SMALL - Telemetry SMALL - Radio
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Old May 27, 2015, 08:19 AM
BluIIs is offline
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I certainly hope the reduced stress helps restore you to full health, Don. It is good to have you back.

I think purhaps your thoughts about airflow over the ESC may have played a part in my loss of control accident. Motor and ESC wires did not allow placing the ESC back under the wing, so it was attached to the side wall of the bay under the hatch. The battery just fit across the floor of the bay and nestled next to the ESC. The arrangement probably did not permit enough cooling air flow across the heat sink.
I'll have to consider another placement this build. Or maybe add 1/8" to the fuselage width up front?
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Old May 27, 2015, 10:20 AM
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Depends on how things are arranged.

Putting something alongside the fins can actually help. Given the choice, the air would rather pass over the fins instead of flowing between them, where it can do the most good. On air-cooled engines they have extensive baffling systems to force the air to go through the fins instead of around them. The same principle applies here.

However, if there is something in front of or behind the fins that obstructs the air from getting into or out of the fin array, then you have a serious problem. Also, if the fins are so shallow thatblocking the passage down to only the dimensions of the fins does not allow enough airflow (which indicates that the fins were designed improperly), you can have a problem.

So, it's a matter of having just enough airflow, not too much or too little, but also using it effectively. Fail to cover both of those requirements and you have a problem.

As far as that old rule of thumb of having twice as much outlet area in comparison to the inlet area to allow for the expansion of the air due to heating, that's simply wrong. We just don't have anywhere near that much temperature rise to create that much expansion. The inlet and outlet temperatures have to be compared on the basis of one of the "absolute zero" based temperature scales, such as Kelvin or Rankine, to determine the expansion ratio.

For example, let's say we're looking for a 40 deg.C temperature rise in the ESC, which would give us about half that much rise in typical cooling air (there has to be a difference in the air temp vs the ESC temp to get the heat to flow from one to the other). So, that means about a 20 deg C rise in the air temp going through the system.

Now, if we have an outside air temp ("OAT") of 70 F, that's about 21 deg.C. That means the cooling air at the outlet should be at about 41 deg.C, which is where we get the 2:1 expansion ratio, right?

WRONG.

We have to convert those temps to Kelvin. The ratio that matters is relative to absolute zero.

20 C = 293 K and 41 C = 314 K The resulting expansion ratio is only 1.07. Since we ideally want to convert that expansion into thrust (or at least not make cooling drag), we can probably get away with a little bit more than a 1:1 outlet/inlet ratio, which would give us a small increase in airspeed at the outlet, resulting in a little bit of thrust, IF we design the passages to accelerate the flow smoothly and efficiently.

If done wrong, the cooling drag can be quite high. The average General Aviation airplane, whose cooling system is designed to be easy to manufacture (no elaborate ducting if they can avoid it) and provide enough cooling to keep the engine from melting down while sitting on the ground with very little airflow, gets typically about 10% to 30% of its total drag from the cooling system.

OTOH, the P-51 Mustang had a very well designed radiator with a thermostatically controlled outlet door that optimized the flow for each operating condition, and using the expansion of the cooling air as effectively as possible. Depending on which reference you go by, it got somewhere between 65 and 300 pounds of thrust from the radiator!
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Old Jun 01, 2015, 09:55 AM
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Welcome back Don.

Yesterday I came upon a gaggle of about 100 seagulls, a half a dozen Mississippi Kites, and a lone Swallowtail Kite, all in one huge gaggle, and happened to have the camera. This shot isn't great, but it sure looks like nature's version of your Chrysalis!

Another neat plane with negative cooling drag was the Republic Rainbow (XF-12). They did it with air cooled R-4360s, but the four engine nacelles were each as long as a P-47 fuselage.
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Old Jun 01, 2015, 11:58 AM
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Quite a few of the air-cooled radials managed to achieve zero cooling drag, or even cooling thrust, but the first planes to achieve thrust that way were all liquid cooled. Apparently the job of precisely controlling airflow through a radiator is easier to manage than through an air-cooled engine.

Oddly enough, one of the planes that never got the variable-flow, high efficiency radiator treatment was the Spitfire. It would be interesting to see how much improvement it could see if someone had taken that extra step. It was already superior in a number of ways over its contemporaries. For example, the wing design actually had a surprisingly high critical Mach number, so it had significantly less trouble with compressibility than its rivals.

Most of the post-war piston airliners were able to achieve cooling thrust.
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Old Jun 01, 2015, 01:18 PM
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this is just too much fun! now with my low stress job, I can think thoughts to these questions- and get paid!
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Old Jun 18, 2015, 06:42 PM
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Chrysalis 2m ordered :D

Hey guys,
I just wanted to say how much I am looking forward this build and flying of this plane. I appreciate the wealth of information here and in other posts and it is reassuring to know if I get stuck or have a question I can get help. Thanks and I can't wait to get posting some pics and stories. Also Don it is good to see your health is improving and your back to work on the fun stuff
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Old Jun 23, 2015, 09:20 PM
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Chrysalis maiden

Finished my Chrysalis 2E build a few days ago. I bought the kit along with a 1.5M HLG version back in 2009. The electrified 1.5M was built soon after purchase and has done plenty of flying but the 2E remained on the shelf till now.

Build is pretty standard, I feel kit has been very well designed by knowledgeable people dedicated to the hobby, unlike many of the big name kits out there. So I see no great need to wander from the build path Don has set out, just a few tweeks to satisfy my need to personalise it! Its a great glider to build and I am amazed by the small number of laser cut parts compared wth stock balsa sheeting etc yet still producing a very straight, clean build. To me thats one of the clever parts.

Using a Turnigy Park480 850kv, 40amp Turnigy esc, 1800mah 3S battery and 11x7 Aeronaut prop. CG came out just ahead of the rearmost point of the CG range without any lead, battery was as far forward as it would go. AUW around 985gm.

It was a bit windier than I would normally fly but there I was, at the park with a new model so... Flies nice and straight, little in the way of trim tho in the conditions trimming was not easy. Penetration seems reasonable. CG point seemed fine (wind permitting again) and the "dive test" revealed it could go further back if necessary. Climb out good on full power, fine tho not what I have got used to with some of my other other models. Maybe a 13x6 prop in future as the motor with 11x7 prop only draw 18amps and the motor is capable of 28amps. But its fine as it is really. Need to mix in a little down elevator with the throttle, I have yet to build a glider that didn't need this.

Bottom line - all good and waiting on a better day (winter down this way at the moment) to see how it really flies.
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Old Jun 24, 2015, 11:15 AM
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Great color combo Chriss!
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