Hi Mike,
The issue with low ceiling is that you want to use the last portion of the turns, and there is only so short you can go with the loop length before you start having tension problems with the motorstick.
Also, a given prop diameter and pitch distribution can also only go so low in pitch before it starts producing less thrust rather than more.
So, one option is to sacrifice some efficiency and use a smaller diameter that can go to a lower pitch at the end of the flight. My 17.5" was formed on a 30" block, and was happy at about 19" of low pitch.
I think this was actually too extreme, though. I ended up using 9.5" loops that took 1940 turns. The best I could manage was a 30:00. Brett was using a similar diameter, but I think a less extreme block. His motors were much shorter, and he finished with several flights over 30 minutes, the best at 31:34.
The biggest challenge we both found was that the high pitch and preload was extremely sensitive. We are used to a turn or two of the preload making a difference in taller sites. Under 50', a 1/8 turn of the preload or high pitch was the difference between hanging up and only going halfway. Finer pitch on the screws would be handy. After a little research I've discovered that in addition to 00-90 (90 turns per inch) screws, there are also standards for 00-96 (96 turns) and 00-112 (112 turns). There are also metric versions, such as M1.2x.25, which has 102 turns per inch.
The only pre-made screws I've seen are the the nylon 00-90 type. Small parts doesn't seem to have them anymore, but this website shows them:
http://www.microfasteners.com/catalog/products/NYLNBB.cfm. I've never ordered from there, so I can't vouch for them. If someone does try, let us know how it works out.
But anyway, I digress. You are asking about what you should build. For USIC, something around 19" diameter seems to work well. Normally I'd recommend something smaller (i.e. more thrust) for people's first F1Ds, but yours flies very nicely. I think you'd do well with 19".
--- In Indoor_Construction_at_yahoogroups.com, "mkirda@..." <mkirda@...> wrote:
>
> Hi, John.
>
> So what is the thinking on props versus ceiling heights?
>
> Is 17.5" better for lower heights due to lower torque (smaller motor cross-section), slightly longer motor, slightly faster rotation balanced with lower overall expected flight times?
>
> I'm kind of grasping in the dark. I still expect to build just one size for awhile. Being a newbie and all.
>
> Regards.
> Mike Kirda
>
> --- In Indoor_Construction_at_yahoogroups.com, "John Kagan"
> >
> > Ok ok, I'm just procrastinating on my last bit of preparation for Kent. I still have to cover my new low ceiling prop.
> >
> > After extensive research and analysis, I have determined that 17.5" is the optimal diameter for Cat II, so I built one. Of course, the research and analysis consisted of Sanborn telling me that that's what Bennett flew.
> >
>
Received on Mon Apr 16 2012 - 18:45:56 CEST