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Twin power set ups

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john stones 107/12/2017 12:59:22
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8855 forum posts
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Might sound like an odd question this, but you never know till you ask it. wink

Take a model powered by a 60...10cc. Now take the same model (ish) but it's a twin, would the thinking be 2x 30...5cc or does the fact there are two props turning alter the calculation ?

Denis Watkins07/12/2017 14:07:41
2119 forum posts
108 photos

Yeah John, 2 x 5cc will fly a 10cc, but with a twin, 2 x .40s would hold a bit in reserve in the event of a single deadstick

But most twin I/C flyers, cut the motor left running so as for a balanced deadstick

I time and run my twin to land under power

Danny Fenton07/12/2017 16:08:07
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I disagree Denis, at least in electric, the power required for a twin is not the same, in my experience as half a single. There are two props and the blown wing aid this. Typically a hundred watt per pound for a single, the sum of two to make 60 watt per pound for a twin.

But if you are referring to IC then i guess you are thinking about trying to fly on one engine? Not a good move in my book, kill the remaining engine and dead stick land.

For example Brian Taylors 71" Mossquito is designed for 2 x 25's

Cheers

Danny

Jon Harper - Laser Engines07/12/2017 16:25:59
3162 forum posts
129 photos

It is more than the sum of its parts.

If i take my twin which has a pair of laser 70's on it, i am turning 12x7 props at more or less 10000rpm. A 140 4 stroke wouldnt swing a 24x7 at 10000. It is more complicated than that, but in general that is the logic.

As for single engine flying there are many schools of thought. If you kill the remaining engine you turn a model with power into a glider and limit your options quite a bit. Most twins glide like bricks as the weight of an extra engine and structure to support it tends to make them heavier than their single engine brothers. Equally though, if you screw up your single engine flight it can end in a spin.

Personally, i would always keep the power on, keep the speed up and only once i was guaranteed to make the runway would i then glide the final approach. If an engine quits on takeoff abort/land straight ahead. dont try to be a hero.

john stones 107/12/2017 17:48:26
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Thanks all.

Not worried about deadsticks lads, I don't have them. wink So you's think for the same CC a twin gives a bit extra then, but you'd go with halving it ?

Jon Harper - Laser Engines07/12/2017 18:00:58
3162 forum posts
129 photos

my model has flown with a 150 single, and the 2 70's. Its faster with the 70's that's for sure!

Do you have a specific model in mind?

john stones 107/12/2017 18:21:36
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8855 forum posts
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Grumpy Tigercub Jon, in the mass build, don't want to offend you but it'll be two electric motors. wink

Ace07/12/2017 18:48:19
125 forum posts
11 photos

Within reason 2 mid sized engines/motors generate more power than 1 large engine. Therefore total capacity or watts can be lower for the same performance. 2 engines/motors with a combined total of 15% less than a single engine/motor will deliver similar or marginally better performance - in my view. I doubt this level of power will sustain level on one engine, so we are now into cut and glide or controlled powered decent.

However if you want to actually fly on one engine (as opposed to a slow controlled decent) you are going to require 50%+ per engine/motor (due to extra yaw drag) compared to a single engine/motor to sustain flight. Working on the assumption level flight requires 50% of one single.

My 2 cents worth - Australian not Euro wink

Jon I know it was just a point but the swept area of a 12 x 7 is 113 sq in, so the 150 would need to swing a 18 x 7 to be equal cheeky

Mike Stevens07/12/2017 20:03:05
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443 forum posts
57 photos

Reminded me of a recent experience on my Twin Star recently - brushless - just after take off one motor cut out but managed to land it ok with lots of opposite rudder and gently inputs of power on the one remaining motor.

Simon Chaddock08/12/2017 00:20:18
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5009 forum posts
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Now if you have an 'inline' twin a motor out is not a problem provided you have the power.

Complete

I can, and do, switch one off for fun or for maximum endurance.

Jon Harper - Laser Engines08/12/2017 08:50:32
3162 forum posts
129 photos

An electric twin. bleh :P

Ace, yea i know, but i was in a rush! and my point is the same, a 150 wont wear an 18x7 at 10k any more than it will wear a 24x7

Peter Miller08/12/2017 09:04:06
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8617 forum posts
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As I see it the area of the props and rpm are part of the equation.

With GRumpy Tigercub two 15s do seem to provide the same effect as a .32

I normally run the 15s on 8X4 props. the .32 on 10 X 6. Unfortunately I can't tell you the rpm.

However Grumpy is interesting in that it flies so well on one. In fact people who have flown it have had trouble working out which engine has stopped and it can be turned into the dead engine with care.

Chris Walby08/12/2017 10:00:26
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269 forum posts
31 photos

There are more experience people with far greater knowledge of aerodynamics than me, although I'll chip in from the experience I have with electric twins.

I have had two electric models this year present me with one motor running (one shed a blade off the prop, the other the manufacturers motor connector failed) and what I would say is to consider the model and where you are flying.

Number one the model, if its a warbird and has a 3 blade prop on each motor it will present considerable drag if it stops plus as Jon says depends on the phase of flight (at take off, landing or mid flight) as to what your options are, but have a plan.

Number two if you always fly 3 mistakes high you will have the best chance of spotting which motor has quit and time to put your plan into action. Low slow passes with big turns reduce options and thinking time.

After the first black bag incident (its back flying again!) I looked at comments and took advice then sat on the simulator with random engine failure mode and although they are limited you don't need a bin liner for each mistake!

To summarise if it glides like a manhole cover (thanks Jon) you have less options and if it will can fly on one motor nicely then more options, but practice and a plan are ultra important.

I have found most electric models (not just twins) that have their origins in IC have initial rearward CoG and would not suffer on AUW if slightly larger motors were used (to give you a bit of spare power if needed) and if the model itself can handle high drag on one side and motor trying to overcome everything else on the other and still have some rudder authority.

If the Grumpy is a good flyer and the electric set up will be lighter than IC then for the emergency situation of one motor failure just go with the electric equivalent of two 15s (set it up with two channels to the ESC's) and gain experience (at a safe height!) of flying on one.

Danny Fenton08/12/2017 10:22:46
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8395 forum posts
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Interesting topic And nice to hear the IC side of the discussion.

One thing that people are forgetting is that when both or one larger engine are running you are required to take off and climb. When an engine fails, and as Chris has pointed out, even electrics can, then the idea is not to climb but to not lose height and speed too rapidly. So all that is needed is sufficient power to extend the point of impact

A single (less than half) motor should still put out enough power to maintain airspeed but perhaps in a gentle descent back to the strip. But also as others have said, know your model, and if in doubt or low, close both motors and point your manhole cover at a safe and hopefully soft spot

Cheers

Danny

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