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twin motors back to back

Old 06-14-2012, 04:58 PM
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skater_719
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Default twin motors back to back

I was wondering, I am building a Northstar Arrow, and planned on using twin brushless 480's, my original plan was to mount them side by side, but then I got to thinking bout mounting them back to back, both motors are in the centerline of the plane, and one in pusher, and one tractor. My reason for considering this is
1. keep the center of gravity close to the center of the plane
2. having counter rotating props in the middle should mean no issues with torque steer
3. I have never done it, and it would look pretty cool imho.

My only concern is, will all that turbulant air coming from the first motor rob the second motor of thrust? If there are any other ussues that would go along with that setup please dont hesitate to point them out. Thanks!
Old 06-14-2012, 06:02 PM
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Default RE: twin motors back to back

The Cessna 337 flys great in that configuration.
Old 06-15-2012, 01:45 AM
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Default RE: twin motors back to back

I have heard that in this configuration it is common to use more pitch on the rear prop than on the front one.
The rear prop is meeting air that has already been accelerated and so could do with more 'bite'.
Old 06-15-2012, 03:23 PM
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Default RE: twin motors back to back

Hi skater_719
I had a twin boom plane with two engines in pusher/puller configuration and flew it about 7 years. I never noticed any thrust problem. The rear engine used a pusher prop. I did notice that the plane would not do a normal crisp snap roll. It acted like it did not want to do one. I assumed that the reason was the engines were turning in opposite directions so a gyroscopic effect was preventing the snap roll from being crisp. If you use a conventional prop on the rear electric motor and mount the prop opposite to "normal" it may not have that same effect. Please let me know how it does.
Old 06-15-2012, 05:12 PM
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da Rock
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Default RE: twin motors back to back

With electric motors, you can reverse the rotation by swapping two of the leads. So for a pusher installation, you don't have to use a pusher prop. You should have no problem propping the rear motor as you can use tractor props.

For Villa's model, he could have easily tested to see if crisp snaps would result with both props rotating in the same direction.

For conventional twins, you can easily set the two motors up with counter rotating props. That does require one to be a pusher. Moreover, you can easily setup the counter rotation both ways. Remember the P-38 Lightining was issued with innies and outies at different times. They had to swap engines to do it. With electric motors, you just swap the props, then swap two leads on each motor.

pretty kewl things to play around with...............
Old 06-17-2012, 07:28 PM
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Default RE: twin motors back to back


ORIGINAL: alasdair

I have heard that in this configuration it is common to use more pitch on the rear prop than on the front one.
The rear prop is meeting air that has already been accelerated and so could do with more 'bite'.
This makes me wonder about one more thing. I was planning on using separate batteries for each motor, but if I do that, will the front motor pull a significant amount more amps then the rear motor? that is the way i have always wired up twins but for this one would it make more sense to then run both motors off one 4000mah rather then the planned 2000mah dual batteries? The wing is 40''ws by 25'' chord made from 1.5'' pink insulation foam. I just kind of eyeballed the size of the plane to what I thought my two Exceed rc Optima 480, 950kv motors could handle, but I haven't ever used these particular motor before, To anyone who has experience with these types of motors, do you think i'll have plenty of power? According to an online motor calc i found, on 3 cells with 9x6x3 props I have on now, it says i should get over 3lbs of thrust from each motor , and with a larger prop I could get over 4, so according to the calc I should have between 6 and 8lbs of thrust! Not sure if I can believe that, but I went to 2 different calcs and got the same results. Please give me your Guys's opinions!
Old 08-12-2012, 09:34 AM
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Default RE: twin motors back to back


ORIGINAL: da Rock

With electric motors, you can reverse the rotation by swapping two of the leads. So for a pusher installation, you don't have to use a pusher prop. You should have no problem propping the rear motor as you can use tractor props.
Yes, this will work, but depending on the size of your plane and the propeller you are using it may not be the best idea.
If you look at a propeller you will see it is an airfoil with a leading edge and a trailing edge.A propeller not only deflects air but it creates lift. Another thing to keep in mind is the leading edge of an airfoil wants to be in the front, like an arrow.On larger props this can create enough torque on the propeller hub to cause damage.On smaller propellers you are just wasting energy and efficiency – something you really don’t want to do with electrics.

Spinning the prop backwards will move your plane but you will lose efficiency.You have to buy a prop for that back engine anyhow – why not buy the correct one.
Old 08-12-2012, 11:08 AM
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Default RE: twin motors back to back

ORIGINAL: im_a_rcav8r


ORIGINAL: da Rock

With electric motors, you can reverse the rotation by swapping two of the leads. So for a pusher installation, you don't have to use a pusher prop. You should have no problem propping the rear motor as you can use tractor props.
Yes, this will work, but depending on the size of your plane and the propeller you are using it may not be the best idea.
If you look at a propeller you will see it is an airfoil with a leading edge and a trailing edge. A propeller not only deflects air but it creates lift. Another thing to keep in mind is the leading edge of an airfoil wants to be in the front, like an arrow. On larger props this can create enough torque on the propeller hub to cause damage. On smaller propellers you are just wasting energy and efficiency – something you really don’t want to do with electrics.

Spinning the prop backwards will move your plane but you will lose efficiency. You have to buy a prop for that back engine anyhow – why not buy the correct one.

If the following verbage is too much, just skip to the picture in the following post.

With the setup I described, you won't spin either prop backwards. No matter which direction an electric motor faces, forward or aft, the direction of rotation can be selected to rotate the prop in the proper direction for the prop. The leading edge of both props will be rotating in the right direction.

Pusher props were originally sold to be used on reciprocating model engines because the engines rotated in one direction only. If you pointed one of them aft, normal props wouldn't work properly for exactly the reason you mention. You need pusher props when you point our recip engines aft. They aren't needed with brushless electric motors since those motors operate in either direction with equal efficiency and power. Our props work equally well when tightened onto electric motor prop shafts facing "in" or "out". They are compressed front and back equally. If you're putting a tractor prop on an engine facing aft, you simply put the prop on facing the direction of flight and then connect the electric motor to turn that prop as a tractor prop.

BTW, there were a couple of reciprocating engines in days gone by that could be setup to run "backwards". They had a crankcase design with the 'nose' bolted on the same way the backplate is bolted on. The bolt pattern for the nose and for the backplate were identical. You simply bolted the nose on the back of the engine. Then came the really tough part. None of us were used to hand cranking those suckers clockwise. And only a few old timers (older timers?) and some speed flyers had hand cranked starters. The starters I saw were straight geared and could be cranked either direction.
Old 08-12-2012, 11:45 AM
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da Rock
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Default RE: twin motors back to back

Since a picture is worth a thousand words, I took a picture of my Twin Engine, Pull-me-Push-ya Wildcat scale model.

Few people know that Grumman tested an upgrade to the Wildcat during WWII. They installed an advanced electric motor in the rudder. In order to save tooling costs, they slapped a wooden tractor prop on the booster motor. As you can see in the photo, both props on my model rotate in the same direction and both are the less expensive tractor types.

My electric motor came from the factory setup to rotate in the wrong direction for my model, so I simply swapped two wires, which is what we do, right.

With electric, we can use tractor props with no effort at all.

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Old 08-13-2012, 11:45 AM
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Default RE: twin motors back to back



ORIGINAL: da Rock

ORIGINAL: im_a_rcav8r


ORIGINAL: da Rock

With electric motors, you can reverse the rotation by swapping two of the leads. So for a pusher installation, you don't have to use a pusher prop. You should have no problem propping the rear motor as you can use tractor props.
Yes, this will work, but depending on the size of your plane and the propeller you are using it may not be the best idea.
If you look at a propeller you will see it is an airfoil with a leading edge and a trailing edge.A propeller not only deflects air but it creates lift. Another thing to keep in mind is the leading edge of an airfoil wants to be in the front, like an arrow.On larger props this can create enough torque on the propeller hub to cause damage.On smaller propellers you are just wasting energy and efficiency – something you really don’t want to do with electrics.

Spinning the prop backwards will move your plane but you will lose efficiency.You have to buy a prop for that back engine anyhow – why not buy the correct one.

If the following verbage is too much, just skip to the picture in the following post.

With the setup I described, you won't spin either prop backwards. No matter which direction an electric motor faces, forward or aft, the direction of rotation can be selected to rotate the prop in the proper direction for the prop. The leading edge of both props will be rotating in the right direction.

Pusher props were originally sold to be used on reciprocating model engines because the engines rotated in one direction only. If you pointed one of them aft, normal props wouldn't work properly for exactly the reason you mention. You need pusher props when you point our recip engines aft. They aren't needed with brushless electric motors since those motors operate in either direction with equal efficiency and power. Our props work equally well when tightened onto electric motor prop shafts facing "in" or "out". They are compressed front and back equally. If you're putting a tractor prop on an engine facing aft, you simply put the prop on facing the direction of flight and then connect the electric motor to turn that prop as a tractor prop.

BTW, there were a couple of reciprocating engines in days gone by that could be setup to run "backwards". They had a crankcase design with the 'nose' bolted on the same way the backplate is bolted on. The bolt pattern for the nose and for the backplate were identical. You simply bolted the nose on the back of the engine. Then came the really tough part. None of us were used to hand cranking those suckers clockwise. And only a few old timers (older timers?) and some speed flyers had hand cranked starters. The starters I saw were straight geared and could be cranked either direction.

Ahhh… very well explained.I had not entertained the idea of turning a the prop “in” or “out”.
Sometimes the simple things escape me.

The photo of your plane is awesome!That is the coolest thing I have seen in a long time!
Old 08-13-2012, 12:09 PM
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rmh
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Default RE: twin motors back to back

Am I the only one to see thru your gag setup
Old 08-13-2012, 03:31 PM
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ORIGINAL: rmh

Am I the only one to see thru your gag setup

Jeez, I hope not....

After all, everyone knows that Grumman used a V-8 that turned out to be the famous Chevy 283. Heck, there weren't any electric motors back then small as that V8.
Old 08-13-2012, 07:29 PM
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Default RE: twin motors back to back

On the pusher/puller I describe above, one of the most fun things I did was to attempt a take-off on one engine. The plane was a 40 size plane, with a 48" WS, and used two Magnum 28 glow engines. I don't remember the weight, but my guess is 6 pounds. For a one engine take off I only started one engine. All conditions had to be perfect; freshly cut grass field and a good head wind. I needed the full runway. Once the plane reached maximum ground speed I would reapedly flip to max elevator and literally jack the plane up to ride just a little higher on the grass. If I ran out of runway I tried again. Once I made a take off the plane was fully stalled so I had to give it down elevator and hold at maybe 4" elevation. By this time I usually was nearly out of runway so I had to make a left U-turn without dragging the left wing. High tension yet lots of fun. I tried it with either engine, and never found a difference, but the wind conditions were always different.
Old 08-14-2012, 04:06 AM
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Default RE: twin motors back to back

Never overestimate your audience

Henny Youngman?
Old 08-14-2012, 04:12 AM
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Default RE: twin motors back to back


ORIGINAL: da Rock


ORIGINAL: rmh

Am I the only one to see thru your gag setup

Jeez, I hope not....

After all, everyone knows that Grumman used a V-8 that turned out to be the famous Chevy 283. Heck, there weren't any electric motors back then small as that V8.
Wow -good thing they didn't use the 307- it would never have gotten off the ground
that one gets my vote as the worst version the 283 that GM ever did
In all fairness -it was an attempt to meet the upcomming emmission regs -
a complete and utter failure.
Old 08-14-2012, 05:36 AM
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Default RE: twin motors back to back


ORIGINAL: Villa

On the pusher/puller I describe above, one of the most fun things I did was to attempt a take-off on one engine. The plane was a 40 size plane, with a 48'' WS, and used two Magnum 28 glow engines. I don't remember the weight, but my guess is 6 pounds. For a one engine take off I only started one engine. All conditions had to be perfect; freshly cut grass field and a good head wind. I needed the full runway. Once the plane reached maximum ground speed I would reapedly flip to max elevator and literally jack the plane up to ride just a little higher on the grass. If I ran out of runway I tried again. Once I made a take off the plane was fully stalled so I had to give it down elevator and hold at maybe 4'' elevation. By this time I usually was nearly out of runway so I had to make a left U-turn without dragging the left wing. High tension yet lots of fun. I tried it with either engine, and never found a difference, but the wind conditions were always different.

There is a huge attraction to push-me-pull-yas for me. I think it's the number of problems they inherently have. Some problems come with pushers by themselves. But they still seem like they should work better. I guess it boils down to what each of us consider better to be.

I wonder if the first test pilot to climb into the fuselage between two engines looked around just before taxi out and said, "Hey guys, watch this." (Maybe the most famous last words ever.)
Old 08-14-2012, 05:55 AM
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Villa
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Default RE: twin motors back to back

Hi da Rock
You mentioned the problems that puller/pushers have. The reason I designed and built my Pusher/puller is because I was flying my conventional twin engine plane one day and realized that it was making me a nervous wreck. I was not having any fun. I realized I was waiting for one engine to stop. When I would loose an engine I had maybe 1/2 second to realize it, decide which engine, and take the proper corrective action. That is when I made it a pusher/puller. Sometimes when I suspected an engine out on the pusher/puller, I had to bring it closer to see the stopped prop.



Old 08-14-2012, 10:46 AM
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Default RE: twin motors back to back

The Cessna - was marginal on one - BUT good ol Uncle Sam bought em anyway
Theay also bought the Ospry-
good performance is not a criteria for government purchasing

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