chris-s

Hi folks,

I have rebuilt a new fus for a Modeltech Dragon Lady. This is a .90 sized low wing vintage style sportster aircraft, the previous fus got slightly damaged during a bad landing but was so badly fuel soaked it was safer to build a new one. I did not build the original and drew up my own plans for the replacement after disecting the original.

The engine is an old OS 90 MAX FP, turning 13x8 prop.

I have refitted the engine and noticed a significant amount of side thrust, about 5 degrees to the starboard/right that I never noticed in the original. The thrust is introduced by the engine mount, engine is mounted side-winder style and the mount is the original (for me) single piece metal mount. This looks like helluva lot, but I'm not experience enough to say for sure.

I can't decide whether I just didn't see it before or whether it took a knock when it hit the deck. The bump occured as it stall from a few feet on landing onto a tarmax strip. The only damage was the tail feathers got knocked off, wing bolts broke, and the undercarraige mounting plate came off, it was completely fuel soaked so probably not surprising. The cowl was scraped on the just below centre and to the left side of the spinner, and prop broke.

I have double checked the original fus sides and parts, and there is no sign of opposite thrust being built in.

Suggestions would be greatly appreciated!

Cheers,

Chris

Should I just fly it and see, or fit a new mount and square it up.

majortom-RCU

My Feedback: (40)

It would be great if someone with the original plans could tell you what the kit recommendation was for thrust offset. Lacking that, I will say that five degrees sounds like rather a lot, but not entirely out of the ballpark (maybe just over the fence). Most low-wing models like two or three degrees of right thrust, so five is not at all unflyable.

Most sport fliers are pretty much oblivious to thrust offset anyway. If it's worth your while to get this parameter lined up with some precision, you can check the trimming out procedures at http://webpages.charter.net/rcfu (select "Model Trim Problems"). I know lots of models where the builder ignored the thrust offset recommendations from the kit instructions, and they think their plane flies just fine. And it does, but it's not competing in pattern events or otherwise under the scrutiny of contest judges. In the worst cases a little rudder trim makes the problem invisible.

chris-s

Thanks for the reply majortom, I realised late last night that I had calculated the offset angle incorrectly, in fact it is just over 7 degrees! I've looked back at some photos of the original taken before the bump, they're not terribly clear but it doesn't look like there is anywhere near as much side thrust if any at all.

I reckoned on 2-3 degrees being reasonable, but 7 has to be too much.

I have emailed the UK suppliers of the kit to see what they have to say.

Cheers,


Chris