Airco DH2

Look at this, starts slow,,,
 

Going back to my Boy Scout days nearly 70 years ago the reef knot was the standard general purpose knot. The sheet knot was for joining two ropes of different sizes, I remember the rabbit coming out of its hole, round the tree & back down its hole.
The videos linked by Flytilbroke use an overhand knot which is simpler & seems to be just as effective.
I read recently to dab some cyano on the ears of the knot which I hadn't done so I'm convinced the knot came undone.

I've watched the videos & they're very informative though I struggled hearing the audio.
If I heard correctly he was making up a motor for a 150 sq inch wing area model which is only just over half of my DH2's area. He used 25g of rubber wound in 8 strands.
Again if I heard correctly it was his Cessna CR3 which is probably a sports car compared to my DH2 HGV.
My whole pack of rubber had 5m of 3/16" which allowing for my snapped piece would have weighed 25g so I have been severely under-using it.
Before I saw the video I had fitted a 100cm length in 4 strands & it did pull the DH2 across a tiled floor but definitely not fast enough for take off.
Another video came on which showed rubber being pre-twisted before being looped so thats another lesson.
So its back to the twisting with more rubber, the question being how much tension can the booms take.

I fear it is the booms strength that will limit the amount of rubber you can use.
The problem you have compared to say a similar sport plane of the same weight is the DH2 has a lot of drag coupled with questionable prop efficiency. The result is it may need twice the rubber giving only half the power duration.
Note the increase torque required to achieve the prop RPM may also give you some flight difficulties when its under power.
Pre winding the rubber is more to do with increasing the power duration than increasing the power available  For most rubber powered planes the issue is not the maximum thrust , it tends to be too much anyway, but how best to get the longest and most even power duration.  
A rubber powered DH2 is not an easy model subject.

On account of we're back to snow I'm stuck indoors so I've been having a look at the DH2's weight problem.
All in it now weighs 235 gram. I have calculated or estimated the weights of the materials I've used & it comes out close to that.
The wood comes out at 115 gram. I've used spruce for the booms & wing struts. Using balsa for the wing struts would only save about 10 gram. The booms have to be spruce, aluminium tubing would work out at the same weight & I don't think would be as robust.
The miscellaneous stuff comes out at 120 gram. This includes 32 gram of nose balance weight, the 12 gram tail at 360 mm from the C of G and 10 gram of booms accounts for nearly all of this.
So I don't think I can save much, no wheels & undercarriage might save 20 gram, no paint posssibly 10 gram so only 40 gram overall which would still make it nearly 200 gram.
What I need is more power.

If this works these are a couple of photos of the beastie.
The tail is held on with a rubber band & the nose is not covered until I try & fly it!

Your DH2 certainly looks the part. Very nice.
Before you pile on more rubber you may have to consider the strength of the wings between the tail booms.
The tail booms may be able to handle the compression forces but can with wing roots resist the forward forces from the booms and the rearward pull at the prop? 

Winding up the rubber earlier hooked onto the prop, but not involving the booms, with quite a bit of tension applied the rubber snapped. I don't think the booms would have taken that much compression, I think any sideways flexing & they would have folded.
If the fuselage was being driven forward by say an electric motor in the fuselage I think the wings between the booms would be able to take that amount of drag, that doesn't seem to be the weakest point to me.
The tail would be pulling the booms back in tension when they would be able to take more than in compression.
The rubber tension would also be helping to cancel out the tension in the boom caused by the tail.
I think the rubber tension would be far greater than the the drag forces & would be the nemesis for the booms.
 

Magpie
My point exactly. The rubber forces will far exceed any other except perhaps those resulting from a crash.
I can see the attraction of using the booms but a rubber motor inside the fuselage would be self contained in a rigid structure. It would allow the rest of the airframe to be built light enough to handle just the flying loads. 
The only comparison of weight I can make is my original 1970 20" KK Auster Arrow weighs 39g complete less rubber but including a 7" plastic prop.

The fuselage can only hold about 15 cm of rubber.
I understand torque is independant of rubber length but how long a flight would that give unless it could shoot the model up high enough to have a nice glide down. Like rear wheel drive cars too much welly would probably just result in somersaults.
I'll have to make a test-rig to try it out.