Air resistance / density / humidity changes ?

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PHXcoach
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Air resistance / density / humidity changes ?

Post by PHXcoach » May 20th, 2013, 11:03 am

Last week was our first trip to nationals, and we had a great time (thanks to all of the events supervisors) but our helicopter times were disappointing.

At home (Phoenix, AZ) we had tuned our helicopter using thin 3/32" rubber to allow us to use a long band with over 2000 turns and keep the motor weight down. We were getting times of about 2:45 with a flat 8" ceiling, but at nationals we only got 1:19. We later found some damage to one of the helicopter blades, but both helicopters had similar low flight times so we think that the change in humidity between AZ and Ohio was the biggest factor.

We also had a weight gain (one helicopter added 0.14g) and some of the balsa glue joints broke, lifting the surface layer off the balsa motor stick rather than the joint itself. Again we believe that this was the humidity.

We can keep the helicopters sealed with desiccant to reduce the weight gain, but that won't enable us to test or adjust for the change in flight times.

Given that next year will be in Orlando (and hoping that we win state again) how do other groups manage this variability ? Do you just go early with extra kits and retune or are there better solutions ?

chalker7
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Re: Air resistance / density / humidity changes ?

Post by chalker7 » May 20th, 2013, 8:01 pm

So humidity could have played a big role in the issues. Specifically, if any joints broke at all, then you have no idea about the flying characteristics of the helicopter. All of your trim settings would be lost.This is a classic issue in any flying event, but it is more commonly a problem in going from areas of high humidity to low since materials tend to contract in those circumstances, which can be particularly devastating with paper covering.
One solution can be to adjust your glue. Don't use brittle glues like super glue, instead use something with a little toughness like water-resistant wood glue (I like titebond.)
National event supervisor - Wright Stuff, Helicopters
Hawaii State Director

erikb
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Re: Air resistance / density / humidity changes ?

Post by erikb » May 20th, 2013, 8:20 pm

PHXcoach wrote: Given that next year will be in Orlando (and hoping that we win state again) how do other groups manage this variability ? Do you just go early with extra kits and retune or are there better solutions ?
Build a new glider on site. That is a time consuming process in the middle of chaos. The goal is to get to nationals with only minor tweaking and repair.

A few things, look into duco or ambroid. Not only are they a little more giving. You can release the glue with a thinner like acetone to replace broken pieces or relieve pressure from swelling.

Next look at solutions that allow you to change the pitch of the blades. The density of the air changes lift of the wings.

Finally, take a variety of rubber bands.

Tweaking the helicopter to find the right combination of prop pitch and torque for variations in tempature, humidity and density due to altitude is hard and takes a lot of practice and knowledge. But, if your kids put in the time they will be able to compete in any venue at anytime.
--
Poudre High School, Fort Collins CO.

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