Predicting 2 Blade APC vs 3 Blade Vario On Motocalc?

[Talon Driver]

Motocalc does not seem to accurately predict 3 blade Vario performance versus 2 blade APC performance. I am wanting to use Motocalc to compare the Vario prop performance to the APC. I have flown the 2 bladed APC and the Projeti is a screamer. According to Motocalc the 3 blade Vario should even be faster.

However static tests for these two different props do not accurately show the performance estimated by Motocalc 7.08 for the Vario 3 blade.

Here is my Projeti's setup. Mega 16/15/4 and CC Phoenix 25. For flight I use 8 cells of 4/5 FAUPs. For Ground testing I use 8 cells of Sanyo 1250 SCRs. BTW both packs are apples to apples at the initial run of the motor, they have about the same internal resistance. The only difference is the NiCd 1250 drops RPM quicker than the NiMH FAUPs (lower capacity vs larger capacity and chemistry) Compared Props are:

- APC E 6 x 5.5
- Vario 3 blade 6 inch set at 4 and 5 pitch

My Projeti has a skid providing additional clearance for the 6 inch props.

All bench testing was with a fresh pack of 8 cells of 1250 SCRs, same temperature. I took several samples of measurements for accuracy. All RPM measurements were with a "Hangar 9 Micro Digital Tach" using either a DC flash light or sun light. Current readings were measured with a home made current meter calibrated at 5 amps using a Dymond Super Turbo Charger charging at 5 amps. I consider current readings to the nearest 1 amp for sure. My altitude is 1,012 feet MSL and room temp about 65 deg F. Here are the bench test results:

APC E 6 x 5.5

16,000 RPM and 21 amps

Vario 3 blade 6 inch

4 pitch: 14,500 RPM and 24 amps

5 pitch: 14,000 and 26 amps (Don't worry the CC 25 can handle this)

Here is what Motocalc 7.08 estimates for a CC 25, 1250 scr and Mega 16/15/4. (All calculations used P Const = 1.11 and T Const = 1 and Motor K, RPM Const, of 2200 RPM/Volt versus default of 2300 RPM/V. Reason is 2200 is what Mega USA reports for the 16/15/4).

APC E 6 x 5.5

16,900 RPM and 19.6 amps

Vario 3 blade 6 inch

4 pitch

16,700 RPM and 20.5 amps

5 pitch

15,700 RPM and 24.8 amps

Results of comparison:

Motocalc's prediction for the static performance of the APC is somewhat close to reality. Still off by 1,700 RPM.

Motocalc's prediction for the static performance of the 3 bladed Vario is even more off the mark. For the 4 in pitch off by 2,200 RPM.

Some of this could be explained by the T and P Constants (I used those for an APC even for the Vario).

Can someone run this setup on P-Calc for the 8 1250 SCRs, 16/15/4 and CC 25 with these two different props using the same P and T consts?

Anyone have any tricks for Motocalc that will provide more accurate results not just for static loads but also for dynamic loads?

Has anyone done any testing to etimate the Thrust and Pitch constants for the Vario?

The Vario Blades have much more area than the APC which would mean that it has a more effective pitch to the theoretical than the APC which would create a heavier load.

In the end I really want a Projeti that with the Vario 3 blade has about the same static thrust but has a higher top end speed.

Help!

Cheers,

Chuck

[Bruce Abbott]

Originally posted by Talon Driver
Some of this could be explained by the T and P Constants (I used those for an APC even for the Vario).
This is your main problem. It is not valid to assume that all props have the same constants, even from the same manufacturer. Each prop should be individually measured to determine its characteristics. From your data I estimate a prop constant of 1.29 for the APC 6x5.5, and 1.49 and 1.38 for the varioprop at 4 and 5 inch pitch respectively. Since you didn't measure thrust, thrust constants cannot be determined.

With revised prop constants Motocalc's predictions are closer, but still a little high on rpm. Unfortunately you did not measure battery voltage, so it is hard to say where the error lies - maybe the motor's Kv is less than 2200, or the battery is performing worse than predicted, or wiring (and meter) resistance is dropping significant voltage (or a combination of these factors).

However, since all the results are affected similarly, you can still see the effect of changng the prop. Motocalc predicts a slightly higher top speed with the 5 inch pitch varioprop, with more midrange thrust, but also increased current. This is the typical result of going to a 3 blade prop, as the extra blade gives more thrust, but rpm goes down due to the higher loading.
Anyone have any tricks for Motocalc that will provide more accurate results not just for static loads but also for dynamic loads?
The dynamic characterstics of most props have never been properly measured, so performance at speed can only be guessed. Motocalc does make an attempt to compensate for dynamic effects eg. a high-pitch prop creates less static thrust than expected. As the airspeed increases, current draw initially increases, until the prop gets out of stall. How accurate is this algorythm? I don't know.

[Bill Glover]

BTW, the 6" Vario blades are limited to 17,000 rpm. So it's just as well you were getting less performance than MotoCalc predicted, otherwise you'd probably have thrown a blade or two in flight!

PS in the past I have flight tested 2-blade Vario against 3-blade Vario. With the same pitch set, static amp draw and rpm were very similar (6" blades on the 2-blade hub and 5.6" on the 3-blade), but in flight, the 2-blader definitely had a slight performance edge. I am using the 3-blader now on an old Pico Jet but only because (a) it's quiet and (b) the smaller diameter means it is clear of the ground.

[Talon Driver]

Bruce,

I greatly appreciate your feedback. You are correct on a great deal of this with regard to battery voltage etc.

I will experiment with the different Thrust Constants.

BTW what is the "P" Const? I know "T" is thrust but I do not know what "P" represents.

Cheers,

Chuck Kowitz

[vintage1]

Power. Its that crude.

You just adjust it till it seems to load up teh motor right.

I have found 1.3-1.5 seems to suit APC E prpps and up to 1.8 for slofly props.

Thrust? I can't measure it either! But again towards 1.5 for SF props. E are defnitely less thrusting, as it were, but don't draw the current...?

Only way to get what you want is with a thrust rig and tach and whattmeter.

[Talon Driver]

OK, help me out here a little bit.

T translates to thrust. Meaning the propeller cranks out more thrust with a higher T coefficient.

P is for power. So does this mean that the higher the P coefficient the more power that the propeller consumes?

If so jacking up T to being a really high number and turning down P to a really low number would mean that the propeller would probably produce more energy in thrust than it would consume in energy in electrical watts. Is all of this correct?

Cheers,

Chuck

[Bruce Abbott]

yes, that is correct.

Assuming equal efficiencies, a low prop constant would be associated with a low thrust constant, and a high prop constant with a high thrust constant. However, an inefficient prop could have a high prop constant, but a low thrust constant.

The minimum prop constant Motocalc accepts is 0.5, and the maximum thrust constant is 2.0. The thrust constant seems to be highly non-linear, with 0.82 producing almost zero thrust, and 2.0 only producing about 15% more thrust than 1.0.

[vintage1]

Yup. Its all crazy, and its the weakest link in Motocalc IMHO.

Follod closely by no really accurate data on many motirs, and the lack of advance timing sjustment on te algorithms.

Doesn' make it junk, but it does mean you have to watch out and not believe in it implicitly. Its a very good tool, not a complete ssolution.