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Center of mass on an airframe

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Center of mass on an airframe

Old 04-10-2002, 01:44 PM
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stevezero
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Default Center of mass on an airframe

Here's something that's been bugging me for a while now, and was hoping that maybe one of you aerodynamic experts could offer some insight.

Lets say you have two identical airframes, one with more weight on the outer edges of the airframe (nose and tail), and one that has the engine pushed back a lil closer to the cg, as well as all of the radio gear closer to the center of mass. Would there be a noticable difference in the stability and handling characteristics between the two, assuming the weight and cg placement were the same? I would think that the airframe with the weight at the endpoints would take a lil more energy to tumble, but the inertia would keep it tumbling more violently, and it would take more force to recover. The airframe with everything at the center of mass would in turn, tend to feel lighter, take less energy to induce a tumble, less inertia buildup/transfer to continue it, and less energy to recover. It should also make the plane a little more stable in straight n level flight, because it wouldnt tend to be as twitchy.


Feel free to think about this and throw in your $.02 worth. I'd love to see a dollars worth of input on this.


Thanks,
Steve
Old 04-10-2002, 01:59 PM
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Default Center of mass on an airframe

not sure about the more violent tumble, a lighter plane will change direction quicker and that would make it more violent, but you got it right, think of an girl ice skating... now put her clothers back on ... when they spin, as the arms come in so the mass becomes central and the spin increases in speed. with planes i had a fun fly with a short fuz and it spins quick, i have a longer version now, its spins are much slower, but it has more stability. the same gos for wingspans and where you put your servos. i suppose this is a good argument for getting the design right and not shoving the batt down the tail.

Taking this one step further, i've always wondered what would happen if the engine was on the C of G, no air flow over the wing tho for prophanging......
Old 04-10-2002, 02:48 PM
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Default Center of mass on an airframe

Steve,

In another post on this forum, I said that the ideal airplane has most of its weight at the Center of Mass, and very light extremities, for all the reasons you have surmised. I mentioned a Goldberg Jr Skylark I had once-- went to a bigger engine and had to add lead to the tail to compensate, and it developed a hopeless flat spin tendency. By going back to the lighter engine and taking out the tail weight, the problem went away.

The Pitts Special changed the complexion of World Class aerobatics forever with its inception back in the '60s. Before this, the popular aerobatic steeds had long wings and tails, such as the Zlins and Yaks and Chipmunks, for smooth rail-like performance. The saucy little Pitts was short-coupled, and introduced the world to exciting tumble maneuvers like vertical snaps and Lomcevaks and avalanches. Taken further, with the advent of the Sukhois and Extras, the real show-stopper maneuvers became what we call today "3-D", extremely high roll rates and zero airspeed gyrations, all relying on low wing loading, high power loading, and the mass concentrated at the center as much as possible.

Simply put, two identical airplanes with the same AUW and CG location do NOT necessarily fly the same, depending on their mass distribution.
Old 04-10-2002, 03:18 PM
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ChuckN
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Default Center of mass on an airframe

I agree. Centralized mass is better. This is referred to as the "polar moment of inertia". Racecar designers pay very close attention to this as well because it makes a big difference on how fast you can go around a corner.
Old 04-10-2002, 04:06 PM
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Default Center of mass on an airframe

To help your intuition on this point, take a yard stick and fasten 2-3 oz weights to either end, then grab the stick in the middle and twirl it like a baton. Now take the same weights and put them in the middle of the stick, grab it as before, twirl as before, notice the difference. Moment arms make a big difference.
Old 04-10-2002, 04:13 PM
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Default Center of mass on an airframe

i'd rather imagine the naked girl skater :spinnyeye:
Old 04-10-2002, 05:52 PM
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stevezero
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Default Center of mass on an airframe

Yeah, thats what i thought. I know lighter flies better, and having a racing background can understand the center of mass concept (thats actually where I got the idea from). Same thing as sprung vs unsprung weight in the suspension/drivetrain of a car/truck. The less weight you have to turn, the better off you are.

Has anyone tried to use a crankshaft extension, or machined a longer crank to mount the motor and move it back to centralize the center of mass? Or would the stress and vibration be too much for it or the airframe to take?

I see alot of guys who seriously overpower a plane for hovering purposes, but how many of them seriously negate the built in performance of the airframe by all the lead/servos in the tail? I may be doing that to myself by putting an Enya 80 4stroke under the cowl of my Majestic pattern plane, but I intend for it to be a testbed for alot of motor/prop/wing/tail combinations. It should weigh in around 5- 5 1/4 lbs, anything over 5.5 and im gonna be upset LOLOL. I studied Christophe's planes and he doesnt have anything in the tail on his TOC plane. I think I will end up needing to add about 100 squares and a slightly fatter airfoil on the wing to give it more of the performance I'm expecting of the plane, but thats down the road a lil. I plan on getting quite a good baseline on it before I go cutting wings and stuff.


Keep the input coming guys, its great to actually find out my brain is actually functioning LOLOL

Steve
Old 04-10-2002, 08:02 PM
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Default Center of mass on an airframe

Has anyone tried to use a crankshaft extension, or machined a longer crank to mount the motor and move it back to centralize the center of mass? Or would the stress and vibration be too much for it or the airframe to take?

I once examined a fairly large P51 (might have been 1/4 scale) powered with a good size gasser, with the engine mounted well back in the fuselage, prop shaft fitted with a pulley wheel which drove a substantial belt (~ 1" wide) which drove a shaft which extended maybe 12" out to drive the prop. In other words, the engine was shifted back maybe 12" from where it would be with the prop fitted directly onto the engine prop shaft. The power transmission unit was a commercially manufactured product, with a nameplate on it (not a home-made device). I made a "mental note" of the mfr name, but can't find the note anymore. It was something like "Acme Power Transmission Products" located in Massachusetts.

The fuse was in the back of a pickup, and I never did find out who the builder/pilot was. I was curious as to how this rig was balanced and damped against vibration. The installation looked pretty solid. I would like to have seen it fly, but didn't happen. My recollection is that it was not a straight ratio, but speed reduced maybe 5/4 as a guess, which would have torqued a bigger prop.
Old 04-11-2002, 12:20 AM
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Default Center of mass on an airframe

Remember the P-39? Talk about long driveshafts!

I have wondered myself about the large engines people are putting on their aerobatic planes for "hovering" and 3D. From what I have witnessed the folks that fly these planes hardly ever exceed an airspeed of 30 mph. I guess in that small of a performance envelope inertia moments are a moot point. Sorry, but that slow stuff just ain't my cup of tea.
Old 04-11-2002, 01:39 AM
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Default Center of mass on an airframe

The old ww1 Sopwith Camel was about as close as you could get to centre of mass to centre of gravity. It could torque turn to the left almost in its own length. Disconcerting to a lot of its adversarys.
Old 04-11-2002, 07:05 AM
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Default Center of mass on an airframe

Im aways surpurprised Americans seam to have 46 + sizes in their fun flys when we stick with 36's and less.
When i see all up weights of 5.5 pounds or more quoted it doesnt make much sense to me when you could use a lighter engine and get the same power to weight ratio. most fun flys here can prop hang on 32's.
Is it just because you want to swing a bigger prop and get more air over the surfaces?
I have noticed the term 'fun fly' is very diffrent for US & UK
Old 04-11-2002, 03:43 PM
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Default Center of mass on an airframe

Philly, I agree with your assessment of the 46's. I have a tt.36 heli motor on my magic, which weighs under 4lbs, and it hovers at less than half throttle. Don't get me wrong, I am a power nut too, but you hit the nail on the head. Power to weight. Less power with less weight is going to fly just as good, if not better, than more power and more weight. I had an os 32 sx in a model tech extra that was ballistic vertically. The plane was just too small. In its original configuration, built mostly to stock, was just a hair over 4lbs, with about 3 1/2 oz on the tail to make the cg a lil more lively. After breaking it in half, I removed all the plastic pieces, replaced them with balsa, moved the battery back about halfway down the fuse, moved the tank and throttle servo back closer to the balance point, and was able to remove all of the lead from it. All up weight was over 4 oz less with, and it showed in flight. What I thought was great vertical before, turned into even more. I am now trying to keep them as light as possible, and build them to fly, not crash. If I can get more power out of the same weight motor, I will try to go with the larger one, but if the weight negates the power increase, I will skip it.

Keep the input coming.
Old 04-11-2002, 04:02 PM
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Default Center of mass on an airframe

Its surprising what a small weight change can make, i had my Cougar 2000 out the other day, a slight breeze and i was just trying to prophanging, i must of used 4 of my 6oz of fuel and i was hovering into wind over the patch, i pulled it up and there i am making my porphang get lower with full control for the first time ever, i actully mannaged to touch the tail 3 times without causing it to land, before a sideways one caused it to flip over and crack the firewall off.
Im not sure if this was because the CofG was further back, or because it was lighter.
Does CofG apply when the plane is hanging? does it become point of rotation for elevator and rudder? and how does moving this effect the plane? forward for more control back for more responce?
Im thinking if the CofG ( center of rotation ) was right behind the engine you'd have alot of fine control over how the plane moves in the airspace once its hanging, something like a helicopter. The further back it is, its more like trying to balance a pool cue on your finger.
Movement of engine relative to angle of fuz.. if you see what i mean.....
Now if i could have it same as it was on empty tank i'd be happy, move the batt back.. umm no, now we going away from center again.
What a game!

If you want light weight and not designed to crash take a look at these
http://www20.tok2.com/home/rc/data3/2002-3-8-2/1.htm
theres a post somewhere that has the cad files for this. funfly for a 25 fp ........i think.......

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