefficient but fun)
My climb rate in the evening so without any thermals is approximately 436 feet/min.
If you use Paul’s minimum battery configuration so only the 2 times 22000mAh Bonka batteries, then you can full trottle only 5 minutes before the batteries are completely empty.
5 minutes full trottle brings me with my heavy fresh breeze super thorix 200 cc polini motor to a height of 5 * 436 feet = 2180 feet.

So the answer to the question of tombusey is Yes! With a less heavier openppg motor on your back in combination with a more efficient thermalling mountain wing you will easily reach the 2500-3000’ altitude range.


#13

Or you could catch a thermal much lower, 500-1000 feet should be plenty :slight_smile: