In two multi engine Grumman aircraft designs (E2C & C2A) the engines are mounted such that they have 2 degrees of outboard thrust (L engine 2 deg outboard & right engine 2 deg outboard), for the life of me I have never been able to postulate why this was done. Anyone wish to take a educated stab at this?

Simple, it's for engine-out controllability. Think of it as the good engine "assisting" the rudder in compensating for the asymmetrical thrust. Did they ever use a couter-rotating (backward) starboard engine too? Less common, but it's occly done so that the "p" factor on the rt eng has less yaw moment during a slow speed high angle of attack engine out situation.

Side note: P-factor, slipstream effect and torque are commonly collectively (and quite wrongly) referred to as just torque. Torque (a rolling moment) is completely different from the other two (yaw moments), which are mostly different from each other.

> Side note: P-factor, slipstream effect and torque are commonly collectively (and quite wrongly) referred to as just torque. Torque (a rolling moment) is completely different from the other two (yaw moments), which are mostly different from each other. <

Thank you for stating that. Most modelers cannot name any force acting on a model other than the "T-Word" and will blame torque for everything including flatulence. It drives me crazy that modelers insist that that a rolling moment from torque has anything to do with the yaw of a taildragger veering out of line on takeoff.

Ain't dat de truth.

And they most often say it was torque that killed their airplane on takeoff. When it left the ground following the prop way before the airplane had gotten a chance to fly and fight back. Because they slammed the throttle wide open to get the scary part over with as quickly as possible.

Well put.

And a number of recent multi designs have much more than 2degrees. The concept for models has shown up in a number of model magazine articles, about both new designs and mods to ARFs and such. I think one model writer was suggesting 4-6degrees.

It seems to me that unless one has a mechanically[bent] mind ,it might be hard to visualize the reaction to different control,or lack of control that an aircraft would make. Enter the wind tunnel. If one could have access to one and set the model up just so and add smoke,then you could see the visual results of different control inputs etc. That should not be hard to do . There is made a large fan that is used for farm operation's such as drying hay etc. It would be an interesting project it seems to me.

Building on what Highorse said up above. In a high angle of attack, low speed, single engine configuration, the thrustline for both engines, due to P factor is on the right side of the nacelle. The amount of yaw caused by these displaced thrustlines is the moment caused by the thrust and the straight line distance from the CG to the centerline of thrust. If we bend that thrusline in by canting the nacelles outward you shorten the distance from the CG to the thrustline thus reducing the yawing moment. The extreme example of this is the JU-87 two outboard engines.

Tomski, I think you're thinking of the Ju-52 trimotor and not the -87 which was the Stuka. But yeah, huge outthrust angles on the -52.

[] so much for answering before the second cup of coffee. Yes that's what I meant to type

Ju-52 trimotor... I remember that aircraft from the Clent Eastwood movie "Where eagles dare"
I found a plan view of the aircraft & I could see with the naked eye the thrust line. NICE example

Thanks

I'm a model writer and the number for out thrust is 8 degrees on each engine. I have built it and tested it on more than one airplane. My main test was to fill one tank 1/3 full and the other completely full, then take off and fly acro, letting one engine run out and die any time in any maneuver. Other than losing forward speed, you can't tell the difference. I had one plane with enough power (Twin Stick OS .46AX) to continue acro on one engine. Loops, rolls both directions, Immelmanns, Cuban-8s, etc. I did both engines and more than one airplane.

For the engineers or math inclined, cos 8 = .99027. This means you are losing less than 1% of your forward thrust with 8 degrees out thrust. Plus, you are gaining over 13% out force.

The books, at least the ones I read many years ago, used to define "torque" as a collective term that included the twisting force of the engine, prop factor and the spiral prop blast.

Someone also mentioned you had to have the engines synced or the differential thrust would cause trouble. I didn't believe this so I took the old Tiger 2 twin and mounted a Thunder Tiger .46Pro/11-6 with a Tower muffler on one side and an old TT .42GP/10-6 on the other. I couldn't tell any difference. I even flew it at the first Multis Over McDonough twin meet and no one noticed I had 2 different engines.

Here's another little number I almost forgot about that a buddy and I threw together for grins. Flew pretty darn well. It has 2 Magnum .28s so I got to fly engine out a lot.

That's part of the problem...The FAA of all orgs (but then again it's a govt org) made the same mistake in the "official" pub (forgot name of it) that all instructors had to know fwd and back. Dumb Dumb Dumb

Thanks again everyone,
I see now why Grumman did what they did... Between the outboard thrust line & the high tech rudder system (differnt post) I would hope that this thing will have good control on one motor.

Regards,
Dave

I may have to build a foamer crasher & see if 2 degrees out & the high tech rudder/s provide enough control for single engine operation

My goal is to build to scale, but I may find that this is not possible/safe

I have only played with Multi engine aircraft one time (electric), I did have a motor lead come off & I lost one motor, I remember having to have full rudder to keep it straight & only able to turn in one direction.
I did not incorporate any outboard thrust on that thrown together design.
I remember the sick feeling I had when I knew it was doomed… to die a foamy horrific death

Regards,
Dave

Dave, consider the use of a GYRO on the rudder channel to help compensate for a sudden yaw. It will react faster than u can and may help a lot.

The gyro is a good suggestion, from what little I know of them, I understand they help on take-off

I have not done my weight estimation yet for the airframe but I would think the addition of a gyro will not put the airframe over the limit
When I get the design off the drawing board & laser cut I will post some pics

I still have a huge task generating the CAD files & I need to figure allot of things out prior to building anything.

Regards,
Dave

There is a couple of good threads on twins in the twin forum, like htis one

http://www.rcuniverse.com/forum/fb.asp?m=5870781

Major,
I enjoyed reading the thread, it was/is good food for thought…

I gathered several things from reading the thread

(1) the aircraft in that thread suffered from not have enough design control at MFR to fly one engine
(2) Experience, experience experience
(3) Things do not always scale from the big boys down to RC scale

*****My experience on a single engine****
I quickly & instinctively gathered I had a problem when I lost a motor, I knew I had to keep the wings as level as I could to land.
The reality/sick feeling was I was not going to make it back to the field, I cut my losses and landed it. The landing gear suffered the brunt of the crash/hard landing but other than that all went well.
It was instinctive to turn in the correct direction; the aircraft would not turn in the other direction (design flaw of my doing).
If I had the design info Ed Moorman gave me (8 degrees out) a few years ago when I played with a twin I would have been in fat city


I think the Germans & Ed Moorman have it right, design in stability & control or test the design prior to sinking a ton of cash into something that will be doomed on one motor

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And the Sunderland... due to a c.g. problem, the wings were swept back, -without- altering the thrust line of the motors!

Did some playing around with electric twins, for the ability to stop and restart the motors in flight.
I found with this Gentle Lady mod that with more than 40% power on the running motor, there wasn't sufficient rudder authority to turn the airplane towards the running motor, in any configuration.. tractor, pusher or push pull.
Making the single motor Mirage with the GL wing into a twin, I had to add rudder area to get decent images from the camdorder. The directional variations without the added area were too distracting.
On one of the flights with the camcorder twin, the right prop fell off.. making the plane very difficult to turn to the left with power on.

Paul,
Did you have any outboard thrustline in your test?

Dave

"Did some playing around..............."

Jeez, didn't you ever!
Hats off to your playing. That's what I call playing.

Everybody who is the least timid about the horrendous risk of moving a CG from the mfg's recommended location etc etc etc, really needs to look closely at those photos.

Our hobby offers all of us an excellent opportunity to try stuff. And your pictures are a good lesson in that.

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Yes, and there is a noticeable difference in the amount of power the running motor can go to.
But the amount of offset looks odd.
BTW, the push-pull came about because I didn't like that right motor running right next to my favorite ear on that side of my head when the plane was the twin-pusher, so I moved right motor to the front.
The v-tailed pusher developed a serious almost uncontrollable lateral darting around as a twin.
With a single pusher motor exhausting between the vees, it flies nicely.