The few of you who may know me also know that I'm always interested in shaking things up... I wasted lots of space on the internet with my discussion-rant about trying EDF for CL... I haven't gone there yet, but keep my fingers crossed.  I come with another question now, I'm looking to take a wing I already like and mate it to a fuselage built EXCLUSIVELY for ECL... the Idea I have is this; shortening the nose moment as much as is reasonably by running the battery as far forward as is safe for operation, directly beneath where the motor is, where a cylinder head would normally be... all properly cooled with scoops, vents, louvers yadda yadda yadda... my Idea is to get all of that as far forward as possible, and then chop out the extra 1.5-2 inches that I estimate will be dead space.  My hope is to bring everything so close to the cg that: 1) being nose heavy will be difficult and 2) hopefully increasing turn rate by decreasing the length of the nose in front of the fulcrum. Also, I want to do all of this while trying to make it look as much like a classic plane as possible, not to compete in classic, because I love the look of the Argus, Ares, The Olympic, and the Starlight.  Any recommendations on this topic would be most welcome!

Thanks and Tight Lines!

That sounds like a good idea to me. 

If you get the aerodynamics of the wing and tail correct, and the balance point is correct, the length of the nose will have little effect on things.  Reducing the moment of inertia of the plane(Bob Gialdini's barbell analogy) can be good, especially if the controls and stab/wing size are off.  It's easy to trim/design a combat style plane to fly smoothly and still turn sharply.  Same thing everybody has been doing with stunters for the last 25 years- large tail volume, aft CG to reduce control loads, larger bellcrank to make sure there is always enough control authority, regardless of the wind.

One thing a long nose does is damp the controls.  Putting the prop further forward makes the prop steady the plane.  It tales more control input to start and stop a turn, making the plane respond more smoothly.  Of course, going too far in anything will cause problems.

How do you figure that?

By flying two nearly identical planes, one with a 6 inch nose and the other with an 8.5 inch nose.  Part of it is moment of inertia.  But the prop is moving a lot of air.  Changing direction of motion of that mass also requires some force.

Oh, virtual mass.  Maybe so.  

Decreasing the moment of inertia of the airplane itself (not counting the air that moves along with it) increases dynamic stability, as you said.  

Virtual mass ??  Can i assign some of the physical mass of my ships as virtual mass?? Please please tell me how?

Randy

This is where electrics have a definite advantage over gas models because they do not have to contend with a shifting CG as the fuel loaf is consumed and that battery weight can be concentrated in a way to shorten the nose moment compared to the gas poered counterparts.

Keith, I think much of your problem may come from the form of fuel you burn.  A baguette will cause more CG transfer than a more concentrated loaf such as a brioche.  Hope this helps.

Keith, That is almost exactly what I was thinking, the other issue that I had in mind is that the propeller also works a gyroscope and wants to hold it's position and part of my hope is to be able to slow down controls but still get fast responses... one of the benefits that I'm looking to take advantage of is not needing the battery to be in line with the motor but put it beneath the motor shorten the nose, keep the tail moments close to the same and see if it'll have my desired effects.  I'll build up the tail stab elevator and fin, even use stringers for the turtle-deck and lightening holes in the fuse to keep as much weight down as possible to keep the balance. 

Interesting ideas here. I personally was considering an opposite approach where you move the battery back as far as possible, lengthen the nose some, but also reduce nose area and have your weight mass centered around the GG. My thought was to create a situation similar to the Teds experiments with the Tucker where they added penny rolls at or about the CG and a thus by his description a plane that was really easier to maneuver, positively stable and yet crisper, even in some moderate winds and under heavier than what was considered optimum weight for a classic era design.

That was my hope Howard, hopefully creating an airplane with decreased control throws, but without necessarily having to add pesky tail weight... The primary idea from my point of view is to screw with things because I feel like Stunters are getting really bland and stale, because standard numbers work so well... but why not try to take advantage of some of the quirks of electric power, such as a fuel source that doesn't have to be perfectly aligned with the power source in order to get the desired runs, taking advantage of certain other elements could be beneficial as well, not needing the fuselage to be as wide to have engine mount hard points, making the fuselage wider, for increased airflow, because you don't have to worry about engine mount hard points. I just hope things could be done differently and still achieve desirable results...

Quote from Howard:

"That was addressing a different problem, to wit getting enough line tension to react the hinge moment of the control surfaces."

I am aware of that , but what I found interesting was, a smallish design with a fairly thin airfoil flying better at 50 oz. than at 40 or there abouts, and where the weight was carried. Adding ten oz. to any design and having it fly better is not the norm and the extra weight was carried about the CG or slightly forward I assume. I havent put pencil to paper on this but the idea intrigues me.

I put pencil to paper on this, but I gave up to finish my new stunt plane.  I intend to get back to it one of these days.  I was only working on hinge moments and their consequences, which appear to be a big deal.  You have an interesting point: what happens when one adds mass to the airplane without adding moment of inertia?  Something else for that pencil to do. 

<snip>  You have an interesting point: what happens when one adds mass to the airplane without adding moment of inertia?  Something else for that pencil to do. 

That would effectively reduce the moment of inertia.

How so? 

The only place you could add mass (weight) without increasing polar moment of inertia would be around the CG.
This would effectively reduce it but the weight req'd to do it wouldn't be worth the reduction. I think it would end up too heavy.

Buliding your empennage light and reducing your nose moment would be a better approach.
I'm under the impression that nose moment is soley to balance the fuse./ tail design.
A Sig Banshee is a good example - light Fox .35 with a heavy tail, hence the l o n g nose moment.

I'm not sure I understand your question. A short nose moment is useful on combat planes.  On stunters, we usually size the nose moment to obtain reasonable =balance.  Often, I see stunters with, what seems like a very long nose.  This really doesn't hamper turning.  We strive for smoothness, and not blinding turn. 

F.C.

Hi there
I don't get on often but here are some things I am working with...
Sometime ago there was a discussion about having a shorter moment arm on the engine to reduce gyroscopic precession.  From that thread Bob Hunt suggested a canard design would put the propellor closer to the CG and thus reduce the gyroscopic precession.  Bob built at least a one (probably more) and I took to the idea as well.  I have built 6.  I like the flight characteristics and it does seem that the airplane turns easier.
Obviously balance is the main issue with any CLPA design so where you place the powerplant is optional as long as the design works with it.  Most everyone has seen twins, I have seen a pusher and a pushme pullyou.  I am not saying that they were great fliers but there is potential for nearly any design as long as the numbers work out.
I encourage everyone to keep trying to find the perfect airplane.  It is more fun that way!
Ed Culver

Sometime ago there was a discussion about having a shorter moment arm on the engine to reduce gyroscopic precession. 

It won't.  A moment is a moment.  There is another moment, though, that's due to the aerodynamic force on the propeller perpendicular to the free stream airflow.  That one would get smaller with a shorter nose. 

I keep hearing about the shifting CG "issue". For some reason few if any spoke of this phenomenon until electrics started coming out. Fuel that weighs 6oz at HALF the arm of a 13oz motor has so much impact on CG that such a tremendous "advantage" comes up EVERY TIME the electrics are mentioned. I can see consistent run from flight to flight, I can sea no need for cleaning the model, sure. Bringing out such obscure and tiny effect of shifting CG really makes electronuts appear exactly that: nuts.

   Obscure?  That's one of the big reasons that current stunt planes fly better than classic airplanes, because they tolerate the CG shift much better. If you don't think it is a big effect, fly your hourglass at the beginning of the flight. Be *very careful*. 1/4 ounce of nose weight makes a quite noticeable difference, project 7 oz of fuel to the nose, and it's about like adding 3.5 oz of nose weight that runs out over the period of 5 minutes. It's a HUGE effect.

    This has long been a topic of interest. Its why we have always recommended flying practice flights in the right order, because if you do them out of order, you do them with the wrong CG position. You can actually use this effect when trimming, if you think you want to move the CG, you can do, say, your hourglass at various fuel levels to see what the effect would be.

    It comes up more in electric threads because before, ALL the airplanes had the same issue to varying degrees, and people got used to it. If you take a specific airplane, take out the IC engine, put in an electric motor and battery, you will find out that it matters very quickly. And I am hardly a Howard-level electronut.

   BTW, this also explains the ability or need to have the dry CG further forward on 4-strokes, as well. Much less fuel used, same airplane weight. To get the same average CG the dry CG has to be much further forward.  No magical physics required despite the endless threads on the topic on SSW.

    Brett

Shoot.  Now you tell me.

     Headed down to the basement to build a leaky water tank, are we?

    Brett

So can I put my fuel tank on the cg and pump the fuel?  Full scale single engine aircraft usually don't carry the total fuel supply next to the engine, or do they?

So can I put my fuel tank on the cg and pump the fuel?  Full scale single engine aircraft usually don't carry the total fuel supply next to the engine, or do they?

Most full scale airplanes store fuel in wings close to or on the CG. There are still airplanes(Sonex or Europe XS for example) that have tanks between the pilot and the motor or immediately behind the pilot's seat.

Brett,

I do not argue that this effect does not exist. I do question its "significance". I have a hard time finding any mention of this effect in articles or forum discussions prior to 2006. If this effect is so prominent, surely, it should have gotten more attention, don't you agree?

  We gave it plenty of attention, I searched SSW for "cg shift fuel" on SSW and found a couple or three pages of hits. The difference from beginning to end of a flight on my airplane is literally identical to adding ~3 oz of nose weight, which is DRASTIC change. Go add 3 ounces of nose weight, fly a flight, and tell me if you can notice a difference. As before BE CAREFUL, that wingover might come out a little low on the first flight.

    BTW, I think Paul Walker was toying with us on this topic. He was among the first to notice that 4-strokes ended up with the CG further forward with no impact on the turn, or rather, it turned better than the CG would indicate. It took me about 6 months to figure it out myself, and I never let on, because I didn't want to give him the satisfaction of  me having failed his IQ test.  That led to an epic thread on SSW in 2002 where Brad Walker was trying to explain it in terms of engine torque and ended up in an interesting discussion on the RPM variation in 4-strokes VS 2-strokes. Brad finally called me on the phone about that and other issues, and I mentioned the CG shift issue, but as I recall he either didn't agree or missed the comment.

    Brett

p.s. actually Brad does the work for us all right here:

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=26043&mesg_id=26043 

Howard and Brett,

Wasn't meant that way. Sorry. I have the misfortune being pretty deep into all this, we work in an environment where .01 seconds after 1000 feet is significant with a lot of variables and so I can get obsessive about small details...occupational hazard.

Please accept my apologies.

But back to the fuselage, the moment contribution really is more than you'd expect. If you are so inclined you can PM me and I can copy some real-world examples of it in action during the analysis of an airplane and email you. Cool stuff, but yeah, past the maximum dorkiness coefficient for here.

Chuck

p.s. actually Brad does the work for us all right here:

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=26043&mesg_id=26043 

I guess I was wrong. This is a good read indeed.

Speaking of thread drift-- call me Ben E. King-- I wonder what Chris is doing with his battery location idea.  Having no originality, I look for good stuff to copy, and it looked like Chris had some.

Like a lot of abstract thinking about stunt design, I am skeptical. Too many forces acting on the tethered birds, muddies up the equations. Which is why, I think, the best performing stunters are evolutions of designs that work. Trial and error made the Nobler. I think for instance, the longer lever in the front, increasing momentum, is to an extent, desirable, smoothing out the curves. Even tho concentrating mass as close as possible to the center of lift would, possibly, quicken a turn. Combat planes look like they turn on a dime. Great potential for cutting a tight corner. But planes like that don't win in stunt. Not smooth. Difficult to overlay shapes. Hard to make repeatable figures of a given size. There's a value in dampening the curves and corners. Isn't there. Do longer nose and tail moment do that? To some extent? Is there an optimum range?

. Do longer nose and tail moment do that? To some extent? Is there an optimum range?

Where is Ted?  He wrote once about one of his airplanes that he found a tail moment that was too long.  And I do not remember why he came to that conclusion.  I will have to go to his material and see if I can find that article.

Keith

Please, before I put knife to balsa.

Howard,

I think you are asking for the information I referenced something that Ted Fancher had written where he built an airplane and believed he found a tail moment that was too long.  I have been looking.  (He has written a lot of stuff.)  I am pretty sure it came after his Citation and then that Imitation profile thing of his.  In that two part Imitation article (Sep, Oct 79 Model Aviation), he advocated long tail moments, at least longer than what his Citations had.  So, sometime between that Imitation article and his Trivial Pursuits, there was a continuing series in his "imagination series" development and he found something that made him think that tail moments could get too long.  I looked through his MA columns (May 84 through Feb 90).  He did not have much to say about tail moments in his Excitation article (MA Dec 81) or his special lightweight for the 84 Reno Nats (which he discarded for the competition anyway), he still favors longer tail moments in his columns published in June and July 85. There may be something in a Stunt News prior to the appearance of or when he wrote about his Trivial Pursuit.  I will go into that part if my library next, but hopefully, he will join this discussion and enlighten us with his "too long tail moment discovery".  I am positive he had one and wrote about it but I have not yet found it. 

It would help a lot if he would just come out and explain what I am certain that he wrote.

Just for the record, I still prize my bound leather two volume set, autographed by the author, Ted Fancher, of his epic work titled How to Design a Profile Fuselage.