|Today, 12:31 PM|
Joined Feb 2013
Outrunner in a Hotliner Reloaded with prop simulations
I have recently pimped a Cheetah (balsa foam wing) from Dymond with carbon spares in the wing and polished fiber glass coating to increase torsion stiffness. So, for less then 200 bucks alltogether I was able to make a very robust hotliner:
I wanted an affordable drive for this cheap hotliner that would achieve a vertical climb rate of >50m/s at reasonable overall efficiency and good acceleration. So I did some simulations with 'PropCalc' and DriveCalc to answer a particular question: Given a static thrust and a given pitch speed: Which propeller dimensions would lead to identical results? After some DricveCalc simulations, the following prop dimensions indeed were predicted to have round about the same static thrust and pitch speed ad the same input power at the shaft:
16x16 "slim' RFM (a typical hotliner prop)
I thought this cannot be true since it is a fact that with lowering the ratio of pitch to diameter (P/D-Value), the efficiency goes down. So I decided to make two example simulations with PropCalc, an 11x8 and an 16x16 prop. The results are somewhat related to the DC predictions (Constant pitch over radius, reasonable blade profile, dimensions comparable to the props listed above).
So, here comes the simulation of the 16x16 prop:
And here is the simulation with the 11x8 prop:
Interestingly, at the same power at the shaft (~1500W), both props show very similar behaviour with the 16x16 showing slightly better efficiency and slightly higher thrust at high speed thant the 11x8 prop. The efficiency is relatively 5-10% higher and the thrust does decrease at a lower rate at increased speed compared to the 11x8 Prop. So the 16x16 prop will show a slightly better acceleration, maybe a few percent.
With this simulations in mind, I decided to use an outrunner without a gear box on the pimped Cheetah, in this case a Scorpiob HK3026 1000V on 6s.
Due to the very small fuselage, the capacity was limited to 1500mAh. However, to increase the pitch speed and efficiency a little bit, I decided to use a Cfk folding prop from Freudenthaler with 10x8 dimensions. With a very cheep hobby king brushlesscontroller, the setup was drawing 2000W and about 100Amps statically. For one climb flight of 5 seconds, the average power drain was about 1800W. ScorpionCalc suggests an efficiency of 75% of the motor at this power at 6s, which is quite reasonable.
So, with this setup I achieved in average a climb rate of >51m/s . I logged one typical climb flight:
To avoid overheating, I used quite big cooling exhausts and a turbo spinner with a big hole in the nose, but the motor came down as cool as it was before lift off.
I also did check some typical log files for hotliners in the range of 2kW of input power and a weight of 1.3 to 1.4kg, using gear boxes and typical props like e.g. 15x16 inch. Indeed, the climb rate and acceleration seem to be a little bit better, like e.g. 54m/s @ 1700W of average input power during one climb flight.
So I thought that if you do not fly at competitions, outrunners driven at high voltage (6s minimum) and moderate prop sizes like 10x8 inch seem to be an affordable approach for "fun hotlining".
The simulations seem to confirm that the loss of efficiency comparing a 10x8 prop and a 15x16 or 16x16 is in the range of 10%, but maybe not more?
As always, I am interested in your oppinion.
|Today, 01:24 PM|
the log draw would be easier to read, if you put a grid into the diagram!
it also shows, that your climbing speed still increases, after switching off the drive!
are you using "free energie" for that ?
|Today, 01:51 PM|
Austria, Stmk., Graz
Joined Sep 2013
What would be interesting now is if you would use a 16x16 prop (with lower LiPo cell) count. Its hard for me to believe that a 10x8 setup is almost as efficient as a 16x16 setup.
|Today, 02:01 PM|
The propcalc simulation shows a virtual 16x16 fixed prop.
The simulation assumes the 10x8 and the 16x16 to have same airfoil and pitch progress.
Both blades even seem to have same width...
Based on this the result seem to be reasionable...but reality is diffrent.
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