Motor/prop testing

[Red]

Hey,

I picked up a new Park 450 today and a Phoenix35 esc for next years racer and decided I would take the time and do a spread sheet and try to keep a record of actual numbers under controlled test conditions to eliminate as many variables as possible. It made for some interesting results!

Test conditions:

I updated my Phoenix Link to the latest Rev, 3.220. I aslo updated the esc to the latest Rev 3.00(beta) and to my surprise there was a warning on the download page about a significant performance increase and the need to watch current draw with the changes.  I tested with 2 different batteries, a NeuEnergy 1500/30C and my trusty old Thunder Power 2100/15c packs. I used a stock out of the package APC 9x9E prop as the constant. Then I tried each battery with low, normal then high timing, repeaking each pack between tests. I ran each test at full throttle for 10 seconds to let the current/volts stabilize and then took my readings. While checking RPM with the tach, I used a flashlight to shine on the sensor as flourescent lights will affect the readings.

The results were not exactly as I suspected. I had long been a proponent of low timing on outrunners. This latest software build packs some significant changes to the effiency of the switching algorythm allowing a nominal power increase. I think low timing is no longer the best setting for racing. It is however the best overall setting for efficiency. Normal and high timing yeilded the highest sustained RPM numbers, very close to eachother actually. The difference is the amount of current draw. With high timing the motor heats up a bit more and draws more current. Normal timing gets you essentially the same RPM but with lower temps and less current.

For now I am leaning towards normal timing. I suspect near the end of the race the pack will be happier and still deliver good performance where high timing might have an adverse effect. This may change as you load the motor down with steeper pitches, but it's a good place to start.

Cheers

Later

[Jon_Stychno]

    Woah , cool stuff. Have you decided on an airframe?

[Red]

Rare Bear for now...not sure for the back up.

[Jon_Stychno]

Thunder Tiger Rare Bear? Cool! Nice to see more Reno racers out there.

[Red]

Yeah it's one of my favorite birds. I'm going more scale for the time being.

[Jon_Stychno]

Still have "Big Hummer", she's up next on the racer (re)build list. I still don't know what went wrong, but I'm gonna give it another shot. 

[Red]

I'm thinking a stiffer pushrod may be in order. 

[Jon_Stychno]

The TT Corsair has a ply bulkhead halfway between the wing and tail that the pushrod runs through, and seems to eliminate any flex. The TT Mustang has the same style pushrod set up minus the bulkhead, and elevator responsiveness seems good wide-o. Another test flight (at altitude! ) will answer some questions. How did your tygon motor mount work out on the Hayate? Posting a pic or two might help some warbird racers solve the tricky motor mounting task on most of the kits used in the class. It would also come in handy dialing in thrust angles, which are usualy altered by ditching the stock motor mounts.

[Red]

The motor mount worked well. It was definately quieter, but I'm not sure about power. Sometimes a soft mount will actually reduce power. I didn't tach it after the mod.I did it as you pointed out to make it easier for thrust angle changes. I don't have the plane any more or pics, but if I do it on the Rear Bear I will take some.

[Red]

There have been reports of something ominous rumbling in the air. Strange noises and a rythmic pulsing have generated some rasied eyebrows and melted stopwatches. Time will tell........

[Jon_Stychno]

Has enough time passed yet? 

[Red]

Not quite yet.  Some interesting things for sure....

[patrick_ohalloran]

Dumb question, but is it possible to change the pitch on an APC prop? I assume not. I mean I cannot heat it up and twist it can I?

[Red]

Great question Patrick. I wasn't going to let the cat out of the bag about repitching props, but since you asked, I let on a little. 

Props can be repitched a number of ways. The most common is like you pointed out,  heat and twist. This works for many props like the Graupner CAM series, Master Airscrew, GWS etc. Unfortunately this doesn't work for APC (stranded resin construction) or wood props for obvious reasons.

APC and wood props can be repitched with other methods however. To do any correctly, a pitch guage is a must. Prather used to make a nice one but it has been out of production for a number of years. They still pop up for sale on EBAY, RCU and Ezone occasionally.You can also carefully mark each blade in 1/4" or so increments to make "stations" and plot them manually. I've done it both ways and both work fine, bit the guage is MUCH easier.

With APC and woodys', the simplest way is to cut down a larger prop for more pitch. Since the pitch is helical in nature and follows a progressively curved path, when you cut it down the pitch increases towards the hub. I do this frequently. I requires a bunch of rework however to bring the blade profile back to original or close to it.

Another way is to sand in more pitch. Since the blades are usually pretty thick, you can sand the leading edge further up and the trailing edge further down. I'll do this when I can't find a suitable larger prop to cut down or I really like the way the prop I'm starting performs. This technique is more for adding small amounts of pitch to an already good working prop though.

If you have a prop you would like to twist, use a bucket of HOT water. Not boiling but as hot as you can stand it. Drop the prop in and let it heat for a minute of two. Then grab the hub and twist from the shank. This will ensure the whole blade twists and stays even and just not one local portion gets twisted. This is important because it can create an area of the prop that may stall, while another area may work fine. This makes the whole prop less effecient.