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Permalink Reply by Bill on June 8, 2009 at 4:38pm

"It's true that there is less air resistance up there, but there is also less thrust available for the prop. " - I'm going to take a page from SR-71 Blackbird's book on this one. :) It's engines have an adjustable nosecone that can move in and out of the engine, either allowing the engine a full aperture of intake air, or it can squeeze it down to a small opening. By only having a small opening, the air compresses, and after the nosecone the area is open which slows the air down. Both of these make the air more dense, and make props more efficient. I'm thinking that the ducted fan approach will be better at high altitude, but a variable-pitch prop would work too.

I looked it up, btw... HELIOS was $15 million. Weighted 600kg and had 30kW of solar cells powering 14 1.5kW brushless DC motors. $9 million was spent on the solar cells alone ($300/W). Seeing as the solar cells I just purchased run $100 for 87.5W ($1.14/W), if I continue that trend (1/300th the cost) I could build HELIOS for $50k. Since I only want mine to weight about 10kg, maybe there is a linear weight-cost ratio, in which I can build mine for under $850? ;)

For control, I think at high speeds the regular control surfaces will work fine. If I ever have to start worry about this problem, I'll already be happy. ;) And yes, dives count!!! If I climb to 30k feet and nosedive above 400mph, woohoo!

"GPS receivers are required by law to not function above 60,000 feet." - The actual limitation is 60k feet AND 1000 knots. Some GPS receivers will give you valid data above 60k feet so long as your speed is lower than 1000 knots, but there are many that don't. The SIRF III chipset that is used in the GPS that is recommended for the UAV Devboard works above 60k feet, but it depends on the actual implementation of the board as to whether it will report it... so the only way to know if it works is to try it out. Sad!

"technology has increased the efficiency of power systems and solar cells enough that using thermals might not even be necessary anymore" - VERY true! Solar cells, LiPos, and brushless-dc motors have been making solid strides for the past 5 or 10 years, so it would be very possible that we could just fly in a circle all day and night.

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Permalink Reply by John Gantner on June 8, 2009 at 7:45pm

I think the variable pitch propeller would be the easier of the two methods. They even sell them for RC models already, like this one: http://www.hobby-lobby.com/evp-pro.htm

I used to use a consumer GPS handheld on some of my projects and the user manual said if either of the two conditions were passed it would fail. (Not that I ever got close to either, though. Rocketry guys might have some info on this.) Have you heard of any specific models that have been verified to work under one of the two conditions?

Diving - hey if all else fails, go straight down and design your fuselage so its terminal velocity is >400mph. Once the breakaway wings rip off it should be pretty streamlined, right? Pulling up might be a problem though.

Another good link: I just found out there is a festival of sorts (next one is on July 18 in Kansas) for hobbyists launching high altitude radio balloons. It looks like tons of fun. http://nearspaceventures.com/gpsl2009/

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Permalink Reply by Bill on June 8, 2009 at 10:03pm

Yeah, specific GPS models:

http://showcase.netins.net/web/wallio/GPSrcvrsvs60kft.htm

Gotta love high altitude ballooning websites. ;) Those guys go past 60,000 feet almost every single launch. Unfortunately the list is rather small because of the small size of the high altitude ballooning community. I could never justify the possibility of losing $500+ of electronics if a single element in my design failed or if random chance would drop the payload into the ocean or a lake or on the highway or any of the other million things that can happen to it!

"Once the breakaway wings rip off it should be pretty streamlined, right?" Ahahaha!! Yes, indeed!! :) And with the right aim, I could put a pretty new hole into someone's roof.

I like that high altitude balloon site! Good stuff. Those guys have a lot of really helpful information on long distance radio links and very simple tracking methods.

I'm very curious about those variable pitch props. I know they use them on helicopters too (Which may be a better choice, bigger prop = better!), but I don't know how they control them. Do you put your ESC on a different channel that has a selector switch (0/50%/100%) and then use your throttle channel to control the v-pitch?

If I ever get a chance, I'll certainly put one of those on the prop and pump the solar power directly into the ESC, stick the throttle @ 100%, and then make the v-pitch be entirely altitude dependent for the highest efficiency. It may be necessary to make it altitude + airspeed dependent... no biggie. :)

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Permalink Reply by UAV Pilot on June 8, 2009 at 11:06pm

Bill,

here is a sight it has the BOP it is discontinued but the miniBOP is still available for $65 and might be a design that you can upscale and maybe get the characteristics that you are looking for.

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Permalink Reply by UAV Pilot on June 9, 2009 at 5:48am

sorry here is the link it will have to cut and paste could not get hyperlink correct. www.edgerc.com

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Permalink Reply by Bill on June 9, 2009 at 11:46am

Thanks for the link. :)

I'm going to focus on the slow speed / high altitude stuff now, but I'll keep that in mind once I start targeting faster speeds. I'm hoping that slow and steady and stable is going to mean less horrible crashes and fires. ;)

Permalink Reply by John Gantner on June 8, 2009 at 11:30pm

I think they use a separate channel for the prop pitch. They call it "4D Flying" - it combines helicopter-like hovering and fixed-wing speed. The videos are pretty unreal - they can fly an inverted vertical climb! Take that, hummingbirds. Unfortunately it seems most of these VPPs are built for very small motors.

Basically, it seems that the first experimental VPPs were created by hacking a helicopter onto the front of a plane, so I think that might be the way to go if the little motors can't do what you need. I think it would be pretty terrifying to see a TREX 600 grafted onto a plane, but those motors can really pull.

BTW, are you this person? My net-sleuth senses are tingling... http://uavideas.blogspot.com/

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Permalink Reply by Bill on June 9, 2009 at 11:44am

Nope, that's not me. Sorry. :)

I bet using actual helicopter blades on the plane would be a bad idea though. The inertia from accelerating those blades is going to twist the plane all to hell. You'd almost certainly have to do two counter-rotating props and you'd end up looking like the Osprey ( http://en.wikipedia.org/wiki/V-22_Osprey )

The 4D flying videos are flippin' sweet!! :D

I'm posting a reply up higher in this chain with regards to some new toys I got in the mail today. :)

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ModeratorPermalink Reply by Gary Mortimer on June 9, 2009 at 3:47pm

You should look some more at the Zephyr

http://www.qinetiq.com/home/newsroom/news_releases_homepage/2008/3r...

After all its what you'd be in competition with there are some of the team here and on RCgroups, can't for the life of me remember their names.

I never tire of proclaiming its goodness, I think I once heard it was launched by four blokes and Real Ale ( If your in UK you'd understand that) Love the fact that its trashing flight time records. The US have recently bought a few I believe.

On balance I'm probably more of a Scaneagle fan ;-)

Permalink Reply by Kay on September 27, 2009 at 11:56pm

damn too bad i just moved out of SD, sounds really fun

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