30" Dumas Stinson Reliant SR-10 - Page 10 - RC Groups
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Aug 19, 2010, 03:41 PM
Headed to Naval Flight School!
PiperCub49's Avatar

Got it. I didn't know if 2S would provide the power needed, but alas, it looks as if it will. I'm definately not going for excessive power here, so I have my power setup. Phew!

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Aug 19, 2010, 06:14 PM
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E-Challenged's Avatar
It will likely fly on 2S, if it doesn't go to 3S but check that current draw does not exceed Hong Kong Louie's max continouous amp draw specs. I learned that the hard way, it only takes a second to "fry" a tiny outrunner spinning a too-big a prop on a
3S (3-cell) lipo pack. Selecting a motor/prop/esc/and battery pack for small RC scale models is not easy. Better to pick a setup that has a little excess power than to have to do major nose surgery to install a larger/different setup when a model can't lift off with authority.
Aug 19, 2010, 07:26 PM
Or current resident
glewis's Avatar
Testing would be needed if adding a cell like EC says. If you must add a cell reduce the prop diameter and pitch first, then test the curent. Work your way up in prop size until a little under max current is reached.

Aug 19, 2010, 09:06 PM
Headed to Naval Flight School!
PiperCub49's Avatar

Those are some good words of wisdom for a young modeler. Hmmm. Where does that put me now?

My problem is that I have very little experience or knowledge, for that matter, with 2S vs. 3S power systems with a given pitch prop. On top of that, I now try to calculate for a relatively light airplane that is supposed to fly fast as a scale bird, but should be able to fly slow without stalling on those tiny wing tips...so I want that wide speed envelope, but I don't know what diameter and pitch prop I need to get there...and then will that work with the motor and what will the resulting weight be for that particular system? Ugh! My mind just swirls in circles and there is no end. It's even harder to choose a system than I thought it would be. It would be easy for some to just buy a couple batteries, a range of props, and maybe even a couple motors to test out, but seeing as I'm young and without a job, you can understand why funds are tight. That's why I am just trying to get system as close as possible to ideal the first time, plus the fact that the LHS doesn't carry anything electric!

At the very least, I can get the 2S battery and test it out with the 7x5 and 7x6 props. If that doesn't do it, I'll got down to the 6x4 and 6x5 with a 3S.

I know that this is sort of a silly question to ask, but what is the 7" prop going to do for me over the 6"? Will the 7" prop let me to fly slower or...what...? I know it's sort of basic aerodynamics, but it still confuses me. So at the same given airspeed, are the systems doing the same thing for the airplane (aerodynamically, handling wise, ect)? Maybe the extra airflow over the surfaces from the 7" would give me better control at low airspeeds, for instance?

I calculated some flight times and may have to go up in battery mAh. Any way I look at it, this model seems to take a heavy hit in weight with the battery.

Last edited by PiperCub49; Aug 19, 2010 at 09:19 PM.
Aug 20, 2010, 01:05 AM
Balsa Lover
gupi's Avatar
My Herr Fokker D.VII 24" conversion weighs 219g with the heaviest battery i got (950mAH 2S) and uses this setup:

Motor: Dualsky XM2812-27 (70W max), 26g with ESC/BEC
Prop: 8x4.3
Battery: 600mAH up to 950mAH 2S

The batteries i use are very compact: length 50 x width 30 x height 11 mm for the 600, a bit thicker for the 950. This allows me to put them very far to the front as i use them vertical in my Fokker's nose.

This yields more than enough power for scale flight, most of the time i go half throttle, hand launch start is very easy at 3/4 throttle and it leaps out of my hand with full throttle.

On the 950 batt i fly over 15 mins.

It doesn't allow to go vertical but it helps a lot in critical situations near mother Earth or in stronger winds.

In my Peter Rake Fokker F.II 30" that I'm building currently I plan to use the same setup.

Aug 20, 2010, 10:39 AM
Registered User
E-Challenged's Avatar
I got the impression that your landing gear length would not allow larger than a 6" prop. For the next project, remember that you can make the landing gear and tail wheel wire a little longer so that the model sits a little higher off the ground. Taller tail wheel wire can help control ground looping tendencies by lowering the nose of a tail-dragger. Use of the elevator with nudges of right rudder during first 1/2 of the take off roll keeps tail wheel on the ground to help avoid tail swing/ground looping.

A larger, slower turning, prop is more efficient, allows you to use a lighter 2-cell lipo which is a plus especially on a small model like yours. There may be other reasons too. BTW, don't forget to mount the motor slightly offset so that the prop shaft comes out of the center of the cowl/dummy radial engine while angling slightly down and to the right. thrust angles should be "just noticeable" suject to increase or decrease as needed. You may need to add or subtract washers to adjust down and right thrust angles after initial flights. You will find that a model when cruising in level flight requires a lot less power to fly than one attempting to take off from the ground or in climbing mode.

You have taken on a fairly complex small rubber scale kit conversion project but are
proceeding carefully and learning a lot just like many others have. If your model is a resounding success, consider yourself very lucky. If you have to make changes, do them one at a time and make repairs securely and neatly. Make sure that the model is not at all tail-heavy, that there are no warps in wings or tail surfaces, and that control surfaces move it the right directions. If you are using four servos and a 2.4ghz receiver, using a $20 Park BEC is good insurance against low-voltage-to-receiver loss of control problems. Make sure that all linkage and hinging of control surface is free and non-binding and that all servos are working smoothly with no buzzing or twitching at rest or at extremes. You can adjust control surface throws at the servo arms and control surface horns and with your transmitter's end point, sub-trim, and servo travel settings if your transmitter has these features. For first flights 1/4" max control surface movements may be plenty. Think of these models as "free flight scale with R/C assist".
I just ran out of wisdom and it's only 7:40 AM.
Last edited by E-Challenged; Aug 20, 2010 at 10:49 AM.
Aug 20, 2010, 03:19 PM
Headed to Naval Flight School!
PiperCub49's Avatar

I would offer condolences on your little Fokker, but you got it back together too fast! That's CA for ya! My first conversion, a Guillow's Cessna 170, had the wings torn off three times, and once was before maiden! I used a 10g brushless and a 2S 250mAh in that one. It still flies around at 104g (w/ battery) and looks and flies just as good as every, save for the ever-yellowing, wrinkling Coverite Microlite on it.

Your setup actually sounds very similar to the one I've set up for this build. Your motor is a bit bigger though. The weight, prop, and speed of the model are most helpful to me. Thank you!


Again, I apologize about the prop clearance mistake on my part. Here I am begging for help and I can't even measure prop clearance!!! Maybe I need to rethink this project.

I never thought about increasing the tailwheel size to keep the AOA lower on takeoff. Great idea! In my blog, I have a pic and some info on a 13" peanut that I converted to 3ch R/C after flying it F/F. The positive incidence on the wing(s) is already high and the tail skid is very small (the full scale one was too). If you don't know how to handle it on takeoff, it will favor a ground loop over a takeoff any day. On the 24" Cessna, the mains are a little smaller than the plans call for and the tailwheel is a little longer. It almost never ground loops. (That one was and still is a complete success. It flew magnificently right off the board. Sure, a bit of luck was involved, but I find that relying on attention to detail and knowledge of aerodynamics is far more realistic! Ever heard of the "luck comes to the most prepared" phrase? ) We'll see how this build goes. It is quite different and has the added complexity of ailerons.

Please correct me if I'm wrong, but as a general rule, shouldn't I need more right thrust as I spin higher RPM's on a smaller prop to offset P-factor? This will help me decide if I want to put in more right thrust versus the old "3 and 3 to start with", which is usually what is just noticeable and works well for the rest of the world.

Thanks again,
Last edited by PiperCub49; Aug 20, 2010 at 03:27 PM.
Aug 20, 2010, 03:51 PM
Hoarding balsa is a sickness
Your "P" factor, pulling left on takeoff will be reduced as you speed builds airflow over the rudder and the tail comes up, reducing the AOA of the propeller's starboard side....ask you flight instructor, or get some tail dragger time.
Aug 20, 2010, 04:46 PM
Registered User
E-Challenged's Avatar
There are two factors that cause a plane to yaw and bank to the left during the takeoff run, motor torque and a spiral airlow from the propeller striking the tail surfaces. There have been many learned arguments about how these factors work but we know that proper amounts of right rudder, buildup of airflow over the vertical fin and rudder and changes in angle of attack lessen the effects which disappear with stable level flight. Didja ever watch the amount of rudder used on a naval carrier plane during take off? These had big torquey engines and need gobs of right rudder, same with land based high powered fighters like P-51's. Watch You Tube pics of Delmar Benjamin taking off in his Gee Bee R-1 replica, he does a tap dance on the rudder pedals to keep the beast headed straight. Most tail-dragger models, especially the little ones, need nudges of varying amounts of right rudder to keep a straight heading. Funny how many ARF models don't even have rudder control. How many full scale aircraft have no rudder control?
Rudders also come in handy making smooth coordinated turns when used with ailerons especially with high wing monoplanes. Rudders are also good for levelling wings during slow speeds near touch down and for doing side slip maneuvers. I use aileron/rudder mix most of the time for relaxed flying of my scale models, purists disapprove.
Aug 20, 2010, 05:19 PM
Headed to Naval Flight School!
PiperCub49's Avatar
Maxxnut and EC,

I've been through that time and time again for some reason. Between flight school and R/C, I feel like I've almost become an expert on left-turning tendencies. I can see it now on the written:

When are the effects of P-factor most noticeable?

A. low angles of attack, high airspeed, and low RPM
B. high angles of attack, low airspeed, and high RPM
C. high angles of attack, high airspeed, high RPM

That one was easy! I bet the first person to take a stab will be right. After all, I'm the novice here!

So let's see if I have this...the size of the prop doesn't directly affect P-factor; it's how much propwash there is over the tail surfaces?

I'll start the LG struts tonight.

Aug 20, 2010, 09:35 PM
Registered User
E-Challenged's Avatar

Pee Faktor

Uhhhhh "B" ?
Aug 20, 2010, 10:11 PM
Headed to Naval Flight School!
PiperCub49's Avatar
Originally Posted by E-Challenged
Uhhhhh "B" ?
Right you are, of course, Mr. Aeronautical Engineer, sir.

Here is the first of many LG struts that I'll be making. I just roughed one out tonight to see what I am getting into. I'm using the pictures of N5828Y for my struts/fairings. I start with a template from Paul Bradley's SR-7 plans, which you can see in the photo.

One question- How do you get color onto foam? I can't just cover it with the iron (it will melt) or airbrush it, can I? Does it need to be prepped in any way?

Last edited by PiperCub49; Aug 20, 2010 at 10:18 PM.
Aug 21, 2010, 12:01 AM
Registered User
E-Challenged's Avatar
I just re-read the definition of P-factor and it is a third factor involving the "propeller disk" hitting oncoming airflow at an angle and the downgoing propeller blade taking more of a gulp of air than the upgoing blade which pulls the plane around to the left along with the effect of spiral airflow from the propeller hitting the left side of the fin/rudder plus the affect of engine torque. This is giving me a headache. Now I'm wondering if down thrust lessens the P-factor by making the propeller disk hit the oncoming air mass at less of an angle and right thrust counteracts some of the effects of spiral airlow hitting the vertical fin/rudder as well as countering torque effects this along with giving right rudder deflection as needed. Downthrust is normally used to lessen the tendency for plane to pitch up when full throttle is applied. We'd better drop this discussion or risk a lengthy lecture from one of the aerodynamics guru/s, I'm out of my league.
Aug 21, 2010, 01:05 AM
Hoarding balsa is a sickness
E-C.. you are getting there. PC49, just put the prop on backwards and it will pull the other way, right?? Ya.
Sorry, it's been a long day.......
Aug 21, 2010, 11:44 AM
Headed to Naval Flight School!
PiperCub49's Avatar

You pretty much have it. I never considered down thrust a factor in left-turning tendencies, but logically it makes sense. Now, as far as right thrust counteracting spiral airflow on the tail surfaces, that is definately true in my mind. I don't believe that motor torque is really a factor with inline engines. Now with radials, that's a completely different story because of the huge mass spinning in one direction.


Well, reverse the prop and the rotation...then you would have right-turning tendencies.

-Kody (trying to start a lecture)

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