Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Altitude effects?

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Old 06-15-2012, 04:35 PM
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ftjets
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Default Altitude effects?

In 2010 i took my Weston Magnum to the Carolina Speed Rally. The plane was well flown here in southwest Florida and flew great. On the first launch at the rally it rolled to the left (if i remember correctly) and i almost lost it. It took tons of trim to get it flying straight. When i returned to Florida and flew it again i had to reverse the trim i had used in NC. I don't think the field was at any ultra high altitude but something caused the Magnum to roll stupid. I had a Jett 50LX in it. Does a change in altitude cause that much difference in torque or could the control rods have expanded/contracted? I still have not figured out why this thing went totally stupid. Anyway, i have built a new MagnumR with the same Jett 50LX but have installed two identical servos on the elevators and eliminated the dual-control rod setup that is designed in the kit. I also installed Sullivan inner pushrods in the elevator tubes which fit much more snuggly than the ones in the kit. Anyone have ideas about why the plane would roll so dramatically between a field in southwest florida and north carolina?
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Old 06-16-2012, 06:13 AM
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Default RE: Altitude effects?

Are your pushrods steel or plastic?

I would run steel thru the plastic sleeves (outer tubes). never plastic for the actual pushrod

In the past I have seen "goldenrods", and other brands, change in length with temprature requiring trim adjustment, but not with altitude.
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Old 06-16-2012, 08:29 AM
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Default RE: Altitude effects?

I think Crazy has the right idea. Sounds like the change in heat and humidity either caused the airframe or the control system or both to move and make it go out of trim. Not an uncommon thing with wood planes.

The aircraft flies, stalls & does everything else at the same airspeed no matter the density altitude. What changes is how those speeds look to you.
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Old 06-16-2012, 12:36 PM
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Default RE: Altitude effects?

I have tried to figure out why the trims changed so dramatically and have not figured it out. This plane had several flights on it in SW Florida which was basically at sea level. It flew like it was on rails and showed no bad tendancies. When it was launched for the first time in NC it rolled so bad that i almost lost it. Thanks to Seth who had better eyesight than me i was able to recover it and went on to make my speed runs. Subsequent flights went without any issues and the trim kept it flying straight and true. (I used that excuse for the less than heroic speeds i was able to achieve) When i got back to Florida i launched it and it rolled the opposite way. After removing the trims i had set in NC it once again flew straight and true. I'm sure that i am not the only one to fly the Magnum at more than one altitude. My first guess was that it was the result of a change in torque but if it rolled to the left that would indicate more torque as i understand it. I have always assumed that you would have less torque at higher altitudes which would have caused it to roll right. I thought about installing carbon fiber pushrods on the elevators but my opinion was that they will not bend enough to move freely through the outer tubes. The Sullivan inner pushrods at least take up the slack and make the connection less prone to slop. I figured that installing dual servos also eliminated one more issue. I find it hard to believe that the pushrods could change so dramatically that it would make that much difference. Of course, it was the ailerons that i used to adjust the trim and get it flying straight. Those control rods were all steel pushrods on a common servo so i can't imagine that they were affected. As far as heat, it was hot here in Florida before i went to the Rally and it was just as hot up there but there was little difference in temperature in my opinion. I have been flying fast airplanes for a long time and have never had to figure out an issue like this. I am completely stumped on this one. Expert opinions welcome!
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Old 06-18-2012, 01:47 AM
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Default RE: Altitude effects?

Certainly there must be an aeronautical genius out there that can shed some light on this. Perhaps if i rephrase the facts. Crazy, you are right about the pushrods and i did attempt to use CF rods which is essentially the same thing and thought that there was too much drag on the rods and didn't want the servos to bind. I have limited time to work on this so i can't go through the process of removing the covering and removing the outer tubes. Assuming that the elevator pushrods had anything to do with the change in trim would indicate an "Ailerator" effect and i wonder if a small amount of expansion/contraction on the small elevator surfaces would cause the amount of trim needed to make it fly straight. Again, when i returned to Florida i had to remove the trim and it flew normally. Also again, the ailerons were set up as in the instructions with a common servo and 4-40 size steel pushrods so i doubt that the ailerons caused the change. My focus was on engine torque and/or prop effects. I like to think that i have developed a reasonable degree of knowledge concerning model engines over the years but i simply have never tuned an engine at one altitude and flew it at another so i have no experience to compare it with. I have flown models in different temperature/humidity ranges with the same settings and while there may have been a slight difference i have never experienced the dramatic change that i did here. Someone must have at least a reasonable explaination or even an educated guess. My best guess is that the change in altitude caused this but i'm not exactly sure why. This one will fly and i'll be ready to adjust on the first launch just in case but i hate to lose ten mph or so due to unnecessary aileron trim. After all, this is a speed rally!
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Old 06-18-2012, 06:42 AM
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Default RE: Altitude effects?

Thinner air at altitude presents less resistance to rolling therefore it rolls more. Makes sense to me, how about you?
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Old 06-18-2012, 07:14 AM
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Default RE: Altitude effects?


Quote:
ORIGINAL: BiggerDanno

Thinner air at altitude presents less resistance to rolling therefore it rolls more. Makes sense to me, how about you?
Makes sense i guess. I just never imagined it would make that much difference. But then in all these years i've never flown at higher altitudes except the high desert area. I figured it had something to do with either engine torque or prop effects form the change in air pressure/density. I'll buy that theory. Thanks for the info Bigger.
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Old 07-15-2012, 09:42 AM
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Default RE: Altitude effects?

I was taught to look the planes over really carefully...looking at it from the rear along the trailing edge before a new day of flying.
You can easily spot a twist with your naked eye. Beyond that, trims can be accidentally bumped out of whack, so that part of the preflight "trim" inspection has to be with the radio turned on.
Speed planes in general should be hand launched with the right wing panel slightly low.
I've discus launched speed planes this way...but that takes a huge leap of faith to try it on a nice plane. It is a backhanded "fling" with your left arm as you grab the plane by the left wing tip.
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Old 07-15-2012, 09:52 AM
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Quote:
ORIGINAL: Lownverted

I think Crazy has the right idea. Sounds like the change in heat and humidity either caused the airframe or the control system or both to move and make it go out of trim. Not an uncommon thing with wood planes.

The aircraft flies, stalls & does everything else at the same airspeed no matter the density altitude. What changes is how those speeds look to you.
Try explaining that to a 747 pilot taking off at Sky Harbor in Phoenix in July at 116 degrees.
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Old 07-15-2012, 11:04 AM
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[8D]
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Old 07-16-2012, 04:19 PM
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Default RE: Altitude effects?

Actually, things do thange with temperature, thus the term "Density Altitude"! The Hotter the air, the higher the density altitude, because the hotter the air, the thinner it is, which makes less lift. there are some places where experienced pilots will only fly into or out of early in the morning, or later in the after noon, when the air temprature is lower, and the effective density altitude is also lower!

Density altitude is a term that sometimes causes confusion to the uninitiated. A high density altitude is NOT a good thing. Density altitude is defined as the pressure altitude corrected for non-standard temperature variations. And while this is a correct definition, my definition is perhaps more appropriate: DENSITY ALTITUDE IS THE ALTITUDE THE AIRPLANE THINKS IT IS AT, AND PERFORMS IN ACCORDANCE WITH THIS COMPUTED VALUE.

Suppose an airport is situated at an elevation of 3,000 feet. It is possible with a low pressure system and hot temperatures for this airport to have a density altitude of more than 6,000 feet due to the effects of the pressure and temperature. An airplane operating at the 3,000-foot airport on a day with a 6,000-foot density altitude would have the performance normally found when flying at 6,000 feet during a day with standard conditions.




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