Short S.42 1912 monoplane

[Prosper]

Ta.

[Pete Fardell]

Nice and calm yesterday so I gave the Short mono another go. As recommended by Mr Crossley I'd put a lot of negative incidence on the tailplane (up elevator)to try and keep the wing at a higher angle of attack. It was instantly better; slower and with no nasty sudden dives. It wasn't really climbing though, so I swapped in my Bleriot motor of 5 braided loops (10 strands) of 1/8, in place of the 4 loops I had before. It still doesn't get much height but if I can stop the motor bunching at the rear peg I think I can probably get more out of it. The trouble is it's a smaller model than the Bleriot, but not much lighter so evidently needs as much rubber in an even smaller space. Perhaps though I can rebraid to be even shorter, or else lose a few inches and hope it's still enough. Anyway, here are 3 short flights from yesterday. The drop in at the end of flight two is, I think, the result of the aforementioned bunching. Poor launch on that flight too.

[Squirrelnet]

Looking much better Pete. Without the upset of the outdoor air that's looking much like it will do it indoors. I think I would try re-braiding the motor shorter, you'll loose a few turns doing that but possibly better than a shorter motor ?

[Jmk89]

Applying the JOD principle that scale models are over-rubbered and under-wound, it might be best to reduce the rubber but wind it up some more.

 The upside is that you will then be accessing the high torque of the motor when it gets to 90% of the available turns, and, with suitable thrust adjustments, that should encourage the climb. The downside is that winding up to that level does lead to tired motors, so they have to be changed more often.

[Pete Fardell]

Applying the JOD principle that scale models are over-rubbered and under-wound, it might be best to reduce the rubber but wind it up some more.

Thank you; I have considered this route and to some extent it's what I'm trying to do already. The previous motor (8 strands of 1/8 instead of 10) may still have made it "over-rubbered" but it wasn't under-wound. That's to say, with both this model and the Bleriot I do tend to wind to near max as my quite regularly broken strands attest.
What I haven't yet tried with this one though is to go with even less rubber, say just 6 or 7 strands, before piling on the turns. The trouble is that with all the nose-weight the model isn't very light so I'm doubtful that I can reduce the rubber that much and still get it to climb at all. Might be worth a go though.

[Squirrelnet]

Surely though if you are winding a 10 x 1/8 motor to full turns or presumably 80% of breaking turns then a smaller motor even if wound to breaking turns will not produce as much power and therefore not climb as much ? I wondered if you needed more power ...11 strands ?

[Pete Fardell]

Surely though if you are winding a 10 x 1/8 motor to full turns or presumably 80% of breaking turns then a smaller motor even if wound to breaking turns will not produce as much power and therefore not climb as much ? I wondered if you needed more power ...11 strands ?

I think maybe you're right, but am a bit loathe to try and cram even more rubber in and if I can stop the 10 x 1/8 motor from bunching then it might yet get a bit more height before it starts descending. It climbed out a bit on the second flight in my video but couldn't sustain it.
Everything really is the same as on my Bleriot, which occasionally gets quite high but usually doesn't.

[Jmk89]

Of course, there is another side of the equation that can be looked at. That is whether the prop is correctly dimensioned for the motor.

In the current situation, where you don't seem to be getting quite enough climb from the high torque beginning of the motor run, I suppose there are three prop variables that can be considered:

• reduce the weight of the prop blades  - I've had very little luck are doing this.
• reduce the pitch of the prop - this is the basis of the variable pitch mechanisms used in rubber duration. Finer pitch is used at the beginning to help the climb and during the cruise section, the pitch is increased so that, even though the prop is turning relatively slowly, it is still taking a decent "bite" out of the air with each revolution.
• reduce the blade area - in effect, this makes the proper easier for the motor to turn. It also means that you can make adjustments to an existing prop rather than having to make a new one. You can either slim the prop down by thinning it at both leading edge and trailing edge, or cut an equal amount of each tip or do a combination of both. It will rather depend on how brave you are.

Some flyers that I have met actually have a test prop system which they use when first trimming the model to determine the best prop geometry before making the prop they actually want to use. In essence, it is a hub into which different sets of  blades (with differing diameters and chords) can be inserted and which also allows the pitch of the blades to be adjusted . After a few years, the number of sets of blades can become large enough to allow a pretty good estimate of what the "best" performing prop will be and that prop can then be carved.

One other thought I had is that the lighter motor, if more than 50% of the motor is behind the CG, will bring the CG forward and some of the nose weight may need to be removed to get the CG back to the correct position. You might find that just that reduction in overall weight is sufficient to enable the model to climb better.

As always, there are plenty of things that can be tried to get the desired result!

[Jmk89]

Surely though if you are winding a 10 x 1/8 motor to full turns or presumably 80% of breaking turns then a smaller motor even if wound to breaking turns will not produce as much power and therefore not climb as much ? I wondered if you needed more power ...11 strands ?

If you look at the turns/torque curves (eg on page 24 of the JOD collection soon too be available in the Plans Gallery), the high torque part of the curve doesn't come until at least 90% of the break turns are applied. At 80% you are just in the early part of the cruise part of the torque curve.  Duration models are always wound to 90% or more - that's why you only get 2 or 3 competition flights from a motor.

I think it was looking at the reluctance of scale modellers to wind into the high torque part of the curve that led JOD to say that the motors were under-wound.

[Pete Fardell]

Jeremy, thanks for all those thoughts. Plenty to consider for me there.
As far as winding to 90% goes, I suspect scale modellers' reluctance to do so stems from the fact that damage resulting from a burst motor in a scale model is much more of a hassle to repair than it is with duration models. And winding tubes don't eliminate the risk of leaving the dreaded angry ball of rubber bunched up at the back.