News:




  • July 17, 2018, 08:46:59 PM

Login with username, password and session length

Author Topic: nose and tail moments  (Read 7896 times)

Offline Bob Wiegand

  • Trade Count: (0)
  • Ensign
  • **
  • Posts: 37
nose and tail moments
« on: January 25, 2017, 02:05:17 PM »
This may have been asked and answered already, but quick search did not answer my question, so here goes.

What are the correct (or average) nose and tail moments for a .35 size stunt plane?  I don't want to start a feud here, just get an answer to the question.

Thanks for your expertise and your patience for a returning old guy.

Bob


Offline Bob Wiegand

  • Trade Count: (0)
  • Ensign
  • **
  • Posts: 37
Re: nose and tail moments
« Reply #1 on: January 25, 2017, 02:18:58 PM »
OBTW, I looked at Netzband's numbers and could gain some insight into why planes fly like they do, but I am just looking for a good set of moment arms for stunt.   y1

Offline Howard Rush

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6184
Re: nose and tail moments
« Reply #2 on: January 25, 2017, 03:16:40 PM »
You may get some answers that may work.  You will also get "it depends", which anybody can say, but won't give you a useful answer.  If you are willing to go to some effort, I can probably show you how to size the tail and tail length to match approximately a known, good stunt plane.  If you are using an IC engine, nose length is whatever you need to balance the airplane.  Do you have a wing in mind, or are you starting from scratch?
The Jive Combat Team
Making combat and stunt great again

Online Brent Williams

  • 2018 Supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 573
  • Making America Fly Stunt Again!
    • Fancher Handles - Presented by Brent Williams
Re: nose and tail moments
« Reply #3 on: January 25, 2017, 03:39:53 PM »
Do you have any particular engine in mind?  ".35 sized" is not really that clear in this day and age.
Do you have have a size of airplane in mind?


Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
https://stunthanger.com/smf/brent-williams'-fancher-handles-and-cl-parts/ted-fancher's-precision-pro-handle-kit-by-brent-williams-information/

Offline jim gilmore

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1104
Re: nose and tail moments
« Reply #4 on: January 25, 2017, 03:43:40 PM »
Something I have wondered about also. I realize that mostly the lengths have a lot to do about getting the airplane to balance with the equipment built into the airplane. The one thing I wonder about is that the actual location of the propeller is determined by engine position and the length of the crankshaft <or what ever the part the propeller is mounted to> is called. If you could mount the propeller further forward or aft and have the same c/g has anybody tried it and noticed any differences?

Offline Howard Rush

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6184
Re: nose and tail moments
« Reply #5 on: January 25, 2017, 05:45:06 PM »
Igor, in one of his posts, indicated that he puts the propeller farther forward than he would just to balance the airplane with minimum nose length (or minimum moment of inertia).  My guess is that he does it to minimize angle of attack of the propeller in maneuvers.  You can do this with electric stunters by mounting the battery aways aft of the motor.
The Jive Combat Team
Making combat and stunt great again

Online Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 10182
Re: nose and tail moments
« Reply #6 on: January 25, 2017, 06:09:11 PM »
What are the correct (or average) nose and tail moments for a .35 size stunt plane?

Nose length is more a function of balance than anything else (unless you're building electric).

Tail moment arm is subject to a number of variables, not least of which is the fact that modelers measure it differently than full-size plane designers do (and call it "moment" instead of "moment arm").  Probably a better starting point for the tail moment arm is to get the tail volume coefficient right (roughly, that's the moment arm from the MAC of the wing to the MAC of the tail times the tail area).

A Fancherized Twister, which is a pretty well-renouned example of the ".35 size" breed, has a tail that's 21 x 5 5/8 inches, with the 1/4-chord point 20 1/2 inches back from the 1/4-chord point (measured WITH FLAPS) of the wing.

The typical design process for just about anything is:

  • Start with something you like
  • Think about what's wrong with it
  • Make changes to fix what's wrong with it
  • Try out your changes
  • Go back to step 2 and repeat until your boss is happy or you're fired

So if you want to design a stunter and you're not a total whizTM, then find a design that you know flies well, and shamelessly copy it.  If you're feeling bold, tweak stuff to try to make it better.

I don't want to start a feud here, just get an answer to the question.

Heh heh ha ha hoo hoo waaaaahahahahahahahahahaha!
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline EddyR

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2194
Re: nose and tail moments
« Reply #7 on: January 25, 2017, 07:26:36 PM »
A typical stunt model from the 1960's Fox .35    With a wing 9.- 10" wide at center without flaps the leading edge to the back of the spinner was 9.5" the trailing edge to the stab hinge line 13.5-14". The numbers I gave you are from the average of over 25 I beam models from 1955-1965. ~^ ~^
Ed
« Last Edit: January 26, 2017, 06:57:10 AM by EddyR »
South of Cocord NC -25 miles from the Huntersville field

Offline Mark Scarborough

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5923
Re: nose and tail moments
« Reply #8 on: January 25, 2017, 08:58:40 PM »
A typical stunt model from the 1960's Fox .35    With a wing 9.- 10" wide at center without flaps the leading edge to the back of the spinner was 9.5" the trailing edge to the stab hinge line 13.5". The numbers I gave you are from the average of over 25 I beam models from 1955-1965. ~^ ~^
Ed
THat is great if you want a plane that flies like a 55 to 65 airframe, probably have better luck working from more modern dimensions
For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
AMA 842137

Offline Matt Spencer

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2985
Re: nose and tail moments
« Reply #9 on: January 26, 2017, 04:50:42 PM »
Inches mayve grown , but those metric ones are much shorter .

Offline phil c

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1839
Re: nose and tail moments
« Reply #10 on: January 27, 2017, 10:44:58 AM »
If you want something close to a plane that you already know flys well for you, just copy the basic wing area , tail area, and measurements from the good flying plane.  The actual shapes don't "greatly" affect how it will perform.  When measuring for the engine, measure to the needle valve(if it's in the usual place for a front intake engine)  The center of gravity of the engine is usually very close to the NVA.  Other measurements can vary widely between them- back of the prop, back of the case, mounting holes, etc.  If you're working with a classic design(anything pre mid  '70's) check the size of the stab/elevator.  Many were rather small in the range of 16-17% of the wing.  Increasing that to 20-25 percent makes the plane much steadier and easier to steer.  If you want to be more of an engineer calculate the tail volume coefficient-
                         (tail area X tail moment)/ (wing area x avg. chord)  Tail moment it what Tim W. referred to above- the distance between the aerodynamic center of the wing and the MAC of the tail.  Most  modern stunters are in the range of .4-.5 (no units, it's just a ratio).  Essentially you're balancing a lever- the effect of the wing balanced against the effect of the tail  about the neutral point which is the CG location where the plane doesn't care which way it's pointing, it won't recover by itself at all.

Measurements of LE to prop, flap hinge to elevator hinge, leading edge to flap hinge give a very rough idea of what is actually going on.  That's why so many planes used to be disguised Noblers.  Now they are disguised Impacts or Thundergazers.  Because much like WW-II fighters they all use similar size/weight engines, have a payload to carry, and end up much alike.

There are at least a dozen other compromises one has to make putting a design together that suits you.  Trying to get them all sorted out one at a time takes near forever.  Using well-known principles gets there a lot quicker.  Not to mention "styling" and laying out a nice paint scheme.
« Last Edit: January 27, 2017, 11:04:42 AM by phil c »
phil Cartier

Offline Avaiojet

  • 2018 Supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6480
Re: nose and tail moments
« Reply #11 on: January 27, 2017, 11:33:06 AM »
This may have been asked and answered already, but quick search did not answer my question, so here goes.

What are the correct (or average) nose and tail moments for a .35 size stunt plane?  I don't want to start a feud here, just get an answer to the question.

Thanks for your expertise and your patience for a returning old guy.

Bob

Bob,

My ARGO 2 design, using a T-Bird wing as a base for the design with many mods and changes to the fuselage and tail area, here's what I decided on.

9.25" LE to prop thrust line and 14.25" flap hinge to elevator hinge.

I'm using an OS LA 46.

Charles


Trump Derangement Syndrome. TDS

Avaiojet Derangement Syndrome. ADS

Please visit my updated Website! www.cfcgraphics.com

If you're Trolled, you know you're doing something right.

Alpha Mike Foxtrot.

Owner of CFC Graphics. "Model Airplane Graphics from a Model Airplane Builder."

"No one has ever made a difference by being like everyone else."

Marcus Cordeiro, The "Mark of Excellence," you will not be forgotten.

I look at the Forum as a place to contribute and make friends, some view it as a Realm where they could be King.

"Ya gotta love it when a plane comes together."

Proverb 11.9  "With his mouth the Godless destroys his neighbor..."

"Perhaps the greatest challenge in modeling is to build a competitive control line stunter that looks like a real airplane." David McCellan, 1980.

Offline Serge_Krauss

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1134
Re: nose and tail moments
« Reply #12 on: January 27, 2017, 06:57:49 PM »
Here's the war to be avoided, so I'll try to give both "sides" their due.

The only reason dimensions like spinner back plate to wing leading edge and hinge line to hinge line seem to work for "nose moment" (arm) and "tail moment" (arm) is that you're dealing with planes that are quite similar, meaning that aspect ratios are very close, shapes are nearly the same, and the ratio of horizontal tail area to wing area are similar. The further you go from these "normal" configurations and lay-outs, the worse these ratios will work. As said above, these are not the tail and nose moment arms. If you want to work with dimensions actually proportional to the effects you want to achieve, you need to use the following (per Tim and Phil):

1) Tail moment arm = the distance from the aerodynamic center of the tail to either the plane's center of mass ('c.g.') or the aerodynamic center of the wing. (either works proportionally within reason, but theoretically, the c.g. of the plane.is the point about which all aerodynamic forces work. These a.c.'s are taken as a good approximation to be at the quarter-chord point (25% back from the leading edge) of the Mean Aerodynamic Chord (MAC). Note, there is no distance between MAC's along the fuselage (fore-aft) direction. That was shorthand for "between the quarter-chord points of the MAC's.

2) "Nose moment arm" (assumed, I'd guess, to be inertial leverage about the c.g.) is measured from the c.g. of the nose (engine, prop, tank,...included) to the plane's c,g, Missing this one has caused some noted designers to make faulty adjustments, especially in using shaft extensions on classic planes with light engines like the Fox .35.

What Phil and Tim said about these dimensions is basically sound. But if you are not going far astray from a particularly shaped model, you can generally retain it's characteristics by just scaling the easy dimensions up or down. Caveat: if the plane is to have significantly less thrust or weight, some other design adjustments may be needed.

Generally, larger tails (in proportion to the wing), longer tail moment arms and of course greater thrust are the modern adjustments made to the Classic norm to improve the planes (along with thicker wing sections). The Tail moment coefficient, T = (At x Lt)/(Aw x MACw) should be around .4 (greater doesn't matter too much). One thing not said yet: for planes balancing at the same spot relative to the c.g., the one with the heavier engine closer in is more pitch-able than the one with a lighter engine placed further forward, except that a heavier plane needs more lift to turn. Here you're considering initial rate of turn, which is much more involved than you want to get. (Howard likes this).

All theory here also is based on simplifications, and of course deflected flaps completely affect these simplifications. That does not negate the moment arms given here being better, because they are actually based on what happens with forces and torques on the plane. Rules of thumb from Ted Fancher handle the oversimplifications. After proportioning the plane's aero surfaces and distances right, re-adjust the  per-cent position of the c.g. along the MAC equal to the ratio of horizontal tail area to wing area. For unflapped planes, it seems to work at 15% - 19% of the MAC, with smaller horizontal tail areas. This is awfully wordy; that's one reason we have math. 'sorry about that.

So, as implied in earlier posts, a set of moment arms depends on your plane's configuration. Eddie's numbers should be good for a classic stunter that flies like a classic stunter, and some fly pretty well.

SK

Online Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 10182
Re: nose and tail moments
« Reply #13 on: January 27, 2017, 07:09:10 PM »
One thing not said yet: for planes balancing at the same spot relative to the c.g., the one with the heavier engine closer in is more pitch-able than the one with a lighter engine placed further forward, except that a heavier plane needs more lift to turn.

Except that Igor Burger claims that there's some advantage -- at least on his planes -- to having a longer nose for aerodynamic reasons.  I don't understand it.  I'm probably not good enough to tell the difference.  If I'm ever nipping at the heels of the "big boys" I might experiment with it.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Lauri Malila

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1081
Re: nose and tail moments
« Reply #14 on: January 28, 2017, 10:02:18 AM »
Except that Igor Burger claims that there's some advantage -- at least on his planes -- to having a longer nose for aerodynamic reasons.  I don't understand it.  I'm probably not good enough to tell the difference.  If I'm ever nipping at the heels of the "big boys" I might experiment with it.

I don't understand it either. But I don't care as obviously someone does understand:)
I'm buildind a MaxBee for my .77 engine and Igor made it very clear that I must keep his original nose lenght. Even if I have to add lead in tail. L

Online Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 10182
Re: nose and tail moments
« Reply #15 on: January 28, 2017, 01:51:05 PM »
Buy or borrow a Nobler kit. Use it as a base. Almost anything designed after 1952 and especially after 1957  through 1969 is a copy.  LL~ H^^

I think you'll find that since 1969 rear fuselages have gotten proportionally longer, and tails proportionally larger.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Mark Scarborough

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5923
Re: nose and tail moments
« Reply #16 on: January 28, 2017, 01:51:28 PM »
someone needs to explain to  me why my comment about CHarles plane not having flown was deleted,
does it not seem importatn when a person is seeking REAL information about design, and CHarles airplane has NOT flown its not really valid information is it?
This is getting a bit ridiculaous.
I guess my comments are not as valid as his? if thats the case I guess my input is not longer needed?
For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
AMA 842137

Offline RC Storick

  • Forum owner
  • Administrator
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 11544
  • The finish starts with the first piece of wood cut
    • Stunt Hangar
Re: nose and tail moments
« Reply #17 on: January 28, 2017, 04:09:50 PM »
Here is my take. Smaller tale volume needs a longer moment to have the same effect. The nose only has to be long enough to balance the plane. Bob Galdini's barbell effect article is the best read.

Mark your input is important.
AMA 12366

Online Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 10182
Re: nose and tail moments
« Reply #18 on: January 28, 2017, 04:22:18 PM »
Here is my take. Smaller tale volume needs a longer moment to have the same effect.

Sorry to pick nits, but smaller tail area needs a longer moment to have the same effect -- because that's what maintains the same tail volume.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Online Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 10182
Re: nose and tail moments
« Reply #19 on: January 28, 2017, 06:12:33 PM »
So, is this aimed at me or at the author of this thread?  The reason I picked 1969 is that is the end of the "classic" era and the beginning, more or less of the more modern stunters.
He did inquire as to a .35 size stunt machine, of which one of the most copied and successful is the Nobler in it's many forms, thus my suggestion. He can do with it what he will, but your remarks need to be a tad more respectful and a whole lot less pious. H^^

Ty, I'm not sure if something is eating at you other than me.  If it is, you have my sympathy.  But I'm not trying to be "pious" here, nor, until now, was I aiming anything specifically at you.  I was aiming at accuracy only.  However, since you want things personal, here goes:

I'm a product design engineer.  When I screw up, some poor sod in production has to make whatever I did work right, or worse, my product escapes into customer hands, where some poor sod has to live with my screwups forever.  So when I screw up, and someone points it out, I don't get all bent out of shape.  Instead I say "thank you" with genuine gratitude.  Sometimes I have to work on the gratitude part, but I always do the "thank you" part.  If I don't think I did, in fact, screw up, I say "thank you" for the effort, and then I go off quietly and check my work, and if I was right all along I go to the guy who pointed out my supposed error and show him his error, with a reasonable expectation that he'll be polite back to me.

Now, I just went and checked my suppositions.  I looked at three airplanes in my collection.  A Blue-Box Nobler (ca. 1974), a Fancherized Twister (ca 1989-ish, I think), and Paul Walker's Atlantis, from around the same time period as the Twister.  If you recall, Ted's article on Fancherization was about modernizing old kits with "obsolete" numbers so that they'd be competitive.

The Twister has a distinctly smaller wing than the Nobler -- (480 vs. 550 square inches).  It's tail is distinctly larger, and it hangs on a longer moment arm than the Nobler.  It's tail volume coefficient (TVC in the picture) is 25% larger than the Nobler's -- which is characteristic of post-1969 airplanes, and the entire reason that I put in that comment.  The Atlantis is bigger in absolutely every way than the Twister.  It has a TVC of 0.49, which is close enough to nevermind to the Twister's 0.52.  It is absolutely not, in this regard, "just another Nobler".

So, with neither piety or bile, I saved you from disseminating misleading information today.  If that bends you out of shape, I'm sorry -- sometimes reality hurts.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Howard Rush

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6184
Re: nose and tail moments
« Reply #20 on: January 28, 2017, 07:16:04 PM »
Sorry to pick nits, but smaller tail area needs a longer moment to have the same effect -- because that's what maintains the same tail volume.

I think Robert has a point (except for the name "moment"). For a given tail volume, a longer tail gives more maneuvering stability, plus some other virtues.
The Jive Combat Team
Making combat and stunt great again

Online Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 10182
Re: nose and tail moments
« Reply #21 on: January 28, 2017, 07:30:17 PM »
I think Robert has a point (except for the name "moment"). For a given tail volume, a longer tail gives more maneuvering stability, plus some other virtues.

I am (as I so recently pointed out to Ty) open to correction, but it's my understanding that two planes with the same TVC have roughly the same stability margins.  I suppose that a longer fuse with a smaller tail (thus keeping the volume constant) would make for straighter straight lines, but by the same token it'd require more effort to turn the thing (and do a better job of highlighting deviations from perfection for the judges).

Like everything else Stunt, there's a limit, and I think we've reached it -- otherwise, some kid would come along with a plane that has half the tail of an Impact, hanging four feet behind the wing, and he'd beat the tar out of all of us.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Mark Scarborough

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5923
Re: nose and tail moments
« Reply #22 on: January 28, 2017, 09:02:23 PM »
I see
"tail volume" and Tail volume coefficient" being used and its not clear whether each person is using them interchangably or not.

basically think of a teeter totter, less "weight" longer arm from the balance point to get the same (?) result.
For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
AMA 842137

Online Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 10182
Re: nose and tail moments
« Reply #23 on: January 28, 2017, 09:29:02 PM »
I see
"tail volume" and Tail volume coefficient" being used and its not clear whether each person is using them interchangably or not.

basically think of a teeter totter, less "weight" longer arm from the balance point to get the same (?) result.


My understanding comes partially from the model press, and partially from full-scale, but somewhat dumbed-down aerodynamic stuff, so don't run off and design a man-carrying plane based on what I'm saying.  As I understand it, the tail volume is the area of the tail times the moment arm (the proper one, from CG to the tail's center of lift).  The tail volume coefficient is the ratio of the tail volume to the wing volume, with the wing volume being the wing area times it's mean aerodynamic chord.

So tail volume is a thing in cubic inches (or cubic meters, or whatever), the wing volume is also in cubic whatevers, and the TVC is a dimensionless number.

It's out there on the web, in a confusing array of variously well- or poorly-written web pages, ready to be waded through.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline RC Storick

  • Forum owner
  • Administrator
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 11544
  • The finish starts with the first piece of wood cut
    • Stunt Hangar
Re: nose and tail moments
« Reply #24 on: January 28, 2017, 09:52:22 PM »
Sorry to pick nits, but smaller tail area needs a longer moment to have the same effect -- because that's what maintains the same tail volume.

That's exactly what I said
AMA 12366

Offline RC Storick

  • Forum owner
  • Administrator
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 11544
  • The finish starts with the first piece of wood cut
    • Stunt Hangar
Re: nose and tail moments
« Reply #25 on: January 28, 2017, 10:01:45 PM »
I think you'll find that since 1969 rear fuselages have gotten proportionally longer, and tails proportionally larger.

They have become longer and larger as the plane becomes larger and the engines become heavier.
AMA 12366

Offline Lauri Malila

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1081
Re: nose and tail moments
« Reply #26 on: January 29, 2017, 01:22:56 AM »

Like everything else Stunt, there's a limit, and I think we've reached it -- otherwise, some kid would come along with a plane that has half the tail of an Impact, hanging four feet behind the wing, and he'd beat the tar out of all of us.

Remi Beringer maybe? :)

Online Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 10182
Re: nose and tail moments
« Reply #27 on: January 29, 2017, 09:21:58 AM »
Remi Beringer maybe? :)

Being a typical insular American -- I dunno.  But, if it were hugely better, wouldn't it have spread?
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Lauri Malila

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1081
Re: nose and tail moments
« Reply #28 on: January 29, 2017, 09:32:27 AM »
Maybe not hugely better, but world champion 2006.

Offline Howard Rush

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6184
Re: nose and tail moments
« Reply #29 on: January 29, 2017, 04:12:01 PM »
Like everything else Stunt, there's a limit, and I think we've reached it -- otherwise, some kid would come along with a plane that has half the tail of an Impact, hanging four feet behind the wing, and he'd beat the tar out of all of us.

Some kid did something quite like that with combat planes long ago. 
The Jive Combat Team
Making combat and stunt great again

Offline Target

  • C/L Addict
  • 2018 Supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1471
Re: nose and tail moments
« Reply #30 on: January 29, 2017, 09:22:10 PM »
I can throw an added complete wrench in this discussion, and confirm with my "gut feelings" that what Howard says is true-
Longer tail moment with same tail volume feels more stable than equal tail volume with a shorter tail moment----
In my R/C sailplanes.
I'm not sure how much this has to do with weight distribution, but I do know that I can feel the difference, especially as the airspeed goes down and the numbers get more critical.
Again, all fascinating to me.

When will someone hollow core stab foam cores, or is someone doing that already??!

R,
Chris
Regards,
Chris
AMA 5956

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1900
Re: nose and tail moments
« Reply #31 on: January 31, 2017, 07:39:45 AM »
Igor, in one of his posts, indicated that he puts the propeller farther forward than he would just to balance the airplane with minimum nose length (or minimum moment of inertia).  My guess is that he does it to minimize angle of attack of the propeller in maneuvers.  You can do this with electric stunters by mounting the battery aways aft of the motor.

During time I did longer and longer nose on MAX versions. The first had "classic" nose with heavy (relatively to what we use now) PA. Later I did  MaxII which had such longer nose with piped OS .46 LA. It is the model used on my first WCh in Sebnitz - finished 10th. Now the Max Bee has the same length but larger side area, because it was still not enough.

Reasons are primary 2 -

The one thing is that prop is aproximately at tanget point in corner as Howard wrote. That means the prop does not make any yawing torque in corner (p-factor).

The other is the side area which makes fuselage side stability "neutral". Brett commented it in some other thread. It means that yaw stability is "artifical" from LO guide and controlled rudder. That makes it better controllable in tubulence as model does not yaw in side wind.

But there are several issues to solve with fuselage (the obviouse is higher moment of inertia - therefore 30% tail), it is not just "rock on arm" designing :- )))) ... may be I should write another article one day ... this time about fuselage :- P

So Lauri, yes that is why I told you to copy all small details if possible, not everyone will immediatelly see that also trimming can change design targets - for example Max Bee flies with relatively aft LO , and position of LO will change that fuselage sides balance ... .. every detail counts :- )))

Offline Matt Spencer

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2985
Re: nose and tail moments
« Reply #32 on: February 05, 2017, 12:02:17 AM »
Indeed ,








Hold on a Moment .  ;D >:(

Wont be in the Air anytime soon . Unfortunately . C.G. is o.k. with old Max 80 R I S E fitted .
« Last Edit: February 05, 2017, 12:43:08 AM by Matt Spencer »

Offline Chuck_Smith

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 305
Re: nose and tail moments
« Reply #33 on: February 19, 2017, 06:41:02 AM »
Nose moments on CLPA ships.... the sleepless nights they've caused me.

Why are CLPA airframes saddled with such a proboscis? That thing sticking out there just looks weird sometimes and makes aero engineers thinking about mission segments cringe.

My take? The conventional wisdom is to place the spray bar 1/2" above the engine centerline. That meant a fuel tank had to be 1" thick. The paradigm in stunt was that the fuel tank had to fit between fuselage doublers which meant it had to be the width of a .35 plus 2 x (3/8"), the typical engine mounting rail that was produced in rock maple. That dictated a long tank. So you make the nose long enough to

1) fit the tank
2) fit the plumbing between said tank and engine
3) fit the engine
4) Add 1/2" - 3/4" for a shaft extension so you can sexy up the cowl.

And thus, the classic CLPA long nose moment was born. Not of aerodynamic tuning, but of necessity given the equipment available during the nascent period of stunt ships. Once it worked, it became part of the tribal knowledge of how to build a NATs winning stunt ship. To this day, we see articles written on how to design a stunt ship by using the crutch of a proven design and so it continues.

Now to some aero considerations. There will actually be a an increase in *aerodynamic* pitching moment for a given control input as the nose lengthens. It's a real effect and it's accounted for in full-size aircraft design. It has to do with the flow field around the airframe. On an inside turn there's an upwash in front of the wing and downwash behind it. The upwash will push the nose up and the downwash the tail down. It's a quantifiable effect and the formulas based on work by Max Munk  et. al., are printed in most stability and control engineering textbooks.

But - and there's always a but - it doesn't come for free. Igor and a couple others are likely reaching for their keyboards as they read. The "but" is that the longer nose increases the moment of inertia about the pith axis. So the distribution of mass is now working against you, i.e, the airframe "resists" being pitched due to its increase in moment of inertia. I've never put numbers into a calculator but it would seem that the MOI is the dominant coefficient in the equation.

As to why a longer nose "grooves" better - there may be some aero-thruth to that. It could be due to something we call "inertia coupling" alluded to earlier in the thread as the"barbell effect". Basically it's about the mass being spun around an axis, in this case the vertical axis. Without a lot of math, if you hinged a barbell at it's mid point and spun it so the weights are going around in a circle they will tend to position themselves as far from the center of rotation as possible (for all you old guys, picture a centrifugal governor).  in the case of a CLPA airframe - or any aircraft - it's important to note the center of rotation isn't the center of the circle it's the center of mass of the airplane. There's a lot a weird vectors in stability and control! Long-winded, but basically the argument can be made that the inertia coupling stabilizes the pitch axis in level flight due to the constant yaw. Enough to feel? Meh, I doubt it but it is there. Our rotation rate is pretty darned low. The only time inertia coupling in pitch seems to be a concern is in spin recovery of jets.

Back to reality - I often muse about building a short-nosed CLPA ship. It should corner better. It should look like a real airplane. If we simply ditch the idea that the fuselage needs to be as narrow as possible and that the tank needs to fit over the mounting rails - the sky's the limit. Back "in the day" we were saddled by the powerplants we had available. Not anymore. Thrust to weight isn't as big a concern now so hopefully we'll start to see some diversity. I can't speak for everyone, but after 40 years over oversized Noblers dominating I'd like to see something new.  S?P

Maybe a 800 in^2 Nemesis with landing gear and a cockpit with a piped .77. Should make the last turn of the hourglass child's play.

Interesting stuff everyone.

All of the above is IMHO and feel free to offer peer review.

Chuck




 
« Last Edit: February 19, 2017, 07:18:14 AM by Chuck_Smith »
AMA 76478

Offline Target

  • C/L Addict
  • 2018 Supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1471
Re: nose and tail moments
« Reply #34 on: February 19, 2017, 09:10:23 AM »
Simply termed, I'd say your short nosed version is more like a combat model. And we all know how maneuverable those wicked little beasts are!
R,
Chris
Regards,
Chris
AMA 5956

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 8669
Re: nose and tail moments
« Reply #35 on: February 19, 2017, 09:39:34 AM »

Back to reality - I often muse about building a short-nosed CLPA ship. It should corner better. It should look like a real airplane. If we simply ditch the idea that the fuselage needs to be as narrow as possible and that the tank needs to fit over the mounting rails - the sky's the limit. Back "in the day" we were saddled by the powerplants we had available. Not anymore. Thrust to weight isn't as big a concern now so hopefully we'll start to see some diversity. I can't speak for everyone, but after 40 years over oversized Noblers dominating I'd like to see something new.  S?P

Maybe a 800 in^2 Nemesis with landing gear and a cockpit with a piped .77. Should make the last turn of the hourglass child's play.
 

    Why are we still concerned about turn radius?  Even the "Nobler Clones" at 13-14 oz/sq ft can turn far tighter than you would normally want to try. You don't see anything like the full capability in flight, or at least not in a competitive flight. Not because the judges like "smooth and soft" but because when you try to turn it tighter, you make too many other mistakes. It's not a technical issue with the airplanes, it's a limit of the pilots.

      Brett

     

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1900
Re: nose and tail moments
« Reply #36 on: February 19, 2017, 10:08:35 AM »

Now to some aero considerations. There will actually be a an increase in *aerodynamic* pitching moment for a given control input as the nose lengthens. It's a real effect and it's accounted for in full-size aircraft design. It has to do with the flow field around the airframe. On an inside turn there's an upwash in front of the wing and downwash behind it. The upwash will push the nose up and the downwash the tail down. It's a quantifiable effect and the formulas based on work by Max Munk  et. al., are printed in most stability and control engineering textbooks.

But - and there's always a but - it doesn't come for free. Igor and a couple others are likely reaching for their keyboards as they read. The "but" is that the longer nose increases the moment of inertia about the pith axis. So the distribution of mass is now working against you, i.e, the airframe "resists" being pitched due to its increase in moment of inertia. I've never put numbers into a calculator but it would seem that the MOI is the dominant coefficient in the equation.
 

Well I wrote it already in that post above - any extra moment of inertia from longer nose can be compensated by more effective tail - and plus that larger tail will donate also little bit of static stability, so it has dual positive effect.



Offline Howard Rush

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6184
Re: nose and tail moments
« Reply #37 on: February 19, 2017, 11:39:37 AM »
Maybe a 800 in^2 Nemesis with landing gear and a cockpit with a piped .77.

You peeked.  It will be electric, though.
The Jive Combat Team
Making combat and stunt great again

Offline Chuck_Smith

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 305
Re: nose and tail moments
« Reply #38 on: February 19, 2017, 12:24:24 PM »
You peeked.  It will be electric, though.

I have some lightweight electric retracts I'll donate.
AMA 76478

Offline Bob Wiegand

  • Trade Count: (0)
  • Ensign
  • **
  • Posts: 37
Re: nose and tail moments
« Reply #39 on: March 03, 2017, 01:04:31 PM »
Thank you EVERYONE!

This has been one of the most spirited and informative discussions I have seen since retiring from my scientist job. 

The points made by all seem to have validity (to the best of aerodynamic knowledge, which is limited) and the practical sense of things seems equally valid.

Has anyone ever actually tried doing the PAMPA stunt pattern with a combat plane?  How must that look?  Bouncing out of corners, maybe?

I know we cut a very thin line between smooth and controllable and quick and reactive.  There is the dilemma.  How to best meet these goals and produce an airframe that will do this with all the constraints we have to deal with.  Those constraints of course include my ability to fly the current pattern.

I do not own a hobby shop with or have unlimited resources, so I find myself working with .35 or .40 size motors and limited knowledge of stunt plane design, having been away since the 1970's.  I do agree with the gentleman who said he is tired of seeing redesigned Noblers (a very good design obviously) at every contest.  Yes, I know they are there because they work, but I was recently lamenting the fact that designs from Red Rheinhardt and Larry Scarinzi and others from the fifties were more diverse and creative designs.

Again, I am digesting everything that has come to light during this discussion, and I hope it will be of use to others of us who are coming back into the hobby.

Now I'm off to build a combat plane to try to do the stunt pattern with. n~

Offline Howard Rush

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6184
Re: nose and tail moments
« Reply #40 on: March 03, 2017, 04:01:03 PM »
Has anyone ever actually tried doing the PAMPA stunt pattern with a combat plane? 

Yes.  My conclusion was that stunt planes are better at stunt than combat planes.  Here is a crude modification to an F2D plane that resulted in a tolerably good stunt plane.
The Jive Combat Team
Making combat and stunt great again

Offline Chuck_Smith

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 305
Re: nose and tail moments
« Reply #41 on: March 04, 2017, 04:58:07 AM »


Has anyone ever actually tried doing the PAMPA stunt pattern with a combat plane?  How must that look?  Bouncing out of corners, maybe?



Not only done it, but recommend it. It makes everything seem like it's in slow motion when you go back to the big ships. It's very likely I flew my first complete pattern with either a Voodoo or a Sneeker.
AMA 76478

Offline Bob Wiegand

  • Trade Count: (0)
  • Ensign
  • **
  • Posts: 37
Re: nose and tail moments
« Reply #42 on: March 04, 2017, 07:51:32 AM »
So, it sounds like it is time to break out the old 'slow' combat planes!   y1

Offline Chuck_Smith

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 305
Re: nose and tail moments
« Reply #43 on: March 05, 2017, 05:51:20 AM »
...

Not because the judges like "smooth and soft" but because when you try to turn it tighter, you make too many other mistakes. It's not a technical issue with the airplanes, it's a limit of the pilots.

      Brett

     

Clearly, you never flew Fast Combat in the 70's and 80's.

Chuck
AMA 76478

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 8669
Re: nose and tail moments
« Reply #44 on: March 05, 2017, 11:13:24 AM »
Clearly, you never flew Fast Combat in the 70's and 80's.

   No, as a matter of fact, I didn't. But I have flown in at least hundreds of stunt contests, and frequently encountered "cornering experts" who really really cared about minimum turn radii.  Who later left the field in 20th place pissed off because they managed closer to 5 feet than anyone, but inexplicably lost to almost everybody. On very rare occasions, they will manage it without screwing everything else up, and they do really well. But most of the time, they jam every corner as hard as possible, the airplane wallows all over the sky at apparent random, and they lose to a kid with a Banshee and a K&B40* who bothered to make the maneuvers recognizable and repeatable.

      I have also met many combat guys to figure "stunt is easy" who nonetheless struggled for years and eventually failed miserably, and only the smart ones (like Howard and PTG) realize that it is a different skill set and bothered to figure out what to do to be competitive.

     You want to fly a combat plane in a stunt contest and show everyone how tight you can corner? Go right ahead -  the contest organizers always appreciate donations.

      Brett

*As an aside, I chose this example because I saw it happen at a bunch of contests in the early 80's - some kid and his dad would show up at a bunch of these contests with a sort of junky-looking Banshee with about 3" cut off the nose and a K&B40. But it ran every time, it was in acceptable trim, and the kid actually listened to what people told him. No one would pay much attention to him, while a bunch of self-styled "super-experts" with full-house airplanes and great paint jobs would spend all morning bragging about how good they were. At the end of the day, there's the kid winning Advanced and the "super-experts" all looking at each other wondering what happened.

Online Dennis Toth

  • 2014 Supporters
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2039
Re: nose and tail moments
« Reply #45 on: June 14, 2017, 07:00:29 PM »
Guys,

Lots of ships have different "numbers". Attached is a little Excel spreadsheet that you can have a little fun comparing different ships.

Spreadsheet use:
The numbers in Black are the inputs, the numbers in Blue are formula output numbers.

Best,  DennisT


Tags: