Tutor II

[JustinShackleton]

A friend gave me a Tutor II that was setup with Cobra 2820/14 840kv. Running on 4s 3000mah. Looking to buy some electronics for it.  Was wondering if that setup is correct.  Seems like the mah rate is little low for 4s for a 5.5 minute flight.  Based on the formula I found (Battery capacity = 1.2 x Watts x Minutes x 16.66 / Volts ( 16.66 = 1000 mAh/A/60minutes/hour ). Looks like the battery should be around 4000 mah.  I was looking at the cobra 2826/12 760kv. and running at 5s.  Very new to electric airplanes so really not sure.  Could someone point me in the correct direction. Thanks Justin

[Tim Wescott]

I don't know where your numbers came from.

Weigh the plane, then do some math.

Multiply the weight in ounces by 7 watts/ounce.  That's the average power if you're using a Hubin timer and an ESC in helicopter or CL mode, and probably the same if you're running a KR timer (the Igor timer uses more, and gives you more).

Multiply the resulting figure by 0.1 hour (6 minutes is 0.1 hour).  The resulting figure is the watt-hours of energy needed.

Divide the resulting figure by 0.75 (i.e., multiply by 1.33).  That makes sure you're leaving some reserve in your battery -- if you suck it dry each flight, it won't live long.

Divide the resulting figure by 3.7 volts per cell, and then again by the number of cells (four in your case).  That's the amp-hour rating you need.  Now, if it's not obvious, multiply that by 1000 to get mAh.

For a 48 ounce plane, which seems reasonable for your Tutor, I get just a hair over 3000mAh.

[Keith Renecle]

Hi Justin,
If you look at Mike Ostella's thread on the Cobra 2820/12 and his Vector close by, you will get some more info that could help you. Your 2820/14 with the Kv of 840 is a bit low unless you can use more pitch. The really good prop for a .40 size e-stunter is the APC 11 x 5.5 E prop but it will need to spin at close to 10 000 rpm so you need a little more Kv. I have an E-Max GT 2820 with the same Kv as your Cobra and I needed a cut down 12 x 6 APC to get a good flying speed. It is then a little heavier than the 11 x 5, 5 so it is not ideal but it will work and at least you can then play with e-power and get the hang of it.

The average 4S 3300 battery weighs a lot.......about 300 grams and more. Mike Ostella just pointed out that the Thunder Power 2700 pack weighs much less at around 234 grams so that is a huge saving. The pattern takes about 5 minutes to fly so a 5:30 flight is plenty. With a 3300 pack you can fly about 9 minutes with your setup so that is a huge overkill. The extra weight however will ruin the performance. I would guess that something like the Tutor should weigh around 42/43 oz. and then you would have a good setup that could be flown with a smaller battery pack as well. Maybe the Tutor II is bigger than the old Tutor, but even then a good flying .40 size profile will fly a lot better if the weight is kept down to 45 oz. or less.

Keith R

[Target]

I'd think that the 2826/10 on 4S would be a better choice, but I'm a newbie.
Just based on what I have observed, 875-975 Kv on 4S seems like something to strive for.
Experts, what say you?

R,
Chris

[Keith Renecle]

Chris, your Kv estimate is correct, but the motor is too big for a .40 size model. A good thumb-suck on the size is to get the approximate all-up  weight of the model and divide it by 10 for the weight of the motor and you will be in the ballpark. The 2826/10 really came from the very popular AXI 2826/10 and it weighs around 180 grams. This was the first really popular motor that was used by the guys that led the way in electric stunt. A .40 size motor should weigh from 130 to 150 grams, and there are quite a few good choices available. We need to find the most popular size motors from a bunch of R/C motors for obvious reasons. This is very much the same as  what we did for our stunt engines, and of course there were many (and still are) modified R/C engines for flying good stunt.

There are also the custom stunt engines made specifically for us in relatively small quantities. The only readily available custom wound stunt electric motor that I know of, is the AXI 2826/13 which Igor Burger has made for running on 6 cells. With batteries getting smaller and a lot better these days we can use the highest voltage that is practical because the higher the voltage the lower the overall current in the system. I look forward to seeing the development in power storage. What we have now is pretty amazing, but looking at things in the pipeline, we are in for some even better stuff in the next few years.

Keith R

[Tim Wescott]

There are also the custom stunt engines made specifically for us in relatively small quantities. The only readily available custom wound stunt electric motor that I know of, is the AXI 2826/13 which Igor Burger has made for running on 6 cells.

Just an observation:  To make a custom stunt engine you need a fully-equipped machine shop, and you need to be a seriously experienced machinist.  To rewind a motor, you need to have enough hand tools to disassemble and reassemble a motor, you need a soldering iron and various sizes of magnet wire, and you need to be able to wind neatly.

So -- where are the motor rewinders?

[Keith Renecle]

A couple of years ago on this forum, there were a few that did some rewinding. Alan Hahn comes to mind. These days, there are quite a few options of weights and windings, especially from Cobra, so this tedious job is not that popular. I can remember that Scorpion motors offered winding kits as well. As a kid (a slightly smaller one than now) I tried my hand at slot car motor rewinding.......I smoked a few going down the main straights as well!  The neat thing about these brushless out-runners is that the coils stay fixed and the drum and magnets spin, so you don't have to balance the armatures and epoxy the windings.

Keith R

[JustinShackleton]

So is the 2820 12 a better choice for the plane? And that can be paired with the 11x5.5 prop.  What about battery mah and c rating.

[Keith Renecle]

So is the 2820 12 a better choice for the plane? And that can be paired with the 11x5.5 prop.  What about battery mah and c rating.

Yes, that will work very well. The battery can be a 2200 mAh 4 cell with a 25C rating. We do not need higher than 25C for stunt and the higher C ratings just weigh more. The next popular size battery is the 2700 mAh, and if you don't mind paying a lot more, then the Thunder Power 2700 is great. It weighs the same as some of the regular 2200 packs.

Keith R

[Dennis Toth]

The Tutor II is a significantly larger ship (wing area is just shy of 600 sq's). IC power would be an LA 46. The weight is north of 54 oz. I have flown my converted to ECL for several years. Have use and AXI 2826 10, used a TP Pro Lite 3300 mah, 4S pack that is just a bit short for long term battery life. What is needed is a 3500 mah pack that weights similar to the Pro Lite. The next size up is ~1.5 - 2 oz heavier (the TP 3900 Pro Lite). If you use other brands you get hit with a bigger weight penalty.

One option is to work with prop sizing. Most people will agree that diameter is king and low pitch is queen. In the IC world current power plants turn 11K to 13K + rpm very happily. This allows them to run 3.5 to 4" pitch which pulls out of the corner. For ECL in order to run the big diameter we need to run high (5.5 to 6") pitch numbers to keep the amp draw and pack size reasonable. In some simple tests that I did diameter costs more amps than pitch. The problem is that as we reduce diameter we increase slip for a given size/weight ship and require slightly more rpm to hold lap time. It would be interesting if someone has the time to do some tests with smaller diameters, low (4") pitch props to see how we could get the amps down (by reducing diameter) and what rpm we need to maintain lap time for a given size ship.

Best,       DennisT

[Keith Renecle]

Thanks Dennis for the info on the Tutor II. I did not realise that it was that big, so Chris Behm's idea of the 2826/10 might just be a good idea. A motor that might just be ideal is the Black Tiger  BT 3548C - 900Kv from RSM. I have one and it is just smaller than the 2826/10 AXI at 160 grams (5.64 oz.) I tested my motor in my 1800 gram Newtron and it ran well but was just a tad small and became a bit too warm for me. I plane to use it in a .46~.51 size plane.

My charts based mostly on the figures and calculations from this forum plus a lot of data recordings, indicate that a 2700 mAh 4 cell would have enough reserve for a good 5:30 flight (actual motor run) on that size of model. You really don't need more motor time than that for a comfortable pattern. My Newtron was flying the F2B pattern in just on 5:00 minutes and the 3300 pack was landing with at least 18% left over on the Hyperion battery checker. This was also without a 100% full charge.

Dennis, are you running the APC 12 x 6 E thin blade or what? I have not done a lot of testing for prop slip, but just recently I did some comparisons using my Eagle Tree data recorder on my .15 size model. The props were APC 8 x 6 E and APC 9 x 6 E. I was flying at 11 000 rpm on the 8 x 6 with a lap time of 4.2 seconds. The static current draw was 20 amps. With the 9 x 6 I fly at the same lap time turning the prop at 10 100 rpm with just a tad more current, but it flies with so much more authority through the pattern.

Keith R

[Dennis Toth]

Keith,
Interesting information from your tests. I have been flying the APC E 12x6P. I also tried the APC E 11x5.5. The larger 12" pulls strong as you experienced. What would be interesting is if you could try a 9x4, I think they launch at 12,500 rpm. If you can compare this with the slower 9x6 rpm would give us another piece of data, also how much each setup draws down the battery pack down. If you can let us know.

Best,     Dennis

[Keith Renecle]

O.K. APC makes a 9 x 4.5 E prop and I tested this with an R/C TX/Rx setup. On my 1400 Kv motor it maxes out at 11200 rpm and draws 21 amps static. I did not try to fly it because that is not enough rpm for such a flat pitch. On my governor with a bit of headroom I get 11000 rpm. What I have found practical for e-stunt is that 5 to 6" of pitch works well. I know that the 13 x 4.5 APC does work for some people as well but on the smaller diameter props the rpm needs to be too high so you need a higher Kv or more cells. For the .15 size airplane, I don't really want to use more than 3 cells. Just for testing sake, I will try to fly the model with the 9 x 4.5 prop sometime soon  and I'll report back.

Keith R

[Keith Renecle]

O.K. I managed to fly the setup with the 9 x 4.5 E prop, and it was not too bad, but as I thought, a bit on the slow side and I was getting 4.7 second lap times. I could still manage a vertical eight with reasonable line tension, but then the weather was pretty good. This was turning at 11 000 rpm. I put back the 9 x 6 prop and it was just soooooo much better. I dropped the rpm slightly from my previous 10 000 ~ 10 200 right down to 9800 and flew really nicely at 4.4 second laps. Solid through the pattern and great line tension all the time. I get the feeling that I could even use longer lines. The other thing is that our club here in Johannesburg South Africa is at 5800 ft. ASL so at a lower altitude I could lower the rpm even more.

Keith R

[Dennis Toth]

Keith,
Interesting, to bad you don't have a higher K factor motor to get the 9x4.5 up to around 12000. The 6 pitch at the rpm you were running is in its range. The 4.5 is just outside its range at 11K. I found the 12x4 I used way back and plan to try it this weekend on my Yak-Yak bipe (two Yak 9 kits put together - twice the wing area in level flight 1/2 the wing area coming out of the wingover - haha).

Best,      DennisT

[Keith Renecle]

Sure I could go up in Kv, and I think that the next available motor of a similar size will have around 1500~1550 Kv. The reason why I don't like that idea is twofold. With e-power we are always looking for overall efficiency and for the same airspeed, a finer pitch prop will require more power. The 2nd reason is noise and to go from 10K rpm to 12K pushes up the noise factor. The neat thing about e-power is that we are not stuck with the problem of trying to keep an IC engine happy with prop pitch. We have governors that can hold the rpm and this lets us juggle stuff like motor Kv, esc size, battery size, and available props like APC that really don't cost much.

Coming back to Justin's Tutor II, you can certainly use a bigger motor and prop with bigger batteries than you need, BUT the thing to remember is that you only need "enough" power and not excess power, because all you are doing is adding unnecessary weight and that creates a snowball effect. An efficient system will mean that you can use a motor, esc and battery pack that will land without being really hot and that the battery will have a nice reserve left over, so that it can be used for many good cycles. I just looked at my other post on my Newtron and saw that I wrote that I had 18% left over after a flight. I checked my notes and I had 20" at low altitudes and 24% up where I live at 5800 ft. ASL.

So Justin, I hope that you will keep us posted on what you end up with in your Tutor II.

Keith R