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iFLYrc_Vic 06-17-2014 09:45 PM

Cessna Skymaster 337
 
1 Attachment(s)
I am building a Cessna Skymaster 337 model made by Seagull. This plane has two in-line motors, one forward (tractor) and one rear (pusher). It has the following specs:
  • Wing Span: 76.8 in
  • Wing area: 806 sq in
  • flying weight 10-11 lb
I will be converting this model to electric and using two Scorpion S-3032-8 motors. Using the Scorpion Calculator, these motors will generate 2081 gf static thrust with an APC 10 x 7E prop (635 Watts IN). My questions are:
1. Should I double these numbers to calculate the total thrust generated by the two in-line motors OR is there some thumb rule for calculating the total thrust for in-line configured motors?
2. How do I calculate the power generated (watts) for the in-line configuration?

Thanks,
Vic

hayofstacks 06-17-2014 09:48 PM

The first motor will pull way more amprage then the.rear. you should pitch up the rear prop.

its all trial and error.

iFLYrc_Vic 06-17-2014 09:59 PM

Twin inline motor configuration
 
Quote:

Originally Posted by hayofstacks (Post 950723)
The first motor will pull way more amprage then the.rear. you should pitch up the rear prop.

its all trial and error.

I thought the same thing. However, all of the information I have seen showed the same motor and prop for front and rear.:confused:

fhhuber 06-17-2014 10:09 PM

to load the motors (close to) equally 25% more pitch on the rear prop while both are running... but you have to watch out because if you lose the front you can burn up the rear power system.

Most people will just go ahead and put the same prop on both front and rear. It works.

336/337 will fly much like a typical traditional .40 glow trainer.

quorneng 06-18-2014 12:09 PM

On my 53" Skymaster I used exactly the same motors with identical diameter and pitch props front and rear. The rear has the LH prop.
The static current draw to each motor is very similar and probably within the tolerance you might expect from two motors.
In flight I would expect the turbulence from the front prop probably negates any benefit the rear prop may gain from the moving airflow.

The complete lack of torque is quite noticeable particularly when compared to flying on one - which mine does quite nicely.

iFLYrc_Vic 06-18-2014 12:14 PM

Quote:

Originally Posted by quorneng (Post 950789)
On my 53" Skymaster I used exactly the same motors with identical diameter and pitch props front and rear. The rear has the LH prop.
The static current draw to each motor is very similar and probably within the tolerance you might expect from two motors.
In flight I would expect the turbulence from the front prop probably negates any benefit the rear prop may gain from the moving airflow.

The complete lack of torque is quite noticeable particularly when compared to flying on one - which mine does quite nicely.

Thanks for the info. Which 53 inch model do you have?

xmech2k 06-18-2014 04:36 PM

Just curious, are the 'rule-of-thumb' things people are quoting for counter-rotating props that are one right behind the other? I imagine the distance between the props on a plane like this greatly diminishes the front props effects. Just thinking out loud. ..

fhhuber 06-18-2014 05:09 PM

Distance diminishes the effect but the distance effect becomes less as airspeed increases.

Ground checks might say they are pulling nearly the same load with the same pitch and dia prop on front as rear. Then in flight test can show the rear motor unloading significantly.

But due to the possibility of "single engine out" (even with our very reliable brushlesss motor systems)... putting the different prop on the rear asks for an overloaded rear motor if the front motor fails (prop adapter comes off or some other reason you get no thrust from the front, but the rear still works)

Full scale covers this by use of "constant speed" (automatic variable pitch) props. Essentially the pilot can dial in the rpm he wants and if the front quits the rear prop changes pitch to unload the motor (and you have just enough power to get to the crash site, because full scale 336 and 337 couldn't hold altitude on just one engine)

xmech2k 06-18-2014 05:12 PM

Thanks fh. And surprising to learn that about the full scale mix-masters.

hayofstacks 06-18-2014 07:10 PM

The rear motor will do next to nothing with the same pitch on rear.

playing with my slowstick bipe with twin motors, it easily flew on only the rear motor. a watt meter test on only the rear pulled 260 watts (just over max) and 380 watts front only. both running pulled 448 watts. I wasn't running out of battery. i even tried powering each motor seperately while testing.

when they were the same pitch, i pulled 380ish watts and the plane was quite sluggish. running a reverse or pusher prop didn't change much, just countered the torque even though both motors spun the same direction.

the motors were about 12" apart and directly inline with the top wing. If you have a larger distance, it will probably negate the effects some.

hayofstacks 06-18-2014 07:14 PM

It was interesting when trying to "3d" or prop hang it. you could hear the rear prop stall out and they plane would immideately drop. varing throttle would keep it up, but i had 55oz of thrust front motor only and somewhere around 38oz on rear with a 2.5lb air frame. a 45 degree angle climb was something to watch. the low speed thrust and 1000sq inches of wing area made it very interesting and quick to climb.

fhhuber 06-18-2014 07:42 PM

We generally have power: weight near or greater than 1:1 in our models. An airplane such as a Cub can fly on 0.3:1 power:weight (even our models)

Loss of 1 in an in-line twin isn't going to force you to land. If you use the same prop front and rear

The cause of most single-engine-out disasters in modeling is excess power overcoming the rudder's ability to correct for the yaw imposed by just having one motor running. (or failure of the pilot to even try the rudder)
For RC twins with the engines out on the wings its often best to just chop throttle and glide in.

I had a 337 (Nitroplanes big one) with motors roughly equal to the AXi 4120-/14. (long ago discontinued motors) It could loop from level flight on one using either motor.

If you want to optimize for both running... remember that loosing the front means shutting the rear down.

hayofstacks 06-18-2014 08:27 PM

Or oversize both motors and underprop the front...

fhhuber 06-18-2014 09:08 PM

Quote:

Originally Posted by hayofstacks (Post 950839)
Or oversize both motors and underprop the front...

You know that no one will ever do that...

iFLYrc_Vic 06-19-2014 12:12 AM

Quote:

Originally Posted by hayofstacks (Post 950839)
Or oversize both motors and underprop the front...

I think that is a bit extreme. The plane is already at 30 oz/sq ft wing loading and does not need the extra weight of oversized motors. Most modelers are not have any issues with same size motors and props. I will follow the majority for the first few flights.

quorneng 06-20-2014 12:37 AM

1 Attachment(s)
iFLYrc_Vic
My 53" 337 is actually an O-2 and is entirely scratch built.
Attachment 176168
Being made largely from 3mm Depron it is very light (22oz all up) so no shortage of performance on its 240W combined total.

To say the full size 337 doesn't fly on one is not quite true as it must have achieved a positive single engine climb to gain US certification.

hayofstacks 06-20-2014 01:54 AM

Id at least throw a 10x5 up front and a 10x7 in back. otherwise i doubt the rear motor would do anything.

fhhuber 06-20-2014 02:42 AM

Quote:

Originally Posted by quorneng (Post 950946)
iFLYrc_Vic
My 53" 337 is actually an O-2 and is entirely scratch built.
Attachment 176168
Being made largely from 3mm Depron it is very light (22oz all up) so no shortage of performance on its 240W combined total.

To say the full size 337 doesn't fly on one is not quite true as it must have achieved a positive single engine climb to gain US certification.


Just telling what people who have actually flown the 337 and O-2A said about it...
We have up to 4 of them show up at the local air show every year, and every one of the pilots operating them said that if one engine failed you had to start looking for a place to land quick because its not staying up long.

iFLYrc_Vic 06-20-2014 12:37 PM

Twin in-line stuff
 
I have gotten a lot of information here and on other forums about these twin in-line power plants. It seems that there is a definite lost of efficiency in the rear motor due to turbulence. The question remains, how much? The only sure way of telling would be to collect data while in flight using a "data logger". My Pheonix 80 amp ESCs do not have data logging so I will not be able to get an accurate measurement of the power used by both motors.

My plan is to just use the same prop (10 x 7E APC) on both motors and do some preliminary static measurements on the ground. If there is not a large difference in power in the static tests, I will use the same setup for the initial flights.

I will post the static results when I get the bird ready for testing - standby.[popcorn]

hayofstacks 06-20-2014 07:46 PM

I am very interested.

like i said, i had a 350 watt motor up front and a 260 watt motor in the rear if i ran them one at a time. running them both gave me. 380 watts, even using different batteries for the static tests.

one i went from an 11x5.5 up front and a 11x8.5p i managed 448 watts total. a regular 11x7 on the rear only gavee about 400 watts. i think there is some gain going to a pusher/reverse rotation prop in the rear.

your results will probably vary as you have a whole fuse and probably more then 12" between motors, but i am still very interested none the less.

i mixed one channel for throttle to a switch so that i could use my front motor only or flip a switch for the rear powered on as well. there was a drastic differance after playing with props.

iFLYrc_Vic 06-20-2014 07:50 PM

Power Usage
 
I have purchased (used) a Eagle Tree Data Logger to record in-flight power usage. This is becoming an expensive project - but VERY INTERESTING.

hayofstacks 06-20-2014 07:56 PM

For static testing, it would be easy to mesure each motor and then combined using a simple watt meter. won't tell you what its doing in the air, but it will give tou an idea of the losses.

measure each motor independantly, and then both y'ed together and compare the results

iFLYrc_Vic 06-20-2014 08:03 PM

Watt Meter
 
Quote:

Originally Posted by hayofstacks (Post 951008)
For static testing, it would be easy to mesure each motor and then combined using a simple watt meter. won't tell you what its doing in the air, but it will give tou an idea of the losses.

measure each motor independantly, and then both y'ed together and compare the results

I have an "E-Meter" that I will use for the static testing. However, I will take static readings with the e-Logger for comparison. Got to finish my TopNotch Aerocommander, then I will start work on this project.

quorneng 06-20-2014 08:42 PM

I do wonder if as planes get older, like cars, their performance falls off more than people realise.
The Cessna 337 manual states: (taken from an NTSB accident report)
"Single-engine climb performance with the rear engine operating is better than with the front engine operating by approximately 85 FPM"
and
"Rate of climb at sea level, 4,000 pounds gross weight, best rate of climb airspeed, landing gear retracted, rear engine feathered, front engine operating only = 425 ft/min."

Interesting!

hayofstacks 06-25-2014 07:43 PM

https://www.youtube.com/watch?v=Ac7R...e_gdata_player

figured id post a short video of my bipe when i had it powered as a twin. with the same pitch on the props, it barely climbed. the differance in this video is obvious.

also, i mounted motors on the top wing hoping to.kill some of the slow sticks tendancy to pitch up. it was overly sucessful.


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