Per Strommen

On my 1/4 scale Kerner Vampire the main wheels are 6 inch and heavy. The 5/8 in Robart leg and wheel weighs 580 grams(22oz)!
This has given me trouble with consistent gear retraction. The Robart 635 would sometimes be unable to lift the assembly, worked in the shop but could fail when flying.
I tried a hydraulic system, but apart from the WOW factor things did not improve as the maximum pressure was the same as in the pneumatic system. Scale factor improved considerably however.
So I needed stronger rams. Reading about electric retracts here on RCU drove me in the direction of LADO actuators, and after some e-mails the order for 3 ea 666 electric actuators was placed. I also ordered the programmer and the die tap for the 6-32 thread. Very positive experience with LADO, Christope and Stephane answer mails promptly and even tested my weight and published the video of the test on uTube for me to see. Excellent service!

Per Strommen

Changing the actuator on the Robart unit is rather straightforward. Three screws, a C-clip and a locking ring and the unit comes apart.
I removed the T-bar from the pneumatic ram, drilled and tapped for the 6-32 thread on the electric actuator, and assembled the unit with the electric "cylinder"

Per Strommen

Time for testing:
Using a servo tester and 4.8V battery the unit was struggling. It barely lifted the leg and sometimes stopped. I noticed the T-bar would sometimes have more friction on one side, and rotate out of the 90 degree angle it is supposed to have to the screw. This would increase friction and to a point where the motor stopped. I changed to the airplane power system, running on A123 cells 6.6V and the retracts seemed do operate as advertised. Success!
Mounting electric retracts in the plane is easy, only one "servo" wire going from the unit to the receiver. Removing the old air(hydralic) hoses took longer.
Doors remain pneumatic for the time being as the speedbrakes and wheelbrakes also are air driven, I saw no need to change this. But I needed more delay on the doors as the electrics are slower than the previous setup. No sweat, just hit the "+" on the Jettronic Door-Sequencer. But no, I could not get the required 12 second delay no matter how many resets and +pushings I did. This fiddling took the gear through approx 40 cycles and the left leg started to act up. It would stop half way every time.

Per Strommen

Problem 1: Whats up with the sequencer? Why will it not give me more than 9 sec delay? The instructions says "adjustable from 4-13 sec"
A mail to Horst Lenerz cleared it up:
"You certainly have one of the newer transmitters with a higher pulse repetition time, which causes a much shorter maximum delay time.
At the time the sequencer was developed, 20 Milliseconds have been usual. Today they are 14mSec and shorter."
I also got a good offer on an update to fix the problem and extend the max delay to 40 seconds. Again excellent service.
Problem 2: What is wrong with the retract?
Removing an electric retract from the model is easy, and I soon noticed my problem: The threads on the screw were worn at the place of most resistance. More testing out of the model revealed more of the same I noticed with 4.8 V- the T-rod was pulled out of 90 degrees to the screw.
My answer to this: I need longer threads through the T-rod to prevent the pinching. So I found some 8mm steel rod and made a new T-rod on my mini-lathe. Finally useful!
Tests were promising, but I decided to go one step further and made another T-bar from 12mm brass rod.

Per Strommen

I feel confident this last change will do the job. The brass can be exchanged for steel if flying (poor landings) should bend the T-rod
The nose unit has worked flawlessly the whole time, but the nose wheel is 5 inch and the assembly is a lot lighter.
Pros and Cons:
Electric stronger, more consistent, weight saved on model, easy retract installation, no air leaks, no valves, no air lines, scale retraction speed
Not so good: A bit long retraction cycle (12 sec), more expensive

Per Strommen

Will keep you posted on test flights!

My way or you can wait for the Robart Electric retracts that I just saw on their homepage....

Jgwright

Per

This is a very informative post and useful for me. I have 3 planes with large wheels. I anticipated having similar problems to you. I have used Behotec C50 retracts which have been very strong and reliable so far. My Horten has a 6 inch nose wheel and a long leg. However it has a second long air ram to assist the noseleg. The mains are 4 inch. This arrangement has worked without any problems. The mains wheels and brakes were made by me.

Recently I made the Arado 555 which has a 4 wheel bogie. I used very light weight wheels on the inside and on the outside I made my own 4 inch wheels and brakes using Dubro tyres. So far this works in the workshop but it has not flown. It is asking a lot though of the pneumatic system.

I have just finished the wheels on the Boulton Paul P111 which are 6 inch mains (scale 1: 4.2). Again they are made by me using the dubro tyre I ended up at a weight of just under 300 grams for the wheel hub and axle. My tests showed that the wheels would lift up at 60 psi leaving plenty in hand. The wheel rims are as thin as I thought sensible so there is no chance of making them lighter.

If I have problems with either the Arado or the P111 I was planning to install electric retract motors in place of the cylinders so your post is very helpful.

I love the Vampire and especially like the nose leg you have used.

John