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Author Topic: Moment Arm Ratios  (Read 2490 times)

Offline Garf

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Moment Arm Ratios
« on: November 11, 2010, 10:06:26 PM »
What ratio of nose moment to tail moment would you use setting up a flapped plane vs an unflapped plane. I copied the moments from the Nobler for my last 2 creations, both unflapped. It didn't work very well.
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Offline Kim Mortimore

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Re: Moment Arm Ratios
« Reply #1 on: November 11, 2010, 10:47:02 PM »
Quote from: Garf on November 11, 2010, 10:06:26 PM
.....It didn't work very well.

Could you provide some specifics?  The problems may not have been related to moments.
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Kim Mortimore
Santa Clara, CA

Offline Tim Wescott

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Re: Moment Arm Ratios
« Reply #2 on: November 11, 2010, 11:31:58 PM »
Tail volume matters, so the wing area, tail area and tail moment arm are all important.  To a lesser (but still somewhat significant) extent the wing chord also matters.

A plan view of the plane would satisfy most of the sticklers, particularly if you comment on what airfoil you're using.
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Offline Mark Scarborough

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Re: Moment Arm Ratios
« Reply #3 on: November 12, 2010, 08:40:39 AM »
Garf,
the nose moment is basically to balance the model with the engine selected,, the tail moment, wing area, MAC, and tail area will impact the way the airplane flies. These are the factors you want to control.
what are you trying to accomplish? are you trying to build a non flapped plane based upon a flapped design, or are you designing from scratch?
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Offline Garf

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Re: Moment Arm Ratios
« Reply #4 on: November 12, 2010, 03:03:46 PM »
This involves my last 2 creations. The stretched PT-19, and the stretched Galaxy, both flapless. I stretched them to the moments of the Nobler. The PT-19 is stock except for the fuselage. The Galaxy has the stock wing, but a larger, higher aspect ratio stab/elevator. The Galaxy will not turn well but has no bad habits. The PT-19 is a flying brick, but turns better than the Galaxy. The PT-19 was the Hangar 9 unit with the dihedral removed.

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Offline Tim Wescott

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Re: Moment Arm Ratios
« Reply #5 on: November 12, 2010, 03:23:50 PM »
Quote from: Garf on November 12, 2010, 03:03:46 PM
This involves my last 2 creations. The stretched PT-19, and the stretched Galaxy, both flapless. I stretched them to the moments of the Nobler. The PT-19 is stock except for the fuselage. The Galaxy has the stock wing, but a larger, higher aspect ratio stab/elevator. The Galaxy will not turn well but has no bad habits. The PT-19 is a flying brick, but turns better than the Galaxy. The PT-19 was the Hangar 9 unit with the dihedral removed.
I'm still getting my head wrapped around control line design after years on the dark side.  So take this with an appropriately sized grain of salt:

  • If I'm gonna copy something, I'm gonna copy a known flapless stunter, like a Flight Streak, a Sig Skyray (which is, I believe, a copy of a Flight Streak!), a Primary Force, or Allen Brickhaus's Fairey Barracuda from the recent Flying Models
  • Those horizontal surfaces look awfully small, and the moment arm looks longer in relation to the wing chord than a Nobler.
  • In general, a long tail with respect to the wing size means gentler turning
  • Have you played at optimizing the CG?  Were I in your shoes I'd see what happens when I moved the CG back.  You should get snappier turning, at the expense of stability
  • Something that Allen Brickhaus mentioned in his Fairey article was that for good performance a flapless stunter needs to be really light.  Have you achieved this?
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Offline Kim Mortimore

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Re: Moment Arm Ratios
« Reply #6 on: November 12, 2010, 06:08:12 PM »

Hi Phil,
I just read the PhD thesis in aerodynamics they sent you on Stuka.  Whew!!!  n~  I had to lie down till my head stopped spinning.  "A bit more complex that I bargained for" for sure!  With all due respect to those guys (and I respect them tremendously--they are some of my heroes), I don't think the approach they're taking is very helpful at this point.  You need practical help with these planes. 

I hope you're not planning on giving up on these planes yet.  The pictures definitely help.  I think what you're trying to do is basically sound.  I'm actually doing something similar with Primary Force wings.  The PT-19 fuselage looks like the proportions are in the ballpark just by eyeballing it.  The Galaxy's fuselage looks too long.  So let's concentrate on the PT-19 to start with.

Please excuse me if it sounds like I'm talking down to you.  That is definitely not my intention.  Since I don't know you and it looks like we need to get back to basics, I'm going to keep it basic.  On the PT-19 wing, find the rib nearest to halfway between the fuselage and the wingtip.  Measure the chord of the wing at that point.  Then find where the plane balances (center of gravity--CG) with your fingertips underneath those ribs.  Measure the distance from that point to the leading edge.  Then let us know what you get for the chord and the CG distances.

Next, have you weighed the PT-19 with engine, prop and tank?  Weight is very important on flapless planes, and it would help to know what you're working with.  By the way, what engine are you using?

That's enough for starters.

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Kim Mortimore
Santa Clara, CA

Offline Garf

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Re: Moment Arm Ratios
« Reply #7 on: November 12, 2010, 09:55:15 PM »
The Galaxy is light. I brought the CG back and it started hunting. The PT-19 weighs a ton. The fuselage alone weighs 13.1 oz. It was an experiment that didn't work out. If it survives long enough, I will replace it. The engine on the Galaxy is a Johnson Stunt Supreme. The engine on the PT-19 is a Super Tigre G21 40 with a fresh Bowman ring and new bearings.
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Offline Mark Scarborough

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Re: Moment Arm Ratios
« Reply #8 on: November 13, 2010, 09:14:57 AM »
Ok, you broght hte CG back, and it started hunting, where did you bring the cg back to? because the hunting isnt neccessarily a result of the cg, it could be an alignment or incidence problem. It could also be a leadout position problem as well and looking at the photos I would guess that is at least part of it. set the CG where it should be, set the leadouts an inch behind the cg , double check that the controls are free and smooth even with line tension applied, and then fly it. Oh and Make sure the stab is on zero with the wing chordline or at worst that the leading edge of the stab is nose up slightly relative to the wing. and that the engine thrustline is at 0 with the wing, or if anything a touch downthrust,,,
then fly,,
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Offline Kim Mortimore

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Re: Moment Arm Ratios
« Reply #9 on: November 13, 2010, 01:14:28 PM »
Quote from: Garf on November 12, 2010, 09:55:15 PM
....The PT-19 weighs a ton. The fuselage alone weighs 13.1 oz. It was an experiment that didn't work out...

What was the goal of the experiment?
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Kim Mortimore
Santa Clara, CA

Offline Serge_Krauss

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Re: Moment Arm Ratios
« Reply #10 on: November 13, 2010, 02:26:12 PM »
Quote from: Kim Mortimore on November 12, 2010, 06:08:12 PM
I just read the PhD thesis in aerodynamics they sent you on Stuka.  Whew!!!  n~  I had to lie down till my head stopped spinning.  "A bit more complex that I bargained for" for sure!  With all due respect to those guys (and I respect them tremendously--they are some of my heroes), I don't think the approach they're taking is very helpful at this point.  You need practical help with these planes.

Kim, Kim, Kim! Ouch!

Phil just asked "What ratio of nose moment to tail moment would you use setting up a flapped plane vs an unflapped plane". The simple answer is that I wouldn't, but I don't think anyone would have appreciated a glib, "There aren't any such generalized ratios" or "That won't work." If Phil had asked then about his planes themselves, I might have (edit: tried to)answered that question.

Aircraft dynamics are complex (I didn't make them that way - really!), but they are why generalized moment ratios are invalid. Fortunately we have some good shortcuts and rules of thumb to circumvent some of the complexities, but nose:tail moment ratios are unfortunately not among them. The more we actually make the effort to understand some of these inconveniences, the better we use the short cuts. I think that understanding what I wrote is worth the effort and not as difficult as we sometimes want it to be. But then I'm probably dumb enough to think that. Ha!

SK  
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Offline Mark Scarborough

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Re: Moment Arm Ratios
« Reply #11 on: November 13, 2010, 04:08:56 PM »
Just my opinion, but, the tail moment, ( measured from the CG, not the flap hinge line,,) controls stability and turn rate,,
the nose moment BASICALLY controls weight and balance, and beyond that any effect is pretty much lost in the noise on something like this.
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Offline Tim Wescott

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Re: Moment Arm Ratios
« Reply #12 on: November 13, 2010, 05:54:51 PM »
Quote from: Mark Scarborough on November 13, 2010, 04:08:56 PM
Just my opinion, but, the tail moment, ( measured from the CG, not the flap hinge line,,) controls stability and turn rate,,
the nose moment BASICALLY controls weight and balance, and beyond that any effect is pretty much lost in the noise on something like this.
Well....

I think it's more that tail volume controls stability -- for level flight, a small tail on a long stick will do the same as a big tail up close to the wing.  But that small tail on a long stick (think RC pattern, or gliders) makes it hard to turn fast, while the short tail moment arm lets the plane turn sharper -- and that's the limit of my knowledge so far!!

"Circular Airflow" by Carl Goldberg Frank Ziac* is pretty much about stability and how small-radius turns affect an aircraft's effective geometry; he's mostly going at it from the free-flight point of view, but he does go into what happens to a control line plane during stunts.

* Thanks, Ty -- you can go ahead and correct me in public: I'd rather have the real virtue of getting things right than the illusory one of always looking right.
« Last Edit: November 13, 2010, 11:41:52 PM by Tim Wescott »
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Offline proparc

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Re: Moment Arm Ratios
« Reply #13 on: November 16, 2010, 01:18:22 PM »
Quote from: Mark Scarborough on November 13, 2010, 04:08:56 PM
Just my opinion, but, the tail moment, ( measured from the CG, not the flap hinge line,,) controls stability and turn rate,,
the nose moment BASICALLY controls weight and balance, and beyond that any effect is pretty much lost in the noise on something like this.

This is it right here-period. There is NO ratio. One controls one parameter and the other, controls another.
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Offline phil c

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Re: Moment Arm Ratios
« Reply #14 on: November 17, 2010, 08:36:13 PM »
Quote from: Mark Scarborough on November 13, 2010, 04:08:56 PM
Just my opinion, but, the tail moment, ( measured from the CG, not the flap hinge line,,) controls stability and turn rate,,
the nose moment BASICALLY controls weight and balance, and beyond that any effect is pretty much lost in the noise on something like this.

I thought the same, until I built a semi-scale P-51 and a same size FW-190.  The long nose on the P-51 has a definite steadying effect on the flight.  The short nose on the FW made it less stable in level flight.  The long nose on the P-51(a good 2 in. longer) didn't affect how well it turned at all.  Otherwise, the planes had similar tail volumes, with the stabs pretty much exact scale with the wings.
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phil Cartier

Offline Serge_Krauss

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Re: Moment Arm Ratios
« Reply #15 on: November 17, 2010, 11:56:47 PM »
Phil-

I would expect the greater polar moment of inertia to inhibit pitch changes for the Mustang, but that really ought to work for both intentional and unintentional pitch changes. Are you sure you didn't give that Mustang a bit more handle input? I'd think that precession from flying in a circle would be compromised too, and that's a pitch-up tendency. Narrow as the nose is, can its aerodynamic destabilizing effect be enough to counter the inertial forces in a loop...? I suppose that (from nose angle) would depend on flap useage anyway. Hmmmm...

Edit: I'd written another paragraph, but I'm thinking about that now - so it's gone. 'sorry.

Anyway, nose length is pretty well determined by component weights and the necessity of balancing the model at the proper c.g., unless one of these other effects can be isolated and the plane ballasted to accomodate it. I'm skeptical though, since moment of inertia is more affected by distance of masses from the c.g. that by their magnitudes (Kmr2).

SK
« Last Edit: November 18, 2010, 12:25:19 AM by Serge_Krauss »
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