Spring Shut-Off

$/!\$ No more development or experiments will go into this - there are now much better mechanical systems available (see Viko on the links page).

Pete Athan's spring-based shutoff appeared to be a quick way to put a shutoff onto an existing plane to conform to the new shutoff rules until a better, more permanent solution is found. I built a version of this shutoff with some modifications:

Put a "bell-crank" in (rather than the sliding coupling) to eliminate line movement friction.
Allow a normal bladder tubing length and tubing type by using the right type of spring-constant and a better tubing retainer clip.
Allow easy connection to lines using just line loop hooks (so that the shutoff can be moved easily from model to model).

Does it Work?

This design works and has no noticeable friction during normal flight. However, at launch and when gliding, there is essentially no control, due to the leadouts being bent up (or retracted) due to the spring action on the untensioned control-lines. You can get a response by applying full up/down and tugging the handle to momentarily straighten the models leadouts.

A partial fix might be possible by shortening the spring and setting the length of the "sensing" wires to the leadouts so that the leadouts are almost straight with no line tension. With line tension the leadouts and the spring only move a further 5mm out. A short spring (perhaps 2-3 coils) will still open the fuel line with this movement. However, getting the sensing wires exactly the right length and getting the nail in the right spot (+/-1mm) so that it keeps the fuel flowing but still allows the sensing wires to pull the bellcrank a little further to pop it off the nail.

Summary

it works well while flying with no friction,
in theory, the shut-off characteristics should be reliable and not "fiddly" (but I haven't let go of the handle),
it is terrible at launch and gliding,
it could be made nicer at launch/gliding, but at the expense of making it even more "fiddly" to get it working properly.
works well enough (just) to act as a stop-gap shut-off until a better mass-producted shut-off is developed.

Other Observations

Mike Wilcox's electronic shutoff is looking good (as of March 2009, and despite my initial reservations of it being able to be produced cheaply enough in small quantities). I'm sure this will be almost intolerably "clunky" to start with, but if people support it, it may be refined over a period of years to be exactly what we want.
The H&R shut-off also looks good, although:
- it must be inside the wing and incorporated into commercial models as an off-the-shelf combination (i.e. licenced very cheaply to these manufacturers), and
- how to reasonably route the fuel tubing from the bladder through the shutoff and to the engine needs to be improved.

Photo Construction Details

Here are the main parts of the shutoff. The spring was carefully selected to provide sufficient force at rest to close off my favourite standard fuel tubing (needs 1kg force on spring to allow normal fuel flow). The length was set to have a little extension when lines were under tension, but not enough to pull and kink the leadouts when released. One end of the spring is tailored to clear the webbing of the engine mount, but allow the spring to sit close to the surface of the wing. The other end is looped to hold the "bellcrank". The bellcrank is essentially a wire version (to reduce wind resistance) of a normal bellcrank. This replaces the sliding fishing-line loop around the spring hook, seen on Pete Athans' original version. Two small hooks ready for tieing fishing-line to (or perhaps better to use control-line wire), so that these can simply be hooked onto the standard looped ends of the control-lines. A part not shown is the fuel tube retaining clip. It is made from standard paper-clip wire to have a straight part and bent into a push-clip at the end. Look for it in later photos.
Here is the completed shutoff installed to a 2.5cc Speed-Limit plane (Viko F2D model). Notice the small nail (actually two, for experimental purposes) to lock the shutoff in the open position for starting and launch. The centre loop of the bellcrank is hooked over the nail to lock it in place. The nail has to lean a little towards the wing-tip and a little towards the front of the model, so that when the bellcrank is pulled towards the wing-tip, it releases itself towards the front of the model (where the line of the spring to the leadouts is pulling it).
This close-up view shows the fuel tube retaining clip in place and clipped onto the spring end-loop. Notice the end-loop is made to clear the webbing on the engine mounts. The clip extends all the way through the spring and further so that even with the spring extended, it is still longer than the spring. Using the clip allows us to avoid using a loop of fuel tube lengthways through the spring to retain the "active" part of the fuel tube in the spring. The single pass of the fuel tubing through the spring means that the normal length of tubing on a bladder is sufficient.
Here are the line-hooks in place, hooked onto the control-line end-loops under the usual fuel tubing retainers. It is very difficult to tie the fishing line onto these hooks while at the same time getting the length of fishing line exactly correct. It would probably be better to use control-line wire (which wouldn't stretch as much as fishing line) and use crimps to get the length correct.

PeterMills 2009-03-06 CategoryComplete