Experiment with twin motors on a Tiny trainer.

[TheFlyingBrit]

While I was digging through my parts box, looking for a motor replacement for my 3ch Tiny trainer. I managed to find four MT2204 2300kv motors , one being spoken for, I pondered momentarily as to what else I could use them on (and here is the catch).
They are commonly used on quad copters so come in two variants CW and CCW. The silver prop nut version is the CCW, which is what most of us use for fixed wing planes. The black prop nut is a CW version, I have two CW and one CCW left in my collection.
I had a eureka moment I can mount a couple of these on a Tiny Trainer 4ch wing, with counter-rotating props. I won't need to mess with side thrust issues just sort the down thrust.
Then the thought crossed my mind which option is best for a twin prop system , both props rotating inwards to the fuselage or outwards to the wing tips ?
Rather than trawl through the internet and find the answer. I thought it would be more interesting, to throw this open to the forum and see what everyone else thinks it should be ?
My initial thought was: The props should rotate inwards to the body of the aircraft, but I am open to a logical thought process that suggests otherwise.

 

[FlamingRCAirplanes]

I like this idea! I also think inwards spinning props would be the best option. But if you think about it, all changing it does is change between if the top or if the bottom it spinning inwards. In the end you get the same result. Right? Or am I missing something?

 

[Merv]

.... which option is best for a twin prop system , both props rotating inwards to the fuselage or outwards to the wing tips ?......

Most twin motors have the prop rotating as inwardly, as you put it. I would call the rotation downward as it passes the fuse. I have no idea why.

 

[Monte.C]

Most twin motors have the prop rotating as inwardly, as you put it. I would call the rotation downward as it passes the fuse. I have no idea why.

If I remember right it's that way on most full-size planes too. I'll stick with that so I have nothing to try to remember. Like "fuzzy fuse". Ok, "down @ the center." I can picture that easily.

 

[TheFlyingBrit]

I was thinking more about the impact of air movement and turbulence across the wing would there be less or more depending on which variant you select. Lets face it we moved away from bi-planes because the twin wing system, forms two turbulent air streams which impact each other. That's why later designs step the lower wings back slightly, to reduce that effect.
I was wondering if twin prop wash has a significant impact on flight characteristics ? depending on which direction they turn, is that why a lot of twin prop planes still have both props rotating in one direction ?
I can see why contra-rotating props on the same shaft are efficient, because one prop will cancel the other out with respect to turbulence.
I can see your point, regards its either the top or bottom turning inwards. But it doesn't tell me if one is better than the other with respect to the performance of a model/plane.

If I remember right it's that way on most full-size planes too. I'll stick with that so I have nothing to try to remember. Like "fuzzy fuse". Ok, "down @ the center." I can picture that easily.

All I can think about is David Brent (Ricky Gervais) from the office doing that stupid dance

 

[Monte.C]

I was thinking more about the impact of air movement and turbulence across the wing would there be less or more depending on which variant you select. Lets face it we moved away from bi-planes because the twin wing system, forms two turbulent air streams which impact each other. That's why later designs step the lower wings back slightly, to reduce that effect.
I was wondering if twin prop wash has a significant impact on flight characteristics ? depending on which direction they turn, is that why a lot of twin prop planes still have both props rotating in one direction ?
I can see why contra-rotating props on the same shaft are efficient, because one prop will cancel the other out with respect to turbulence.
I can see your point, regards its either the top or bottom turning inwards. But it doesn't tell me if one is better than the other with respect to the performance of a model/plane.

All I can think about is David Brent (Ricky Gervais) from the office doing that stupid dance

Good. Then you won't forget.

 

[TheFlyingBrit]

Now we have some sort of agreement on directional rotation, any suggestions as to the thrust angle and pod position in relation to the wing ?
1. Does the angle of downward thrust reduce, the closer the motors get to the CG point ? Is there a rule of thumb ?
2. Which power pod system is the most efficient: In line with the wing, slung below or mounted above ?

 

[Monte.C]

...is that why a lot of twin prop planes still have both props rotating in one direction ?

Planes will have both rotating in the same direction to save time & money. I don't think there's any other good reason to do it that way. That way all parts can be interchangeable. Especially important for warplanes. Many planes can handle it. I'm sure there are many others that would be horrible or impossible to control that way.

 

[TheFlyingBrit]

I seem to remember something about passenger jets having there engines below the wing to help with wing flexing in flight
Again there is variation depending on the aircraft and sometimes the application, Its understandable to have engines mounted above the wing on Sea plane's for example ? As per my previous point, load carrying and larger commercial planes tend to use under slung motors/engines.
Which then leads on to the next question is it dependent on the wing mounting: high, mid or low wing position ?

 

[Byrdman]

Use whatever fuse you want, but I think I would just build it like a Sea Duck or Cruiser wing. They are already designed for twins and you could scale it to any size you want. Are those too powerful for a mini cruiser?

 

[TheFlyingBrit]

Use whatever fuse you want, but I think I would just build it like a Sea Duck or Cruiser wing. They are already designed for twins and you could scale it to any size you want. Are those too powerful for a mini cruiser?

I don't think they would be over powered, in fact they would probably be ideal for a mini cruiser.
However, rather than building a new model I was only thinking about using a Tiny trainer as the base fuselage/tail (I have a few of them) and building a new wing to accept the twin motors ?
Although thinking about it now, a Tiny cruiser might be the best way forward. The design for a twin is already there and it gives me another different model to add to my collection.
Normally I wouldn't build a mini design, as I struggle to see the damn things if they get any sort of distance away from me. Eye sight ain't what it use to be in fact its .
I will watch the build on you tube and make a decision after that.

 

[TheFlyingBrit]

Just typed in FT tiny twin and guess what came up

 

[Scotto]

I remember my dad talking about the xp82 twin mustang being unflyable before they made it counter rotating inward, but I couldnt find anything on the internet to back that up. I did find This conversation interesting though.

Hey if youre building a new wing anyway you could try mounting the motors to the bottom of the flaps. Maybe you would have better luck than I did. It worked fine I just tried to go too slow too many times.

 

[CappyAmeric]

Counter rotating props are used to nullify P-factor - and to eliminate the “critical engine“ issue on full sized aircraft. Some twins, (such as the KingAir which does not have counter rotating props) when the left engine (the critical engine) fails, the P-factor is far more detrimental than when the right engine fails.

On R/C twins, counter rotating props nullifies P-factor very well. I always use counter rotating to wash in at the fuselage.

 

[TheFlyingBrit]

Just watched the mini cruiser build and to be honest, if I was going to build that model it would have to be the full size version not the mini. However, it has given me a few ideas and some things to consider when I get around to building my twin. What has shocked me is the increased cost of ESC's, I use to buy my Simon K ESC's in bulk 20 amp and 30 amps. They where pretty cheap at the time, but now they have jumped up in price by almost 2/3 of the previous cost I bought them at.
Sorry I digress, I briefly watched the Tiny Trainer twin video too and its sort of confirmed that's the way I want to go. However, I won't be 3D printing my power pods. I also need to consider an adjustable down thrust system, for the motors. So I can optimise the down thrust before finally locking it in place.

 

[TheFlyingBrit]

Found my twin prop project the FT Super Bee, watched the build video early hours of this morning. A couple of cans of Red Bull and I bet that will be great fun to fly

 

[TheFlyingBrit]

Building a Bloody Baron and Bushwacker first before I attempt the twin.

 

[Ratcheeroo]

Its weird that you guys are discussing twin prop projects because that is next on my list. Have already built the FT MS P-38 and Vincent Unrau's DH-88 but I found something that really caught my eye the other day and immediately printed the plans just to have on hand when I was ready to start, I have a couple of Emax Rspec 2205 motors that will be perfect for this plane.

 

[TheFlyingBrit]

That is one beautiful plane, maybe I need to rethink my options again regards building a twin

 

[Pieliker96]

Having the props rotate inwards at the top is generally better for engine-out scenarios as the p-factor and gyroscopic precession from the remaining engine will act opposite the moment of the asymmetric thrust, and the prop torque will tend to hold the wing with the dead engine up. The one wild card is how the slipstream interacts with whatever's behind it, depending on configuration this effect may overpower the others and make a outwards-at-top rotation direction best.