Dumas Zero (As a trainer)

[TheLurker]

Make sure that you test that the O Ring is strong enough before using on your model. ...

+1 
I've had them fail as well.  Another possible source, as used by GM, is thin walled, narrow bore neoprene (or similar) tubing.  Take slices off a length of tube as you need them.

The snap is some lengths of tubing G gave me at the last Trinity.

[Piecost]

Yes, thin walled plastic tube is used on indoor duration models, as flown by GM. Not with reverse S hooks, which tend not to be used much on such models.

I have used the hard plastic tube and was supprised that the edge do not need smoothing.

I like the flexible O rings as they deform and kind-of wrap onto the hook.

Lurker, Is the neoprene flexible or stiff?

[Konrad]

Ok guys, we need to go back a bit to some basics. How are the O rings incorporated into the motor?

I assume you find the center point with the strands running in the center of the O-ring. Fold over the  pre wound strands as you would when braiding the motor. The ends of the motor are attached to the rear peg spool as normal. Attach the prop hook to the O-ring and let the motor unwind braiding the motor.

[Piecost]

For a four strand motor; thread the    loop through the O Ring. Attach one end of the loop onto an anchor, such as a door handle.

Position the O Ring in the middle of the loop. Apply the pre-tension turns. Hold the O Ring and fold the motor, attaching the free end over the anchor.

Attach the winder to the O Ring, and let it dangle and turn to braid the motor.

Only use a single O Ring at the propeller end. No need for one at the peg end. I am not even sure how one could be added there. But, a small rubber band can be useful at the peg end to prevent the motor unraveling. The small silicon Loom bands are a convient and cheap alternative to orthadontic rubber bands.

I have not figured out how to do six strand motor.

[Konrad]

Here is the first flight of the Zero with 100 turns. It really was too windy but what the heck, the wind is from your right. We have about 2 dozen test flight when we quit for the day. We got about 4 flights that did not stress the nose cowl. There was no constant pattern to the flights to aid in trimming. I blame this on the wind. But I'm also coming to the conclusion that I might need more decalage. I measure 3 degrees at the wing root measuring from the bottom of the wing airfoil and the stab.

In this flight and all others I could not get a transition from power to glide. You can see the start of a violent stall to the right (viewed from the cockpit). Other times the stall was to the left. I'm thinking of adding a bit more up decalage and add some nose weight to compensate. In far too many flight where the wind caused the plane to nose up into a prop hanging stall with a flip onto its back. This got the zero up to 5 meters. From here the Zero came straight down with no hint of a pull out. This is leading me to think I need more decalage (up). I'd have expected that as the model gained speed in the dive that the elevator (up trim) would have tried to pull the nose towards the canopy. I never saw the self correcting behavior.

The Zero shows promise both in the initial glide tosses and in the power on section of this video.

https://m.youtube.com/shorts/BHcCiixUQT4?feature=share

[Spiros]

From this particular flight, I saw a nice glide path. No nose up and in my opinion the "stall" was due to under power and wind on your left wing. You only had 100 turns

I wouldn't try it on a windy day. You cannot come into a conclusion having such windy weather.

[Konrad]

From this particular flight, I saw a nice glide path. No nose up and in my opinion the "stall" was due to under power and wind on your left wing. You only had 100 turns

I wouldn't try it on a windy day. You cannot come into a conclusion having such windy weather.

I agree that I shouldn't read too much into that day's flights. But two things did appear to be consistent. That is I never got a nice transition from power to glide. I could get some nice glides from a shoulder height launch. And I got a few nice powered flights. But at the end of each powered flight (if I was still in the air) the Zero would transition into a spiral dive, never to recover!

I had about 4 zoom climbs at (200 turns) where the wind drove the nose vertical. The result was that the Zero ended up about 7 meters above the ground hanging on the prop. It then flipped over on its back and dove straight down, making no effort to pull out. It did this about 4 times. I think this is a clue. (I know don't fly in the wind)!

The clue I think I'm seeing is that as the model gains speed in the dive there isn't enough decalage (up trim) to lift the nose towards the canopy. Right now I think the fix is to add up trim to gain this self righting characteristic.  Then add nose weight to find a nice level trim speed. (More nose weight might add some stall margin).

[Konrad]

All that talk about nose dives and I think I should say something about the survivability of the model. Key was the addition of a balsa box to support the thrust button. This should take the load off the fragile plastic cowl.

I also cut a slot into the rear of the balsa cowl ring to allow us to add mass (red clay) as far forward as is practical

[TheLurker]

Lurker, Is the neoprene flexible or stiff?

The black tubing shown is quite stiff, the clear perhaps a little stiffer than shop-bought O rings.

I'm expecting both to wrap around the hook when I get around to using them.

We now return you to your normal programme.

[Konrad]

It's all good. Are you using the tubing as a sheath to go around the prop hook? (Not allowing the wire to cut into the rubber motor)?

I'm using the S hook and rear spool to define the ends on my braided motors.

[Piecost]

A sheath seems like a good idea, but not one that I have bothered with; yet. A bobbin loose fitting onto the peg aids installing the motor and helps to reduce bunching. The bobbin can be seen to be wobbling and partially rotating as the motor unwinds.

[Konrad]

This Zero is being used to reintroduce my brother to the dynamic art of flying models.

Well that was hubris! It has been more than 20 years since I last flew a rubber band scale model. And that was a much larger Comet 24" Hellcat. It is obvious that I'm on the steep learning curve on making a workable power system.

Here is what the power system looks like as flown last weekend. The rubber is about 3 grams (4 strands of 3/32 nominal rubber). The rear spool (bobbin) has been working great. I'm about to disassemble the motor to incorporate the O-ring.

This is a good time to ask what do you use as a motor lube? I was using glycerine. Also how much to you preload the loop before folding it over during the braiding process.

[Konrad]

Yikes! I just found out why the thrust line shims apeared to have little or no effect while trimming. The Dumas thrust button will allow the prop shaft to gimbal 10° or more! I will need to come up with a rear suport on the prop shaft to actually allow the shims to effect the thrust line.

[TheLurker]

It's all good. Are you using the tubing as a sheath to go around the prop hook?

No, when I can be bothered, not often, I use heat shrink tubing for that. This sort of thing.  I used to get mine from Maplins, they sold one or two yard lengths for not much more than pennies, and I'm still working through it.

The tubing shown above is simply sliced to give a custom width "O" ring.