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Old Mar 01, 2015, 12:31 PM
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Joined Feb 2013
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Outrunner in a Hotliner Reloaded with prop simulations

Hello together

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:

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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:
10x6 CamCarbon
11x8 CamCarbon
12x9-10 CamCarbon
13x11 CamCarbon
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:

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And here is the simulation with the 11x8 prop:

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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:

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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.

Best regards,

Dirk
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Last edited by Lightmare; Mar 01, 2015 at 01:44 PM.
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Old Mar 01, 2015, 01:24 PM
444 kph
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Joined Nov 2008
522 Posts
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 ?
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Old Mar 01, 2015, 01:51 PM
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Austria, Stmk., Graz
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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.
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Old Mar 01, 2015, 02:01 PM
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Deutschland, RP, Niederzissen
Joined Dec 2011
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Jay,

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.

Yello
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Last edited by Mreyello; Mar 01, 2015 at 02:11 PM.
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Old Mar 01, 2015, 03:04 PM
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Interestingly, if you 'play' a little bit with the width of the blades in PropCalc and then adept the RPM to achieve the same input power, the differences are not that big. AFAIK that makes sense since the efficiency of the prop also depends on the Reynold numbers.

Given the little 'spike' in the log file, I assume this has to do with pressure changes in the fuselage after switching of the motor. As far as I know, many pilots report about this issue. -A strong reason not to overrate the log results and to compare them with simulations of estimating character.
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Old Mar 01, 2015, 03:26 PM
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Deutschland, RP, Niederzissen
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You may play with number in a simulator,some others play with 16x16 wide and 16x16 slim in reality...the difference is...Big...


Quote:
I assume this has to do with pressure changes in the fuselage after switching of the motor. As far as I know, many pilots report about this issue.
only a few take maximum spikes as the number to promote and to compare...

Yello
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Old Mar 01, 2015, 03:56 PM
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Of course the difference is big. But in terms of efficiency, that is not so easy to measure. You would have to change the entire setup in order to achieve the same input power when changing the width of the blades. And then you need some kind of normalization like keeping the pitch speed and the static thrust constant also.

It is not trivial to correlate the width (or the geometry) of propeller blades with efficiency. I know of publications about micro-UAVs where very small props with a high width compared to the diameter reach an efficiency of ~80%, e.g. here: http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

One must keep in mind that the climb rate (Steigen) is the time derivative of the altitude (Höhe). So as long as the altitude curve is steady, it is reasonable to use the climb rate at the point of switching of the motor as a value for comparison.

To give some more normalized data, I always combine the above mentioned climb rate with the overall increase of altitude at a given running time of the motor, like in my case 360m in 6s.
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Old Mar 01, 2015, 05:41 PM
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Austria, Stmk., Graz
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Quote:
Originally Posted by Mreyello View Post
Jay,

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.

Yello
Thanks for pointing that out!
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Old Yesterday, 03:58 AM
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But keep in mind: The 16x16 prop that I have simulated approaches a maximum efficiency of almost 90% (while the 11x8 prop has 85%). Even if the width of the blade had a "magic influence" and increased the efficiency to 100% (which is pure nonsense), the difference still would be in the range of less than 20%.

To pick up the black widow prop again, here it is:

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Look how small the prop is!

And here is the efficiency diagram:

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These type of measurments can only be performed in a highly sophisticated wind tunnel. This is why I am dubious about evaluating some props with logfiles without comparing them with reasonable (estimating) simulations and then judging the efficiency. As we have discussed, even the pressure drop in the fuselage after switching of the motor reportedly distorts the results.
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Old Yesterday, 07:53 AM
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I'm curious have you flown this plane yet
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Old Yesterday, 02:33 PM
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Yes, I have flown it several times already. Goes like a bomb . The sound is somewhat like a mixture between pylon racer and hotliner, but with insane vertical climbs. Actually I posted a log in the first entry of this thread. The climb rate at the end of a climb is >50m/s.

From my subjective point of view the acceleration is very good, no inrun flight needed. However, I think the acceleration with a 15x16 prop (round about the equivalent to the 10x8 prop at equal power on the shaft, comparable static thrust and comparable pitch speed) would be a bit better, but not so much better. This is what the simulation suggests and what I could find out when comparing log files from other RC pilots using this prop on their hotliner of comparable weight and size.

The best I could find was a Hawk F5B with an ARC inrunner on 5s, gearbox on a 15x16 prop. The weight 1.4kg, 50-100g more than my cheetah. The input power was ~1700W average (100W less thhan my cheetah) and the maximum climb rate at the end of a climb was 54 m/s. (3m/s more than my cheetah). See the example here: http://www.rc-network.de/forum/showt...=1#post2219922. You can translate it with translate.google.com from german into english.

One must take into account that the Scorpion motor has an efficiency of something around 75%, while a good inrunner with gearbox easily reaches more than 80%. So, it seems that the overall efficiency of my entire setup is maybe 10-20% lower compared to a geared motor with a large prop.

If you do not fly competition, than why not simply put a 20% larger LiPo into the fuse and save lots of money?
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