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May 19, 2019, 06:25 PM
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Question

servo size vs. model size


Hi all. You can say I'm old, and old school. I still like making balsa dust and I remember you bought a radio as a set. Servos? You got what came in the box with the transmitter and receiver.
I'm building a 68" span WWI Eindecker kit, guessing 5-6lbs, and wondering if I could use standard servos for the rudder and elevator. I have plenty of them just hanging around, so why not? I'll need to use something a little smaller for the ailerons though.
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May 19, 2019, 06:43 PM
Neophyte hacker
portablevcb's Avatar
Yes. If you are going to electric then you might want to move the servos closer to the nose and place the batteries as far forward as well.

I like HS82 servos for wings in larger planes, or other mini servo with metal or composite gears (no plastic gears as in HS81 servos).

charlie
May 19, 2019, 07:48 PM
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Quote:
Originally Posted by portablevcb
Yes. If you are going to electric then you might want to move the servos closer to the nose and place the batteries as far forward as well.

I like HS82 servos for wings in larger planes, or other mini servo with metal or composite gears (no plastic gears as in HS81 servos).

charlie
Yes, going electric on this one. I hope to keep it down to a 4S battery. Not going to do any 3D flying with this for sure. I've relied on mico servos for my other arf's I didn't know how big I could go with them and still keep it light. Or just say screw it and go for a standard size that comes with the transmitter kit.
May 19, 2019, 09:41 PM
Neophyte hacker
portablevcb's Avatar
For most WWI planes with rotary engines the balance is the hard part. Std size servos aren't that much heavier than mini's in this size a plane.

Depending on motor you might consider a 3s and swing a larger prop. Larger motor also means more weight up front. Look for a low kV motor. For packaging you might even want to parallel two 3s packs to fit close up front.

charlie
May 20, 2019, 03:13 AM
An itch?. Scratch build.
eflightray's Avatar
Many of the modern mini and micro servos can put out as much torque as the old Futaba and JR servos for a fraction of the weight.

It depends on whether you consider weight saving a benifit or handy cap, (some IC fliers don't seem to care, they just fit a bigger engine ).

.
May 20, 2019, 05:30 PM
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Stephen M's Avatar
'jeep, you say its a kit. Is it an electric kit? If it is just ask here and let us know what you have as guys who have built it may have specific advice.

As an aside I built the Balsa USA Fokker Eindekker 40 gas plane but using electric and the only change was the motor, batteries and servos (I used micros in the wing and minis in the fuselage) the rest of the build was stock and pretty heavy. It weighs somewhere around the same as your target and flies with lots of power in reserve on 3S. I use two of the original Hobby King 3S2200 packs, one each side of the motor in the cowl. I run a 500 or 600 kv motor and swing a 14" diameter x 10" or 11" pitch prop. Flights are just above half throttle.
May 20, 2019, 08:05 PM
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Quote:
Originally Posted by Stephen M
'jeep, you say its a kit. Is it an electric kit? If it is just ask here and let us know what you have as guys who have built it may have specific advice.

As an aside I built the Balsa USA Fokker Eindekker 40 gas plane but using electric and the only change was the motor, batteries and servos (I used micros in the wing and minis in the fuselage) the rest of the build was stock and pretty heavy. It weighs somewhere around the same as your target and flies with lots of power in reserve on 3S. I use two of the original Hobby King 3S2200 packs, one each side of the motor in the cowl. I run a 500 or 600 kv motor and swing a 14" diameter x 10" or 11" pitch prop. Flights are just above half throttle.
It's the Balsa USA Eindecker for .45-.60 glow. So I'm converting it. Thinking about the Turnigy G60 500kv motor. This thing has some huge control surfaces, that's why I'm thinking about using std servos. Or some metal geared smaller ones for the ailerons. I hope to turn a 17"-18" prop. Also thinking about a sound system. I've never messed with one and I think this would be a perfect model for one. Plenty of room for a speaker in the fuselage.
May 21, 2019, 08:02 AM
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Stephen M's Avatar
Hmmmmm. Looking at the Balsa USA site they only show an 80" version now. Even my 60" span version is not there. Anyways mine is the 60" span so what shows above worked for me. Yours is not much bigger so may be pretty similar. If I can be of any more help please ask.
May 21, 2019, 05:01 PM
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Thread OP
This is a really old kit, plain brown box, and says designed for Balsa USA in the corner. But it is 68" span. A prototype? Can you tell me the width of the cowling on yours? Mine is missing. Mine is 7-3/8" wide at the firewall. I hope I can find one at Balsa USA or I will have to build my own.
May 21, 2019, 05:38 PM
Registered User
Like the question... same issue for me... building a 120% FT Guinea Pig. 9g servos (1.5kg.cm torque) work fine in the 100% size model I have, but 20% increase in wing length creates 40% increase in wing area. Makes me think about going up one size in servo to 17g (3kg.cm torque).
May 21, 2019, 07:37 PM
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Stephen M's Avatar
My cowl is about 5 1/2" diameter and I cut it to a length of 2 1/4". No where near yours. Sorry.
May 23, 2019, 09:46 PM
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Quote:
Originally Posted by Stephen M
My cowl is about 5 1/2" diameter and I cut it to a length of 2 1/4". No where near yours. Sorry.
Thanks for measuring for me. Now I know I need to call Balsa USA and see what they have that's close to that size. I may have to build one out of balsa. Yuck! Lots of work.
May 24, 2019, 09:33 AM
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Stephen M's Avatar
Well the fuselage is really big. Perhaps to fit olde tyme radio gear. Span is not much larger than mine and mine runs fine on two micro servos, one at each aileron. Since you say you have lots of suitable standard servos then might as well go that way if they fit in the wing at the aileron. For the weight I would be inclined to stick with 3S as the flying speed is so slow I doubt there is a lot of drag. As I said I mounted two 2200 packs one each side of the motor. I made a plywood 3 sided box which went around the motor. The batteries are fixed to it and stay in the plane. The 500 kv motor sounds good. I think mine is 500 kv. I made my cowl just long enough to clear the prop when the motor is bolted to the firewall. It is large enough to hold the batteries without drama. Since all this weight is far forward balance was not an issue. I did leave the distance from the wing front to the firewall as designed which is twice the scale distance. Only us scale nerds notice though. One more thought. I am pretty sure the top of the ply box holds the rudder and elevator servos again keeping weights forward.
May 24, 2019, 07:09 PM
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Quote:
Originally Posted by Stephen M
Well the fuselage is really big. Perhaps to fit olde tyme radio gear. Span is not much larger than mine and mine runs fine on two micro servos, one at each aileron. Since you say you have lots of suitable standard servos then might as well go that way if they fit in the wing at the aileron. For the weight I would be inclined to stick with 3S as the flying speed is so slow I doubt there is a lot of drag. As I said I mounted two 2200 packs one each side of the motor. I made a plywood 3 sided box which went around the motor. The batteries are fixed to it and stay in the plane. The 500 kv motor sounds good. I think mine is 500 kv. I made my cowl just long enough to clear the prop when the motor is bolted to the firewall. It is large enough to hold the batteries without drama. Since all this weight is far forward balance was not an issue. I did leave the distance from the wing front to the firewall as designed which is twice the scale distance. Only us scale nerds notice though. One more thought. I am pretty sure the top of the ply box holds the rudder and elevator servos again keeping weights forward.
As drawn, it only calls for rudder and elevator. So big servos were the norm. You may be right about mounting the servos forward of the wing spar and wing hold down. Two 3S packs would be nice for long flights. I need to get some batteries and servos out to see where stuff is going to lay. I have a short list for the hobby shop this weekend so I can wrap up the wings. Servos, extensions, and hardwood for mounting the servos. My goody box is getting low on scrap pieces!
May 25, 2019, 05:27 AM
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Stephen M's Avatar
For internals I cut up foam to the size of the items. Then use those blocks to determine layout. Lots cheaper than buying and finding a different make or model fits better.


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