Lynn S

The design of the Predator got me to thinking about how this plane is balanced. It is made like a stick with the motor on one end and the camera payload and radio on the other. Like a seesaw the balance point is somewhere to be found on that (stick) It looks to me like what they did in the real plane is put everything they needed for the payload on the end opposite of the motor and found the balance point on the fuse.... THEN put wing ON that balance point. This may be the way it is done and again It may not, it just dawned on me how it could be done.
To apply that to one of our planes you would go ahead and install the engine, battery, radio and all internal parts to the fuse, THEN find the balance point on the fuse. The wing CG would be placed over that balance point. This way no weight would have to be added to make the plane balance.
Anyone ever think about making the wing CG meet the plane's CG instead of the other way around?? Or, am I just going nuts??

ajcoholic

My Feedback: (10)

Nope, thats just fine. Often on rubber powered models I will wait to attach the wing untill I have the fuse/rubber/prop/etc all mounted and then place the wing balance point over the cg of the rest of the works.

This works if the plane is a traditional layout, with a non lifting stab of "normal size".

The only place where you may have a concern is if the nose is extended an extreme amount, you may need to counter the added lateral area with an increased rudder/fin area.

AJC

Lynn S

Thanks, AJ,
...and it would take sort of a box, or tube type fuse where the wing could be moved around. Talk about an extended nose look at the Predator, it has a long one. When I saw where they put the wing I thought this is like finding the balance point of a stick and thats where the wing is.[8D]

nekked_man_2000

I built a gremlin flying wing, and what you do is build the fuselage from plastic pvc, and the wing from foam. You set threaded blocks in the wing, move the body back and forth, with all radio gear and motor installed, until the plane balances and drill the mounting holes and connect the elevons. That's basically the same thing. Thing I wonder about, on a conventional plane with a long fuselage, wouldn't there be a dissadvantage to having the weight all at the extreme ends? At least for aerobatics, but would it make it more stable in pitch??? Hmmmmmm?

Austin

William Robison

My Feedback: (3)

nekked:

The weight distribution effect you are talking about is the polar moment of inertia. And it does affect the response for a given control input.

Where you see the greatest design consideration is in GP and sports racing cars - that's a big part of the reason for mid-engine designs. By keeping the greater part of the weight near the cg of the car (yes, there is such a thing) the maneuverability is enhanced.

Varying the wing locatiion for balance is common on indoor models, and many outdoor free flight modelers use the same method.

Bill.

BMatthews

WOW, you build rubber models too? That just took you up another 10 points in MY book...

Now I just have to wonder what sort of cutter you use for machining the rubber to build the motor out of...

This is how I set up all my P30's, Coupe 'dHiver's, and all indoor duration models models that I've ever built.

Lynn S

Bill-
I guess the Predator is built for low aerobatic performance and high stability.(the reason for the third fin on the tail)
I do know a little about cars NOT having good weight distribution. I owned a rear engine Corvair in the mid 60. I lost control of it once on a gravel road. All the weight on the rear end made for some wild 90 degree(side to side) back and forth swinging of the rear. I'm lucky to have survived that thing.[X(]

matchlessaero

Lynn, like the other guys, I used to build FF planes that way, but I would not suggest it for RC planes which you expect ulitmate performance from. The reason is that as you move the wing back and forth, you end up changing the aerodynamic moments of the airframe and its ulitmate performance (pitch stability).

However, two other methods that are a lot like what you are mentioning.....

On CL stunters, it is common practice to adjust the length of the nose of the plane to get the CG right without adding weight. I did this last summer on my asofyet unfininshed Triumph Stunter- shortened the nose 3/4" to get rid of a nose-heavy condition....

On my RC airplanes, I have started to build this way...First, I finish building the wing, tail and the majority of the fuse.... but I leave either the top or bottom sheeting of the fuse off. I then mount the wing and engine and mark what I figure the balance point to end up needing to be. Next, I put all the components in where I 'think' they should go and move them around until the plane is just slightly nose heavy. I mark their positions, mount them and finish the plane. I also try to give myself some leaway with the battery. Its the easiest heavy thing to move around and fix any CG problem.... Usually, the plane ends up balancing just right, but if not, I move the battery or get a smaller/bigger pack.

Man, I am getting too doggone longwinded...... []

ajcoholic

My Feedback: (10)

Thanks Bruce, I build a ton of rubber FF models (Sig mainly, but a lot from Peck too and some Herr kits) with the kids I work with regularly. Heck, just yesterday I was showing two brothers how we could balance their Sig Mini Maxer's without adding nose weight by moving the wing back on the fuse...

Dont all modellers still build rubber for fun??

AJC

William Robison

My Feedback: (3)

Andrew:

Of course. But how many people playing with these things are really modellers?

Bill.

Jim Thomerson

Having never built a successful rubber-powered model, but having associated with a few experts, I would say that successful rubber model builders are true modelers in the highest sense of the word. Being a kind and sensitive soul, and not wishing to offend those who enjoy buy-and-fly toy airplanes, that is all I will say.

Jim

William Robison

My Feedback: (3)

Jim:

If your rubber jobs have all been scale or semi-scale your lack of success is not in the least surprising. That's where the real experts are.

You need to get a good sport rubber job, around 35" span. That size is easy to build and trim, and pretty cheap in cost as well.

It's been so long since I've bought a kit, or even built to someone else's plan for one, I can't make a recommendatiion to you.

AJC: Do you have a favorite kit or design in that size range? Or in another size that you would suggest?

Bill.

Beeza

My Feedback: (1)

A couple things--

Rubber is a great way to go! That's what eventually got me into gas free flight (back when NOBODY had affordable r/c). I saw the Peck Polymers 1 Night 28 converted into electric r/c on the E-Zone a while back. Cool conversion. The first thing I did was search Wakefield and relive old memories... I used to build the Comet rubber kits.

About balance and the Corvair comment. I drove a Pontiac Fiero for a while. I spun it on ice once at slow speed and it couldn't recover due to it's mid engine design. Pretty scary!

Come to think of it, weren't there problems with the P39 or P63 planes tumbling due to a mid-engine configuration?


Beeza

Lynn S

Hey this is getting off the topic some....but still about weight and balance and even... flight .....
When the Corvair was first introduced ...I remember a Ford Falcon commercial ( I was 11 at the time)that demonstrated how an arrow would not fly straight with the weight at the REAR of the shaft. I don't remember exactly how they did it but they shot one normal arrow at a target and hit it. Then they shot an arrow weighted at the rear instead of the front, I don't remember where the fins were. Needless to say the second arrow didn't hit the target....so the message they wanted to deliver was the Falcon with a front engine was more stable than the Corvair with a rear engine. I hate to say it but that was true. It would turn on a dime, and go in snow... but TAIL HEAVY is not good on loose gravel.
....sorry for getting way off topic, but its a good story

William Robison

My Feedback: (3)

Lynn:

Directiional stability is great for an arrow. An airplane needs maneuverability.

Off RC, but on subject:

I think it's generally better to steer around a corner than to go straight off the road. We all laughed at that Ford ad.

While the Corvair might not have ben the best car in the world, Stirling Moss (remember him?) said the Corvair was without quetion the best handling car the Yanks ever made.

And the Falcon is a car that never had any quality in its entire production run. Not just my opinion, it was the opinion of the market. Almost couldn't sell a used Falcon, while the Corvair prices stayed up until Nadir's book hit the market. (Nadir, incidentally, means the lowest point something can reach.) And the Corvair today, if you can find one, commands a high price. There are no remaining Falcons, they've all rusted away.

Bill.

Digger-RCU

I was intestested in the original question Lynn, because that is how LWR and I build a couple of spads. The plane was complete except for the wing-holdown dowel holes. We rubber banded the wing to the fuse and slid it back and forth until it balanced. Then we drilled the holes. Worked great.
I kinda thought we made that technique up! Now I find out the Goverment has been doing it all along!!!!!![8D]


PS. My Dad had a 62 falcon pickup, and a 64 corvair. I liked them both! Although to me, the corvair felt a liitle strange when I drove it. As for all the old falcons rusting away and being worthless, I think 1/2 of that is true. There may be few falcons left, but the ones that are left are getting pretty expensive. Corvairs on the other hand are considered relativley inexpensive on the restoration market. I predict they will get there though!!!. Now if someone could find me a 62 skylark convertible!!! thats the one I want.

Randy (Digger) Birt

DICKEYBIRD

Here you go Digger! http://www.gaspumpclassics.com/forsa...rnia_1519.html

ajcoholic

My Feedback: (10)

I recommend the Sig Tiger, Cabinair and 29'r for a great flying, and good building model. Also very inexpensive relative to the newer Dumas and Herr laser cut kits. They are in the 24" ws range however.

Any of the Herr larger scale (30" and up) models are awesome, but have a larger price tag to boot.

If you really want to get back into the world of printwood, and more work from a building point of view most of the Peck kits are excellent flyers IME and I like to get them for my kids also that I build with.

The Sig Mini Maxer is a non-scale flyer that is also an excellet build and a superb flyer.

AJC

MR Flyer57

Back to the original topic, I have built the Gremlins also and they do indeed slide the fuse to achieve balance. Also I have some crickets that also have you put the wing on last, just for the balance.
I have to really hand it to anyone who has built a successful rubber powered model. I have built 10 of them and never had any worthwhile flights, the best is a controlled crash. I have had better flights with the little put together planes that come in the plastic wrapping.
So anyone who is into the really little indoor freeflights,,, WOW that is the ragged edge of flight.
MR Flyer57

nekked_man_2000

There aren't many falcons left because they were kinda homely, but there are plenty of its very close relative, the Mustang. Pretty much the same car with pretty sheet metal, and somewhat more popular than the Corvair from what I'm told. And saying the Corvair was the best handling car the yanks had ever built wasn't saying much at the time, even those old vettes didn't have really good suspension... but I do like the Corvair, especially the converts...and Ralph gave a lot of cars bad raps. I read that book a long time ago in high school (unsafe at any speed???), he didn't like fins or points...I guess he wanted a Nerf car. And, by the way, Falcons can command a pretty good price too, especially an all original V-8 sprint or Convertible Ventura, but both the Falcon and the Corvair have a small niche' kind of market, and prices can vary a lot, unlike Mustangs and Vettes if you've got one of those you won't have a problem finding a buyer, and values on specific models are pretty well known...I find all four of these cars(and many others) appealing, but my wife wouldn't be caught dead in the nicest example of the first two.

How'd we get on a car discussion? Sorry, I love cars and can't help but get sucked into these things.

Lynn S

NM2000
The topic was about balance .....so I got to birdwalking a bit about how the Corvair was rear end heavy.

Japanman

....Was that a Rubber powered 180hp convair with rear engine (pusher??) It could land on the road but not on ice???

Boy, am I confused...

J.M

William Robison

My Feedback: (3)

Hey, people.

We have to talk about r/c, not cars.

Let's discuss the weight distribution on airplanes, else we'll have to close the thread.

Yes, I know I made some car posts here too, but that doesn't make it right.

Bill.

ajcoholic

My Feedback: (10)

OK Bill...

One other mething commonly used by me to balance planes, and sort of similar, is to (instead of adding nose weight) block off the engine/mount to get the weight out further ahead. One of my own designs I made the nose way to short. I ended up extending the mount by building a plywood box that went bewteen the firewall and engine mount, and that saved me adding any weight. Since it was an original sprt design, the look didnt matter anyhow!

AJC

nekked_man_2000

I said I'm sorry, really. If it helps, this topic forced me to think about the whole issue of balance. I've never considered the effects of having the majority of the weight at the extreme ends, and this may call for some experimenting, but no more car stuff from me.

Would a plane balanced like a dumbell require a lot of control surface deflection to change directions? I guess I should say, more deflection than a normally balanced plane, with most of the weight concentrated around the center of gravity.