****GWS-38: The Aviation Legend **** - Page 53 - RC Groups
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Oct 26, 2006, 11:37 PM
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crxmanpat's Avatar
Guy, you placed an order for the 38? I thought you were holding out until they were generally available?

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Oct 28, 2006, 08:29 PM
Registered User
kahloq's Avatar
Ok some news to report. Using the DX6 system and setting up differential thrust has proven very frustrating and has so far been far removed from what Horizon stated should be the setup.

The good news:
I have succeeded in actually getting both channels(1 and 6) to follow the throttle stick movement.
I have succeeded in getting the rudder to throttle mixes working right(25% reduction in one motor with a +25% increase on the other).

The bad news:
Channel 1 is the normal throttle channel for all setups. In this situation, with two motors and having two throttle channels basically, channel 1 seems to still take precidence and although both motors will spin up according to the stick movement, there is a massive difference in RPM speed. Its so bad that you can hear it without having to check it using an optic meter.
Now, maybe someone has some good ideas on this cuz Im all ears...so let me give some more info.

In order to get both channels to spin up using the stick(throttle), it was necessary to put the third mix...throttle 1 to channel 6....(and as seen on the radio itself C 1 6)......at -31% both at the low stick position and the full throttle stick position.
If I adjust sub trim for the slower motor(channel 6)....it puts everything out of whack and the channel 6 motor no longer functions except on rudder input.
If I adjust servo travel on the channel 6 motor, same thing....no follow with throttle stick movement but still works for rudder.

So heres some numbers using an optic rpm meter. Channel 1 motor at roughly 1/4 throttle is about 3000 rpm and the channel 6 motor is at 1800 rpm. At somewhat above 1/2 throttle, motor on chan 1 is at about 5900 rpm's and motor on chan 6 is about 3300 rpm's. As you can see, the higher the throttle stick input, the bigger the rpm difference becomes. This is NOT due to variances in motor build or differences due to esc construction. Sure, there would be some difference as a result of either of those, but not to this degree. Further, if both esc/motors are simply y'd off to throttle channel 1, they both spin at about the same rpm(within tolerances). So, this signifies something is up with the radio programming or whatever.

Now, even with these huge differences, I went outside and ran the plane on the street some just to play around. Looking from the tail, the motor on the left side is channel 1 and this is the one that spins faster. At less then 1/3 throttle, the plane will track straight with no rudder input. Once you get past that, you can see a slight veer to the right(left motor pulling more will turn plane right). At 1/2 throttle, there is a good amount of overpowering from the left motor compared to the right one. With rudder input, its easy to keep the plane straight. However, in the air, the overpowering left side would ultimately crash the plane pretty fast. Anyhow, the actual functionality of it works VERY VERY well. I could turn the plane and do a 180 literally on a dime in either direction. Even with the slower RPM side, it could do the same thing.
I was very impressed with this and would be simply great if only I could get the two motors to sync up.

Ok...so heres the actual mizes as they appear on the DX6 (channel 1 is normally throttle, channel 4 is rudder, and channel 6 is flap or AUX)
Mix A 4 1
-25%/-25% <-- this indicates left and right stick movement...so at left deflection, it'll read -25% and also at right stick deflection -25%..Either can be adjusted independently.

Mix B 4 6
-25%/-25% Left/right again

Mix C 1 6
-31% / -31% full down/ full up on throttle

These settings are the only way I have been able to even get both motors to follow the throttle movement.

Throttle trim has had to be set a -22 in order to keep the channel 1 motor from spinning some at down stick position

Travel adjust for channel 6 is 0(changing this screws up everything else)
Travel adjust for channel 1 has never been changed, so it is -100/+100
Subtrim for channel 1 is 0
Subtrim for channel 6 is +125 .(I had been playing with this setting and the travel adjust and the C mix for the last week to try and get something to work).
Oct 28, 2006, 09:36 PM
Outcast outlaw
Wow -- it seems to me a steerable nose wheel would be easier...
Oct 28, 2006, 11:29 PM
Hooper, full throttle!
Tommy D's Avatar
I could be wrong but I believe heís attempting to use differential thrust to emulate rudders.

Kinda hard to do a stall turn with your nose wheel!

Tommy D
Oct 28, 2006, 11:31 PM
Registered User
kensp's Avatar

The answer is to use a bl**dy big nose wheel

Besides, so far my flight-tests with 50 % differential thrust show that it about equal to a sluggish rudder. It will cause a turn but itís not very effective. One out of five attempts at a stall turn was successful but this rate will probably get better with practice. With rudders fitted I would be getting a 5 out of 5 success rate.

Last edited by kensp; Oct 29, 2006 at 01:38 AM.
Oct 28, 2006, 11:34 PM
Foam abuser!
crxmanpat's Avatar
Ken, that was too funny!

Oct 29, 2006, 12:04 AM
Registered User
kahloq's Avatar
Originally Posted by Red 61
Wow -- it seems to me a steerable nose wheel would be easier...
Well yes it would....but then theres no mind challenge either. I am really wanting to figure this out as I may have other planes that could benefit from it(especially if they dont have a steerable wheel).

This issue is the only thing holding me from getting the plane finished and test flown.
BTW, it already has a steerable nose wheel in the plane. Diff thrust could have saved me the weight of the servo, connecting rod, etc.

Kensp...i dunnno about your radio, but I can tell you from the ground tests I did with my DX6, that the diff thrust would make a large difference in turn ability. If I could get the motors synced up somehow, I am betting I could easily do stall turns even at the 25% I have the rudder mix set at now.

Not sure how much longer I will fuss with this though.
Oct 30, 2006, 05:58 PM
Registered User
kahloq's Avatar
Ok....finally got the differential throttle to work correctly with DX6. After all this BS I went through, I still couldnt figure out what was wrong. I even tried it using a JR7202 radio and it did the same thing(one side would spin faster then the other). So, I deleted everything, wiped the model memory and rebinded the rx just to be sure no blips of anything was left over.
So now, it works fine.
Mix A 4 1 +50/+50
Mix B 4 6 -50/-50
Mix c 1 6 +100/+100

If the wrong motor spins up or slows down with rudder input, simply swap the plugs from channel 1 and channel 6 or change the mix on A and B to a positive amount if it was negative and vice versa.

Additonally, you have to set the travel adjust on channel 6 to 0. Now, this applies to both flap toggle positions and I may have overlooked doing this for the up position...but I dont know. I thought I did everything the first time...but heh...it works now.

Horizon had given me a +100/+100 and -100/-100 for Mix A and B, but that made the plane too uncontrollable on the ground cuz it would lift one wing up off the ground due to centrifugal force cuz it spun around so fast in the turn. I reduced the mix to 50% and it still turns very fast but is manageable now.
I hope this helps anyone wishing to do this to save weight of a nose gear servo or have some form of rudder control in the air without cutting the rudders out and adding servos. I will now pull out the nose gear servo all together Yeah!!!
Oct 30, 2006, 06:10 PM
Registered User
kahloq's Avatar
New test data
This is with Park 370 outrunners spinning 9x7x3 MAS counter-rotating props on 18 amp CC esc's powered by 1 - TP 3s 2100

Static WOT
combined watts as read by e-flite wattmeter:
250 watts on fresh charged battery
239 watts after run down on battery to about 11.5 volts
218 watts after voltage dropped to below 11
Amp draw 23.5
Pretty much what I expected after doing single motor tests at about 145 watts per motor. A slight drop of 20 watts per motor when two in use off of same battery.
Mind you it takes quite awile to run down the battery, but I can get a sense that its getting close to below 11v and better bring it in for landing to avoid hitting LVC, but this would be imprecsise at best...so I will still fo course use a timer.

One thing I do have to say though, is that I think tis best to get slightly bigger diameter foam wheels if using 9inch diameter props. There is clearance with the stock wheels, but if you come in for a landing nose down slightly, you will strike the ground with the props(if using 3 -blade). 2 bladers may simply align themselves out of the way. Anyhow, I will pick up some foam 2.5 inch wheels and use one of the stock main wheels for the nose.
Oct 31, 2006, 03:40 PM
Outcast outlaw
Hey -- I think I've finally decided on my power system (still no GWS-38 though).

What is the average weight of the GWS-38's that are currently airborne? I want to make sure I have enough thrust.
Oct 31, 2006, 03:51 PM
Foam abuser!
crxmanpat's Avatar

I just measured again last night. Currently at 34.3oz/972g AUW with two coats of unthinned craft paint. Once final paint and decals are on, I will probably end up at 34.5oz. Using the Hacker A20-30M's and APC 8X6SF props, she flies nice and smooth, with enough power to ROG in about 40', climb out at 45*, and lands with just a hint of power on.

Multiple loops from level flight, inverted flight is fairly effortless, and rolls are nice and smooth. So far, I have not noticed any tip stall tendencies at reasonably slower speeds. Landing rollout is usually about 50-60' on my slightly rough dirt strip.

Last edited by crxmanpat; Oct 31, 2006 at 04:01 PM.
Oct 31, 2006, 05:39 PM
Outcast outlaw
Originally Posted by crxmanpat
...I will probably end up at 34.5oz...
Ugh... This is beginning to remind me of my first (Air Devil) P-38 -- it weighed in at 40.8 ounces.

My currently flying P-38 (a Flying Styro) weighs in at 24.4 ounces... That's a nice weight for a twin.

I am thinking of doing a full fiberglass and airbrushed paint job on this GWS-38... My power system will include Cobri gearboxes (more weight), so it might end up weighing closer to my first P-38!!!
Oct 31, 2006, 06:10 PM
Foam abuser!
crxmanpat's Avatar
My personal opinion only, but I think BL outrunners are the way to go. I loved the sound of the twin gearboxes chattering away on my original setup, but the weight savings when I switched to the Hackers was phenomenal.

Oct 31, 2006, 08:47 PM
Outcast outlaw
Pat -- what's your top speed? Total amp draw? Total thrust?

On a 3s LiPo, I'm looking at 46.7 mph, 44.4 ounces of total thrust, at a total amp draw of 13.5 amps.

The same power system on a 4s LiPo, I'm looking at 59.9 mph, 73.4 ounces of total thrust, at a total amp draw of 21.3 amps.

My goal is to keep the amp draw low, so I'm wanting to stick with the 3s LiPo setup... If your numbers are similar to mine, I will consider switching to the outrunners. However, I currently have outrunners in my Flying Styro P-38, and I'm not happy with the 24.8 amps it pulls at WOT.
Oct 31, 2006, 09:04 PM
Registered User
kahloq's Avatar
Last time I weighed mine...it was at 34 oz I think. Dont remember. I think I also estimated a final weight of 38oz....Im using the LG and heavier props(MAS 9x7x3) instead of APC SF or APC E.

I also put larger foam wheels for better ground clearance, so the weight gain amounted to about 1/2 an oz due to that. With 250 watts for my setup, I can go to 40oz and still be at 100 watts per pound which is still more then scale. I dont want to get that heavy though.

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