Hot weather gives shorter flight times?

Hi, my question is, would the temperature of 95=BA F cause my 3S, 1500 MaH battery to give two third the normal flight time of about 18 minutes for a fully charged battery?

My flights were normal in that sometimes fast, sometimes slow and easy.

Wan

Reply to
wanjung
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My flights were normal in that sometimes fast, sometimes slow and easy.

Wan

I am sure that ambient temperature changes have an effect, Wan, but I'm rusty on the exact effects.

As temperature rises, cell impedance drops. It would not surprise me if capacity diminished along with the falling impedance.

Ed Cregger

Reply to
Ed Cregger

Well, if we're talking about temperature changes, we'd probably want to know what the `normal' temperature is.

| I am sure that ambient temperature changes have an effect, Wan, but I'm | rusty on the exact effects.

Well, as it gets warmer, the air gets thinner, which requires that the plane fly faster to stay aloft. This requires more power. It also makes your prop spin faster at a given throttle setting, but it creates less thrust. Ultimately, you give it more power to compensate, probably without even noticing in a model.

But normally it's a small effect, unless the temperature differences are large. Full scale pilots are well aware of it, but R/C guys generally don't have to worry about it as our planes are generally overpowered. Between 72 and 95 degrees F, I'd expect a difference in air density of about 4 %.

(An extreme example: I'd expect air at -40 F to be about 30% more dense than air at 95 F. That would be very noticible. If your plane (and you) could handle it, flying in Antartica in the winter could be lots of fun!)

| As temperature rises, cell impedance drops. It would not surprise me if | capacity diminished along with the falling impedance.

I doubt capacity diminishes signifigantly, at least in the short term, going from 70 to 100 F. Though with reduced internal resistance, the plane would have more power, and perhaps he just flew more aggressively because he could.

Or maybe his battery is getting weak with age and the temperature change is a red herring.

Reply to
Doug McLaren

not to mention that as the air gets hotter and thinner, the prop has to turn faster to get equivalent thrust, and the plane has to move a bit faster to get equivalent lift. in other words, it will take more throttle to fly

bob

Reply to
Bob Cowell

On Tue, 28 Jun 2005 15:24:06 GMT, snipped-for-privacy@frenzy.com (Doug McLaren) wrote in :

The guys who fly around Denver know a lot about density altitude, I think. I believe they regularly oversize their engines to cope with flying at or near 5000 feet.

There was a story told on the internet of a soldier building a 1/2 A model at the south Pole. As I recall, he said that he started the engine indoors and let the model fly outdoors. It didn't get far before it shredded itself in mid-air. Something in the structure didn't like the temperature difference.

Here in Buffalo, I've flown at a high in the mid-80s and a low around 10 degrees (F). I've done no controlled experiments, but I think I can feel some difference between the colder days and the hotter days. The engines certainly need a little adjustment for the difference in density (richer on cold days--more air comes in with each cycle).

And ice is a lot less forgiving than even the driest earth. Don't ask me how I know. :o(

Marty

Reply to
Martin X. Moleski, SJ

I haven't noticed the effect on current generation Lipos, but older Li-ions would get noticibly peppier several minutes into the flight, as they warmed up. Bill(oc)

Reply to
Bill Sheppard

On Tue, 28 Jun 2005 13:17:43 -0400, "Martin X. Moleski, SJ" wrote in :

GACK!

I got some details wrong:

  1. I was the person who posted the story to the internet.

  1. It wasn't the South Pole.

I read the story in Hal DeBolt's column, "Golden Age of R/C," M.A.N., January 1994. I posted it to the web last year. It doesn't answer your question because the material in question wasn't Monokote.

Here it is again:

One winter, Wesley Pipe of Beaverlodge, Alberta, Canada, went to work in northern Canada, near Ross River. (I should say that the coldest temperature in Canada was recorded there.) Impressed by Maynard Hill's records, Wesley thought that circumstances might allow him to establish one that not even Maynard would want. Perhaps he could fly in a colder temperature than anyone else.

So, with his work gear, Wesley packed a Goldberg 1/2A Skylane, a Cox .049, a pint of fuel and and O.S. "Pixie" single-channel radio. At the barracks, while he waited for the cold weather, he built the Skylane. When it was time to cover it, alarm set in; he had forgottten to bring dope! If he waited for some to be sent to him, he'd miss the coldest weather. Improvisation to the rescue: he sprayed the silkspan with auto paint!

After the Skylande had been assembled and successfully test-glided, Wesley sat back and waited for the temperature to drop. The current -40 degrees Fahrenheit wasn't low enough; the guys in Alaska could easily match that! Finally, the temperature dropped to -70 degrees Fahrenheit. That would be frigid enough; it was time!

The temperature of the barracks was a pleasant 70 degrees Fahrenheit, and the Cox easily screeched its siren song. With a positive radio check, the Skylane was launched through the barracks door. After an initial dip, it climbed merrily. When it reached about 75 feet, there was an explosion like that from a shotgun, and the model fell like a dead duck. Wesley was surprised, yes, but the goal had been accomplished; an R/C plane had been flown at -70 degrees Fahrenheit!

Examination showed that the silkspanned ribs had been shredded, and that caused the loss of lift. Why? The only possible cause was the sudden 140-degree drop in temperature, and this leads to a good question for you thinkers: did the covering explode or implode with the temperature change? Wesley never did figure it out.

+++++++++++++++++++++++++++++++++++++++++++++

Marty

Reply to
Martin X. Moleski, SJ

Ermm, I thought he was talking about his flight batteries...

Ed Cregger

Reply to
Ed Cregger

| Ermm, I thought he was talking about his flight batteries...

He was. To be more specific, he was asking if the high (?) temperature would cause his battery to give 2/3rds of the normal flight time. Knowing how high `normal' temperatures are would help, but I assumed that they were 70 degrees F for the purposes of some calculations, and then repeated the calculations at -40 F for an extreme example, which Martin then took off on a tangent (but an interesting one.)

(Perhaps you were meaning to respond to Martin rather than me?)

I attempted to offer some explanations for his shortened flight time, or, more to the point, to explain why I didn't think the temperature would account for his shortened flight time. (Yes, warmer temperatures would account for some decrease in flight time, but not a

33% decrease, unless the temperature difference was huge.)

And even if I wasn't trying to answer his question (which I was), there's no law that says we have to stay on topic :)

| > Well, as it gets warmer, the air gets thinner, which requires that the | > plane fly faster to stay aloft. This requires more power. It also | > makes your prop spin faster at a given throttle setting, but it | > creates less thrust. Ultimately, you give it more power to | > compensate, probably without even noticing in a model.

More power = shorter flight times, if that wasn't obvious.

| > Or maybe his battery is getting weak with age and the temperature | > change is a red herring.

Personally, I think this is the most likely cause. Either that, or he's not flying the same way after all. Or maybe he just didn't fully charge the battery. Or mabybe his motor is wearing out.

With only a few details to work with, it's largely a guessing game anyways.

Reply to
Doug McLaren

No.

But you should peak the battery up every time you fly it.

Something else may be causing this, but not loss of battery capacity.

Reply to
The Natural Philosopher

Full-scale airplanes suffer takeoff and climb performance when the temperature climbs, but cruise is normal. As the air density drops, the engine gets less oxygen and maximum horsepower (as used in takeoff and climb) suffers, but in cruise where the throttle is reduced significantly there is little difference. We notice no RPM changes with temperature; as density decreases so does drag, and any decrease in thrust is offset by the decrease in drag.

Dan

Reply to
Dan_Thomas_nospam

Doug,

The battery (about 2 years old) was fully charged the day before. The usual flight times had been about 18 minutes, broken into 3 six minutes segments. I normally fly in temperature range of 5 to 8 F. I had made a couple of flights to test the terrain and trees. On the 3rd flight it had lost oomph and I had to do an almost dead stick far from where I intended.

That day was all electric at a private property. It was hot enough (9=BA F+) that another flyer with an 18 cell, 25V Li Poly powered bipe trailed black smoke after the prop made a racket. He was able to land it, upon inspection, his ESC had burned up with only a shell.

I still wonder if the temperature had something with these two incidents.

Wan

Reply to
wanjung

Sorry about the temperatures in my post. "I normally fly in temperature range of 55=BA to 80=BA F"

"It was hot enough (95=BA F+)" that another flyer.....

I don't know how it got messed up. I do wish they have spell check and highlight capabilities here.

Wan

Reply to
wanjung

On Tue, 28 Jun 2005 19:17:17 GMT, snipped-for-privacy@frenzy.com (Doug McLaren) wrote in :

Right. Thread drift happens. :-O

Marty

Reply to
Martin X. Moleski, SJ

As it turns out, I was the one that was confused. For some reason, I thought he was talking about the battery for his radio only and missed the fact that he was powering an electric motor. Oh well.

Ed Cregger

Reply to
Ed Cregger

No that wasn't it. the Covering "shrank" and crushed the airframe. The suddenness was due to the "egg factor". You know, drop an egg on the floor, it breaks easily,...but apply even pressure across the entire surface and the egg will take a massive amount of pressure before collapsing, and when it does it will be violent due to the amount of pressure it took to do so.

The Airframe breaks easy on impact, but under even pressure is very strong (like the egg), until the pressure of the shrinking covering reached the limit,...then "boom".

Rethinking the question: A weather balloon expands as it ascends into the rarified cold air. This brings other questions. Does a balloon expand due to the rarified air or the colder air? Or is it due to both?

Neither,... it expands because air has less pressure as you get higher (why your ears pop). Less pressure outside allows the air inside the balloon to push out and expand the balloon futher than it normally would. The outside air is less "efficient" which allows the inside air to relatively become more effiecient.

Reply to
Phillip Windell

No it wasn't the air. the model is not "airtight". The covering simply "shrank" and crushed the plane.

Reply to
Phillip Windell

No. The air is not "trapped" in the model,..it is not "airtight".

The covering simply shrank as it cooled just like everything else does. But the wood due to the porous and organic nature of wood did not shrink. The shrinking covering simply crushed the plane. It is just the extreme version of the way using the heat-gun on covering when first applying it can twist and distort a model if you get too carried away with the heat-gun. The model covering has the unique characteristic of shinking under both extreme heat (heat-gun) or extreme cold.

In addition to the pressure caused by shrinking, the cold also makes everything brittle.

Reply to
Phillip Windell

Ain't it the truthhhhh. Ain't it the truthhhhh!

Ed Cregger

Reply to
Ed Cregger

On Thu, 30 Jun 2005 09:07:42 -0500, "Phillip Windell" wrote in :

Technically, you are of course correct.

Even balloons are not airtight.

But you can put enough air pressure in a balloon to pop it, even though air is leaking out through the skin continuously.

That seems to be a reasonable hypothesis.

To test it, all we have to do is use the same materials (silkspan and auto paint) and expose the material to the same conditions (something like a 140 degree F swing in temperature).

I've seen that happen.

The record from the original experiment says that the wing ribs were crushed. It may be that they became brittle in the cold.

Marty

Reply to
Martin X. Moleski, SJ

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