On the bold, BINGO! The best you can do via calculations is to get an idea of what SHOULD work.

Try this, I have used it for decades and it is always close to radar clocks, the only variable that affects accuracy besides airframe drag is propeller brands. Some manufacturers always state their props as having higher pitch than is measured, this means the rpms will be higher with their props and inexperienced pilots are influenced by rpms. For instance, if I put on a prop that has an excellent rip sound then I know I can increase airspeed by going up 2 inches in pitch even though static rpm will always drop and there may not be any prop rip with the higher pitch. A prop that has a nice rip cannot fly faster because it is operating at its max mach speed. In this case the plane would go faster if the prop fell off in a dive. This fact is illustrated by the current RC world speed record holder being (not a jet), but a slope sailplane at over 500mph. Now lets see what kind of arguments this provokes.

That is the problem with finding multi blade props, there just are not a lot of options. The problem with these bigger props, is the cheap ones do not maintain their shape under load. So essentially they flatten out losing their pitch. Among my fellow large scale warbird flyers, the Zingers and Top flite props are known as "paint stirrers", as that is about what they are worth in relation to using them for flying.

You may look at APC and see if they have that multi blade hub, then you can replace a single blade if one gets damaged.

I am going to inject this as well. These 4 blade props look no more scale than the optimum 2 blade prop. So to me, I would just get the optimum 2 blade prop, and go fly. If you want it to look more scale in the pits, throw a static prop on it.

mark IX,
I am currently flying a TF P 51B with the rimfire motors. When I had the 65cc version installed, using an APC 26-15E prop and 12s 5300 battery packs, data downloaded from the CC 120HV ESC showed it had reached a peak rpm with this prop of 6345. Amp draw at this rpm was 60 and watts were 2661. Estimates of airspeed by other pilots were 95-105 mph, pitch speed by the chart was 98mph. Peak amps at the beginning of level flight acceleration from fast cruise was 104amps, watts 4696. The Rim Fire 65cc has a kv of 160 the same as your motor. Performance would be similar. My plane weighs 28.5 lbs. I have observed the same numbers using an XOAR 24-14 sport prop and 26-14E prop. To compare my planes performance to other similar planes with gas motors, none have been able to equal it in speed or climb, their only advantage is duration. I have had online discussions with a pilot who uses a Varioprop (Germany) 22 inch 4 blade set at 16 inch pitch. He says he got speeds of 150(?) mph when the motor was in a GS Corsair. I can tell you this- it is necessary to use prop pitch angles higher than 14 inches to get a 160 kv motor to deliver adequate warbird performance. My motors seem to respond to high pitch more than any other factor. Higher rpm would definitely help, the only way to get rpm is more cells-higher voltage. This means 14-15 cell packs and ESCs that can handle them, they cost twice what I am spending now. From the way my motors have responded to blade loading I don't think 3-4 blade props would faze them, they need high pitch to increase performance. I have checked out Biela 4 blade carbon props, they have a 23-14 Corsair prop that I think would pull my plane at 85-90mph, scale speed for 1/5 warbirds. If they had one with 16 inch pitch I would buy it in a heartbeat. I read in another forum about a TF P 51 pilot using the Rim Fire 65cc and the Zinger 22-14 4 blade prop who said he has been operating his plane with this combination for a couple of years and is happy with performance. By my calculations if he is seeing 6500rpm his air speed is 95mph. My motor run on a two blade 22-12 APC or Graupner seem to be running free-no load and speed and climb and amp draw are all low. RPM was 7000, close to what it turns with no prop. After reaching what I considered max perf. with the 65cc I switched to the 50cc version and got speeds up to 110 mph lvl flt using the Graupner 22-12 with a nice rip at 8450 rpm and a little better using XOAR 22-14E but no rip. Amp draw is up to 90 in lvl flt and 135 in verticals. I don't try diving to get high speeds any more after an earlier plane's fuse broke in half after a radar clock of 122 using a FOX 2.4 and an APC 20-12 prop. Hope this will help you with your project. When I started mine I could find very little actual experience that I could use. The calc programs don't seem to be able to give meaningful results on large scale systems, even when I input data from successful power combos I had already tried. I got a lot of useful data from the 3d guys.

Of course it's hard to calculate a drive exactly, but it isn't that hard either, especially an electric drive. And mere estimates may be sufficient:

The motor has 160 Kv so assuming 12x3.5V (battery voltage drop under load) =42V the motor would spin 6720 rpm if idle. With a 14"-pitch prop that would mean 89 mph top speed and with the 16" prop 102 mph. That's a bit simplified but pretty good. An electric drive "unloads" not that much with speed, 10% is a good estimate for such a drive (other drives could have up to 20%). So you can expect 80/91 mph top speed, the actual flight speed depending on the model's drag, as has been said before, and assuming the pitch specification is correct.

Maybe I'm mistaken, but I wonder how E-calc gives that low rpms and speeds. Anyway, I think 50 mph would be quite a good cruise speed, 80 mph being the true-to-scale top speed. The higher-pitch prop would be needed for the required top speed (because it won't reach pitch speed), but it would be good for economic cruise flight as well. I would even look for an even higher pitch.

Since only estimates are possible some experimenting with different props may be advisable. A reasonably accurate calculation would be a bit tedious (though possible) and would require calculating the aerodynamic performance of the model. Could be overkill.

Sjhanc, good data, thank you. Do you know what your flight times were?

Thanx, Mark

I set a timer for 5 minutes at take off and try to be on final when it goes off. My current setup is very easy to land so it usually works ok. If I miss an approach I have enough battery for 2 more tries. Usage is around 3900 mah out of 5300 mah. I only go full throttle for 3-5 seconds for vertical maneuvers but that is enough to get it out of sight. The only planes I have that climb faster are hotliner gliders and nothing else can keep up with a hotliner in a vertical climb. I am assembling a 12s 2p 3200 mah pack that should give me a couple more minutes without additional weight.

Sjhanc, thats pretty close to what E-Calc has predicted for my flight times as well. It said around 4.5 to 5.7 minutes.

Thanx, Mark

I used motocalc and every result was this will overheat. I was already flying it and have solved the temp issues.

Yeah, me too, which is why I switched to E-Calc.

Sorry about the poor quality, it was a gray day. Four level runs produced GPS speed readings from 110- 114 mph. TF GS P 51B, Rim Fire 50cc brushless, Gens Ace 5300 mah 45c 12s LIPO pack. CC120HV ESC. Graupner 22-12 sport prop, 8470 rpm. The chart indicated that this set up would have a pitch speed of 108mph so maybe the prop is a little better than 100% efficiency or there might be some unloading in the air. I was unable to down load data from the ESC. When I hooked up the Castle link software this morning it updated the app then it needed the firmware updated from 4.19 to whatever the latest version is. After getting the plane on the ground and before making a turn to return to the pits it suddenly let out the expensive gray smoke from the ESC. This ESC had had 50 successful flights before the firmware update and the flight duration and maneuvers were no different than before. It either was time to go or it choked on the new firmware. The plane was test driven around the yard for an hour yesterday and was run up to full power with the previous firmware with no problems. Another factor is the rough ground at the edge of the runway tripped the gear and it went up on the prop, stopping it, I shut it down but maybe not quick enough.

It's interesting that using the speed formula posted earlier in this thread and using these specs, the speed comes out at 101 mph. If the motor unloads as suggested, then that puts it right on the money.

Sjhanc, sorry to here about your ESC. I think I would have a discussion with Castle about it if I were you....... BTW can you post some pics of your setup.

Thanx, Mark

in this picture the motor is the RF 65cc. I am now using the 50cc version. Except for the length they are identical. The ESC here is behind the fire wall, I have since moved it to a plywood platform just behind and below the air duct, it gets a lot more air in the new location and never gets hotter than 132F. The plane is inverted on a stand for the electric install.

Why did you switch to the 50cc motor?

Thanx, Mark

Because the 65cc motor with its 160kv will only turn 6700rpms max on 12 cells(26-15prop). This limits top speed to 95-100mph if it unloads all the way. The 50cc motor has a 230kv and will turn 8500rpm (22-12 prop). I checked speed this last weekend using an onboard camera recording GPS speed and got top speeds of 110+ mph with a peak on one run ending in a vertical climb of 114mph. Since I don't do long dives to build speed this means that because the P 51 being a low drag airframe is allowing the prop to operate at slightly better than 100% efficiency It is flying faster than computed prop speed (108mph).

Do you have any experience with the Suppo 7034-8 motor? Kv is 190, W=6600. I was also considering this as well.

Thanx, Mark

mark IX, sorry that's a new one on me. you will have to rely on user ratings.

BTW, is there a reason these elec motors are mounted off of long standoffs, instead of face mounted on the mount ears? Cooling? Or personal preference.

Thanx, Mark

You could build a plywood box to space the motor the correct distance from the fire wall, a lot of the 3d planes use this method. About 15 years ago I built a plane that used 2 ply boxes, one fit inside the other. You bolt the motor to the large box and slide it over the smaller box to get the spinner space you need then bolt or glue them together. The boxes were 1/8 air ply and the motor was a ZDZ 80. It worked fine and had no standoffs. The carb was inside the box and got air from inside the fuse. it had a hole on top for needle valve adjustment The box was also used to mount the ignition box and the servo and micro switch for eng. cutoff. I glued furniture grade hardwood corner stock inside all the corners for strength and aluminum angle to reinforce the motor mount ply wood (aircraft grade) And the box to firewall joint-It isn't necessary if you don't use a ZDZ motor but may be if it is a bigger brushless motor.

mark IX'
I just got back from flying a new battery. Made out of Hobby People 4s 3200mah 30c packs. The assembled pack is 12s 2p (24 cells) 6400mah and is good for 60c. Temps. are lower, rpms 200 higher (Graupner 22-12 sport prop). Highest speed reached was 118mph level flight. The old 12s 5300 45c pack gave me 5 min. (3900ma) with two approaches. The first flt. with the new pack lasted 7 min. and used 4900ma with 1500ma reserve. I will carefully extend flight times with throttle conservation and I think I can go 9 min. with this pack and still have enough for 2 approaches if I miss the first. This 24 cell pack weighs about 12 oz. more than the old pack. The plane doesn't notice the extra weight.

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