Angles for a twin motor airplane

[Ricardo RW]

Hi,

As commonly known, single motor planes place the motors with a right and down angle (about 1 or 2 degs.) in order to avoid torque and (maybe) another fenomenon I don't know.

As I'm about to design and build a twin motor trainer (I will reuse an old trainer wing) I wander if the same principle should be use in both motors: right and down angles.

Both motors turn to the same side (counter cw if you look them from the front).

Any help will be appreciate.

Regards.

[Sparky Paul]

I did this last year..
Looking for single motor performance, as many twins crash when a motor quits.
I found a significant amount of motor offset enhanced the single-motor performance. 5 or more degrees would let the plane fly at higher speeds before the rudder became ineffective in holding heading.
Some of the variations...
The push-pull came about because the normal twin pusher had the right hand prop too close to my head when holding for the launch. :)
No different flight characteristics, same limit on power and rudder authority.

[Ricardo RW]

Thanks Paul, I think I'll try a set up like yours.

Thank you for your 5.000+ words. :)

[Lon Enloe]

I'm building a twin at the moment to use up some old brushed motors. I've got a set of contra-rotating 10 x 4.7 props that'll be a perfect fit for the geared 350 motors. Bye, bye torque problem.

[Sparky Paul]

It's not a torque problem with a twin, it's P-effect, and off-center thrust.
Motor torque acts along the motor centerline.
With the motors seperated as on a twin, the torque effect is minimal on the airplane, whose inertia resists the torque.
P-effect comes from the down-going propellor blade at high angles of attack.
The leads to asymmetric thrust from the prop, also around the motor axis, but with the P-effect being either inboard (good) or outboard (bad), its effect due to the moment arm to the plane's centerline will be a little or a lot.
With the running motor at high power, the moment arm around the yaw axis can be greater than the rudder can control. The direction of rotation of the prop, or which motor has failed has no effect here, it's power and moment arm.
Normally twin electrics use a single ESC. When it shuts the power off, both motors quit.
To get single motor control, I used two ESCs, with each in seperate channels,; 3 and 6, mixed together to the throttle stick.
One motor on channel 3, the other on channel 6, which has a knob for control. I can vary this knob to control that motor without affecting the other one, which will follow the throttle stick.
Another 1000 words... :)
.
On full-scales, the motor which creates the most problems with the other motor off is termed "the critical motor". On the P-38, -both- motors were critical, because both rotated "outwards"... the downgoing blades were on the outboard sides of the motor.
Usually the designer wants both motors to rotate "inwards".. the downgoing blades closest to the fuselage, to reduce the rudder power needed to handle single-engine flight. Both for pilot effort relief, and a smaller vertical.

[harveyweewax]

On the P-38, -both- motors were critical, because both rotated "outwards"... the downgoing blades were on the outboard sides of the motor.
Usually the designer wants both motors to rotate "inwards".. the downgoing blades closest to the fuselage, to reduce the rudder power needed to handle single-engine flight. Both for pilot effort relief, and a smaller vertical.

So what was the reason for having the props on the P-38 rotate outwards then?

Regards,

Sven

[Sparky Paul]

So what was the reason for having the props on the P-38 rotate outwards then?

Regards,

Sven
.
Ask Kelly.
Both directions were tried.
Possibly something about turbulence/erratic airflow over the aft part of the plane.
It's one of many questions I didn't get to ask of Tony LeVier when we were talking a couple years back.