Large Model Association Forum

I wonder if more experienced members could advise on whether to use electric or pneumatic actuation / locking for my 1/4 scale Westland Wyvern. The various mechanisms are as follows :-

Retractable Main gear
Retractable and steerable tailwheel
Double wing-fold (mid panel and tips)
Power canopy open /close
Lowering deckhook

Also, would it be advisable /practical to 'interlock' the wing-fold and undercarriage mechanisms to prevent accidental wing folding whilst in-flight ?

Thanks for any assistance...
Dave C

Dave,

I've never tried electric retracts, air have always worked for me. As long as the system is well made with no leaks (and no dodgy plastic bottles), they are safe & reliable.

For the wing fold interlock, the easiest way would be inside the transmitter, so that folding would only be operational with the gear switch in the 'down' position in case of inadvertent operation in flight. A mechanical interlock would be getting a bit complicated & likely to be more trouble than it's worth.

Hi Rob - thanks for the reply, I guess the main problem with a pure transmitter-based interlock for the u/c 'down' switch position would be that the wings could still be folded during take-off and landing ? I had in mind some sort of oleo leg extension sensor to give the following control logic :-

Main gear up - no wing fold available
Main gear down and oleos extended - no wing fold available
Main gear down and oleos compressed - wing fold available

Presumably, this would mean an electrical micro-switch (or similar) on the main gear...not sure how the air systems are controlled - by miniature servo valves ?
Dave

In full size aircraft we called them WOGS - "weight-on-ground switches". They detect the oleo extension at take off and enable retraction, armaments, full radar power etc. On digital logic aeroplanes like Eurofighter the cockpit button function and its caption change so that there is no up undercarriage selection available if the oleos are not extended or down if the airspeed is too high!

For model work suggest a small magnet on the u/c leg and a reed switch in the wheel well. This would avoid any micro switch spring loads affecting the retracts. Chris

Dave,

You're starting to put in complexity & lots more stuff to go wrong then. In the very high load/vibration environment of a model plane, reed switches & microswitchers are nasty unreliable things.

It would be easier to put the wing fold on a really inconvenient switch that you couldn't reach in flight & mix it to the throttle so it would also only work at idle if the TX is capable.

Thanks for the input on full-size Chris.

Rob - I've had a look at my Tx and there's a rotary knob on it with a full 270 deg's of travel. I like the simplicity approach and I reckon that this knob could be accidentally nudged in flight without any ill-effects ie with wing-fold / unfold at the extremes of travel.

For the various power systems, I'm thinking : Air - Retracts, Gravity - Deckhook, Everything else - Electrical

Thanks and regards,
Dave

Hi Dave,
Firstly, electric retracts are becoming more popular, and offer better reliability. I use retracts extensively and air retracts are a constant source of troubles, not just to me but in my club in general (jet club). Conversly the models we see with electric retracts have been ultra reliable. So, I would say that electric retracts are the coming the thing.
Secondly, Of all the things that you mention, the wing fold mechanism is undoubtably the priority issue here. If you go on youtube and search you will find a significant number of videos of models crashing when wings have folded in flight.
If it were me I would have a locking switch on my Tx that cannot be accidently operated. Secondly I would have electric interlocks in the wing that prevent the throttle advancing if the locking pins are not in place. The locking pins would toggle the switch at the limit of their travel.
Micro switches are not nasty unreliable things and are not particularly effected by vibration. Howerever the items that they are fastened to, or that are used to toggle them, may be. So if the switch is fastened to a piece of 1/16th sheet that flaps in the breeze, do not expect the operation to be reliable.
Some rule for microswitches.
1) take the time to get exactly the switch you need. Do not "make do".
2) Ensure that the switch is electrically capable of dealing with the voltage and current you will be asking in to switch. I look for a rating double that which I intend to use. Avoid using the switch to drive high current devices.
3) ensure that the switch is set up correctly, so that when it is supposed to be toggled on the lever is sufficiently depressed, but never bottomed out.
4) Microswitches are mechanical devices that will wear out. Buy spares, and ensure that you can change them.
Good luck with the Wyvern, sounds amazing.
Regards,

John

Watching models with retracts (air operated) and their constant problems put me off using them for years. When I eventually did I was amazed at how little it took to make them fail to operate.
I have no experience of electric retracts.

As regards the wing fold, I have just started to use mode switches on the DX18 in my CARF Lightning. I set 3 modes - Ground, TOGAL and Cruise. (TOGAL = take-off, go-around & land)
In ground mode the retract function does not operate, so I cannot raise the gear when in Ground mode (I have done it more than once on prevuious jets).

You could use the mode switch to make the tip fold inoperable in any mode other than Ground.