|Feb 21, 2012, 12:13 PM|
Motor for a wind tunnel model
Iím currently involved in a project that involves testing a wind tunnel model with a driven propeller. We need to put 6kW through an 18Ē prop with a tunnel speed of 40m/s to be representative of the full scale case.
Having played around with Propcalc (among other methods) to ballpark some numbers Iíve come up with a couple of motor/ prop combos that will just about do the job when run at peak current draw.
Scorpion HK-5035 (410kV) spinning a 2 blade 18x10 prop
Rimfire 50cc (230kV) spinning a 4 blade 18x14 prop.
The Rimfire is the better of the two, but it would be good to have something with a bit of spare power and a higher kV to give us a wider choice of props.
Are there any motors in the 6-7kW range with a kV of 250-300?
|Feb 21, 2012, 12:47 PM|
If you're counting on the pitch speed as calculated by the various *Calc programs to represent the air velocity in the tunnel -- you're likely to be in for quite a surprise.
Here' some data I collected long ago that shows how velocity off the back of the prop as measured by a Kestrel compares to calculated pitch speed -- the4 blader gets closer!
|Feb 21, 2012, 04:06 PM|
PropCalc returns thrust/ power across a range of 'flight speeds'- that's how I'm estimating performance at the 40m/s tunnel speed. It tallies well with my actuator disc/ blade element analysis so I believe the numbers are good enough for ballparking. Thanks for the reality check though, my analysis and that of the program are 'ideal' so less thrust would be expected in reality... all the more reason for a more powerful motor than the Rimfire!
Of course, there are a fair few outrunners to choose from in the ~7kW range, but those I've come across don't have a high enough kV to deliver that power through even the pitchiest 18" props when the tunnel is at speed.
|Feb 21, 2012, 08:24 PM|
USA, ME, Ellsworth
Joined May 2008
"..The Rimfire is the better of the two, but it would be good to have something with a bit of spare power and a higher kV to give us a wider choice of props..."
The Rimfire motors only look better on paper, if you try to get the amount of power out of them that they say they are good for you'll watch them go up in smoke.
There is a rule of thumb that average quality brushless motors running at their continuous rating will generate 3 Watts for each gram of motor weight. There are some better motors that will get up to 6W per gram or even higher (Hacker, Neu, and Scorpion come to mind first).
If you look at the Rimfire .50 specs those put it at 5.2 Watts per gram for bursts
Burst Watts: 6500
Weight: 44.1 oz (1250g)
This tech note has more detailed specs and rates it at
Max. Constant Current: 110A (at 12S)
Max. Continuous Power: 5000W = 4 Watts per gram
Max. Surge Current: 135A (at 12S)
Max. Surge Power: 6500W = 5.2 Watts per gram
Many of the smaller Rimfire motors had similar ratings and none of them were able to deliver at those levels without overheating. The Rimfires are a house branded and maybe slightly better than average quality motor. Or maybe just an average motor in a prettier wrapper and more cleverly merchandised than the average average motor. But no one is getting the advertised power out of them.
The Scorpion HK-5035-500 motor is a 670 gram motor that has much better specs than the 1250 gram Rimfire 50.
Max Continuous Current 120 Amps
Max Continuous Power 5400 Watts = 8 Watts per gram
Peak Current 160A (5 seconds)
Peak Power 7100 Watts (5 seconds) 10.6 Watts per gram
The fact that Scorpion is getting that much power from a motor that is a little more than half of the weight of the Rimfire testifies to the difference in quality in the two motors you mention.
Scorpion doesn't have a motor that is a little larger than the 50 Series, you would have to move up to the 70 Series to get more power as far as I can tell. But the HK-7050-340Kv motor looks like it will meet all of your needs while running at a very relaxed pace:
Max Continuous Current 200 Amps
Max Continuous Power 10000 Watts = 5.9 Watts per gram
Weight 1700 Grams (51.85 oz)
Peak Current 300A (5 seconds)
Peak Power 15000 Watts (5 seconds) = 8.8 Watts per gram
Not sure about the structure you'll want to mount your wind tunnel prop or props in but using a toothed belt drive would let you vary the effective Kv of the output shaft pretty easily. And it might be better from the viewpoint of removing some mass from the area in the center of the prop if that is of benefit.
Good luck with it, we'd all love to have a wind tunnel like that!
|Feb 22, 2012, 04:42 AM|
Thanks, I had naively assumed they'd deliver the specified power- I should have known better having owned a few disappointing motors! Sounds like the HK-5035 is the one to go for of my original choices. The 7050 will certainly deliver what we need, but (along with the ESC needed to run it) will exceed our budget... not by much though, so it may be an option. For simplicity we'd rather not introduce belts or gears, though that would make motor selection easier.
It's great to use the tunnel, but it would be nice to have more than a day to play with it!
|Feb 22, 2012, 05:44 AM|
Power = weight, so 6-7kW means about 2 kg -2.5 kg
(except if you are running only bursts or short duration)
Instead of speaking desired Kv you could speak about desired prop max-rpm
Because voltage(and ESC) + Kv are also actors for your problem.
Big-heavy motors don't like to turn very high speed (vibrations, unbalance, ball bearings).
Some motors to consider (after Scorpion HK7050 1700g Kv 317 rpm/V)
Keda C80100/06 1750g Kv 180 rpm/V
Suppo A7050 1630g Kv 185 rpm/V
Turnigy CA120-70 2.55 kg Kv 150 rpm/V
Free Air HCS C 12000W industry 3 kg Kv 265 rpm/V
See also Plettenberg and Hacker
Personnally my favourite would be the HK7050
MGM is selling HV controllers (HBC .. 63V or 100V)
|Mar 21, 2012, 04:02 AM|
Sorry to dredge up this thread again! This part of the project has been on the back burner for a while, but I'm now working on it again and designing around the HK-7050 as it seems like the motor for the job. However, the configuration of the aircraft means that there's not a lot of space to accommodate this beast. The motor itself will just about fit, but the fact that the mounting holes are shaft-side means that it can't be attached straight to the firewall, needing a 'cage' type motor mount- it's this that I'm having trouble accommodating. Things would be a lot easier if the shaft could be reversed, but that's not possible.
Another option I'm considering is re-winding a back-mountable low kV motor in the 6-7kW range (one of these for example) to bring the kV from ~160 to ~300. It would be so much simpler to mount and cheaper too. I've never re-wound a motor before so it would be a bit daunting starting on something this big, but there's plenty of help and information here and elsewhere so it seems doable.
Is this an option worth pursuing?
|Mar 29, 2012, 05:46 PM|
Sorry, don't like doing that, but I'd really appreciate an opinion on whether re-winding a motor to increase (~double) its kV could work for this application, and whether there are any potential problems in doing so.
|Mar 29, 2012, 06:11 PM|
For your application I imagine around that
Scorpion HK7050 Kv 317rpm/V
APC 18x10 E
10S LiPo ~37V
Power 6300W Current 172A
Air Speed 160 km/h (44 m/s)
If you want a cheaper motor (to rewind) you have first to dismount it, clear the copper wires, rewind ... re-glue the magnets = a lot of work
Of course it is possible with some expertise.
Result = less efficiency (than with the Scorpion), probably
Risk of damaging the ball-bearings, if there is a third thin BB
Candidate : TGY Rotomax 100cc
|Mar 29, 2012, 07:12 PM|
USA, ME, Ellsworth
Joined May 2008
I saw your question earlier and just didn't have any experience with that motor or feel I could answer the question.
But, if you weren't aware of it, getting an answer from Fourdan is an answer from as good a source as you could hope for.
The Turnigy Rotomax 100 he recommends is a little heavier and a little more expensive than the one you asked about:
A google search for "rcgroups rotomax" will surface a few posts here with some interesting discussion on the motor but there have not really been any first hand reports from users yet as far as I can tell. No one appears to have stripped one for rewinding yet either so you'll be on new ground there too.
One of the posts at that link discusses the differences in the weights between the big Turnigy motors and the more expensive (and more powerful too) Plettenberg and Hacker big motors was kind of interesting. Weight is not a concern for your intended use I guess.
The "third thin BB" that Louis mentions is a bearing located at the bottom of the rotating magnet housing. That is something that has been added to bigger motors because the magnet housings needed more stabilization to keep them from wobbling or deforming in use.
If you decide to rewind one like the Rotomax you'll have an opportunity to make a major contribution to the pool of knowledge here with any photos and other info you come up with. We haven't seen much of anything on rewinding the biggest motors yet.
If you did not know that, the Kv and turn count are linear and the Kv increases as the number of turns decreases. So you may be able to get away with simply removing turns until the Kv is right. To compute the change in turns the math is:
winds_new = winds_old * Kv_old / Kv_new
Of course, you won't know the original turn count until you get the motor stripped. Some motor sellers will share all the details like the turns wound but Turnigy and HK are not among the ones that do that.
If you remove turns you'll be removing some copper mass. And if you become fully addicted to motor rewinding that empty space will nag on you. So you'll probably eventually want to rewind with larger wire to restore the copper mass to the maximum possible amount.
Good luck with it!
|Mar 30, 2012, 11:00 AM|
Thanks for the replies!
It sounds like re-winding is a viable option... perhaps not ideal, but weight and efficiency are indeed secondary concerns for this application. I imagine the bearings could be damaged by running the motor at twice its intended rpm, but durability isn't a huge concern as it will probably only be needed for a total of a few hours (though I'll do my best to get my hands on it once it's served it's purpose ). Being statically mounted, I'd imagine it will have an easier life than flying motors due to the absence of gyroscopic forces.
Since re-winding using the original wire will significantly reduce the copper mass, am I right in thinking this will reduce the power I can put through it? It would be great if the motor was wired in star configuration as I could get the kV I'm after simply by changing it to delta, but I gather most motors are delta wound already...
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