FMS 1400MM P40B

[WintrSol]

RE: FMS 1400MM P40B

OK, still don't see how it is more accurate than the finger tips. As vertical balance is also a concern of mine I still need to perform a few flight test to get the model trimmed out correctly with each  size (weight and shape) battery I plan on using. It might help some of the arguments it someone can post a link to the machines they feel are of value along with the instruction on how to use the machine.

All the best,
Konrad

I use this one: http://www.motionrc.com/great-planes-c-g-machine/#description
The pivot bearings are fairly small diameter, so create a narrow range of repeatability. I find that using fingertips depends a lot on how you touch the model, meaning I can shift the apparent position by up to a 1/4" by just changing the angle my fingers contact the model. I can repeat a measurement with that machine within a mm or two, no problem. As for vertical CG, most of my models have the battery very near the thrust line, so that's not much of an issue. Also, I'm mostly a scale and sport flyer, so vertical CG is a non-issue for me.

[Konrad]

RE: FMS 1400MM P40B

Sorry I missed your comment on page one about pad area. Some of the larger EPO models will dent suspended on finger tips.

How does one avoid cosine error, as I see no means to insure that the model is level or even a defined level datum (bubble glass)? Then there is the dampening effect (negative effect on accuracy) of placing the center of mass below the pivots of the machine.

That CofG in the link is the one I have issues with (I find offers little or no value to the set up of the model).
Or more to the point, the ad is very misleading "An absolute MUST for safe, satisfying flights". This is why I called it a gimmick. We know that 90% of this is false;

"An absolute MUST for safe, satisfying flights. An improperly balanced model-at best-won't be able to fulfill its true performance potential. At worst, it could crash and be destroyed. Why take the risk? Great Planes' inexpensive, easy-to-use CG Machine is the only tool that finds the exact balance without guesswork...and it can be used with virtually ANY fixed-wing aircraft."

True if the mass of the battery is kept on the thrust line its variability will have minimal effect of the vertical hang of the model in 3D flight, or on your more classic up and down lines.

All the best,
Konrad

[Grossman56]

RE: FMS 1400MM P40B

I read in a magazine that a simple CG 'machine' can be made using two pencils and a Workbench.   Just anchor the two pencils in the jaws of the workbench, eraser upwards, and balance with that.  Ever tried it?  I'm thinking I'll try and see.

Grossman56

[Konrad]

RE: FMS 1400MM P40B

Why, for that small an airframe your finger nails will give better data.

[WintrSol]

RE: FMS 1400MM P40B

How does one avoid cosine error, as I see no means to insure that the model is level or even a defined level datum (bubble glass)? Then there is the dampening effect (negative effect on accuracy) of placing the center of mass below the pivots of the machine.

All the best,
Konrad

I guess, one deals with cosine error the same way you do with your fingers. Same answer for damping effect, which is greater when fingers are used, since your skin is elastic.

The questionable text in the ad is just typical advertising hyperbole. A simple statement of the usefulness wouldn't improve sales, now, would it?

[Konrad]

RE: FMS 1400MM P40B

Nope, but it would help keep $26 in the pocket of the modeler rather than in GP's bottom line.

I actually balance in an unstable condition, that is the center of mass is above the fulcrum (finger tips or bearings). This accentuates the tip point.

So we agree that that it really doesn't offer much if any added accuracy to the measurement of the CofG. The machine has too many uncontrolled variable as to be really no better than one's finger tips.

Now when we adjust the balance on full size aircraft these datums and moment arms are well defined. Yet, I was surprised to learn that even on the 747 there is still a field for the flight test pilot to fill out his subjective impression of the aircrafts handling characteristics. If the test pilot's report is less than satisfactory it was up to the engineers and mechanics to try to figure out what tolerance stack ups caused the less than favorable report, and adjust accordingly within the specifications for the variables.

So what I'm saying is that in the end it is all about how the airplane flies not about some number that the plane balances on. Save your $30, if you want to really see an improvement in the flying characteristics of your model spend it for faster servos rather than a balance machine.

All the best,
Konrad

[WintrSol]

RE: FMS 1400MM P40B

I actually balance in an unstable condition, that is the center of mass is above the fulcrum (finger tips or bearings). This accentuates the tip point.  

So we agree that that it really doesn't offer much if any added accuracy to the measurement of the CofG.  The machine has too many uncontrolled variable as to be really no better than one's finger tips.

All the best,
Konrad

Yes, we are in basic agreement; but for someone new to the hobby, the machine can be a quicker way to know if a model is close to being stable. As one gains experience, and knowledge about how to adjust CG to better suit them, then your fingers are always set up, and make a quick assessment much easier.

As for the question about using pencils, that works too, and I have done it that way for a quick evaluation of a small model. They can put dents in foam wings, though, so one must be careful with them. Also, you want to round over the erasers; a flat-tipped one will lie to you.

[fhhuber]

RE: FMS 1400MM P40B

Acceptable CG is a range from forward-most that you have reasonable ability to slowdown for landing to rear-most where control becomes extremely sensitive.

With many tail-draggers, the forward-most is controlled by being able to keep the prop out of the dirt on takeoff... Especially WWII warbirds such as the P-40, P-51, Corsair (virtually all of the single engine tail-dragger fighters)

[Konrad]

RE: FMS 1400MM P40B

Why adjust a flight parameter for ground handling. CofG is used to set the margin of flight stability?

The placement of the CofG is a function of the aerodynamic center of lift (flight forces). Ground handling (ground loops, bounce up, tip overs , etc.) are a function of the center of mass and the wheel axle. Since the center of mass has an effect on the center of gravity it should not be used to adjust ground handling issues as it will have an effect on the flight characteristics. Ground handling issues should be adjusted with the landing gear geometry (toe in, rake, height , etc.).

Avoiding a prop strike on take off should be more of a function of proper throttle management, not the forward or rear position of the center of gravity.

So the short of it is, don't adjust a flight parameter for a ground handling issue.

Control sensitivity is more often a function of too much travel and or control slop. Too far an aft CofG results in divergent flight. If the aircraft flies ok or better without input from you, then your issue is most likely too much control movement. If the aircraft is divergent (usually in pitch) then your CofG is likely too far aft. Divergent is accelerating in a direction without input from you (or flight controls) , for example a tuck or deep stall. Far too many fliers move the CofG forward when their real issue was having too much control throw or slop in the control linkage.

All the best,
Konrad

[fhhuber]

RE: FMS 1400MM P40B

Hand launching a P-51 you can get way with the CG being further forward than trying a ROG. The wheels are back further than where you would place them if not for scale appearance.
Its highest performance CG is still further back, but the useful CG range is limited by the landing gear placement.

It is what it is. If you put the CG to the forward limit for flight... you can't ROG.

The Dynam P-51 is a perfect example. Its common to shim the retracts to have the wheels slightly more forward when down so there's less of an issue with ROG because the aft limit of CG range is almost where the plane can't taxi without chipping the prop. It flies fine with the CG up to 1/2 inch further forward than where just letting the plane sit on the ground without the prop turning it will end up on its nose.
Thus the landing gear placement limits the useful CG range.

[Konrad]

RE: FMS 1400MM P40B

Maybe this is why the P-51s fly so well. Guys are forced to place the CofG aft were it belongs for proper flight trims.
The Piper Cub is an example of having the center of mass aft making it easier to ground loop. While the aft CofG results in a much better flying model. Tip over is a throttle management problem, or the grass being too long.

Actually the P-51 is also prone to ground looping with the center of mass aft. But again it flies so much better with the aft CofG, than being nose heavy (Nose zooming up as the speed gets faster, I.E. coming out of a dive).

All the best,
Konrad

[Dewey H Lee]

RE: FMS 1400MM P40B

Hey guys. The cg machine that he is useing is not the holy frail. I got one and I have always had every plane I checked on it to come up tail heavy. Put weight in the frount and be bad nose heavy. Planes checked on the fingers balanced within 3/4 of the factory recommendations every time. Flew hands off with little trim.

[WintrSol]

RE: FMS 1400MM P40B

Hey guys. The cg machine that he is useing is not the holy frail. I got one and I have always had every plane I checked on it to come up tail heavy. Put weight in the frount and be bad nose heavy. Planes checked on the fingers balanced within 3/4 of the factory recommendations every time. Flew hands off with little trim.

It can do that, especially if the center of mass is at, or above, the bearings. I have a 3D mid-wing aircraft, and the center of mass is above the machine's bearings, no matter if I rest it on the machine right side up, or upside down. That makes it very hard to assess the actual CG with the machine. With aircraft where the center of mass is well below the bearings, it can be accurate and repeatable, when you take the limitations into account. It is all in understanding how it works, and what the limitations are.

[Brandon Moon]

RE: FMS 1400MM P40B

Hi Folks...new here and thought it was time I jump on board :) here is a little of what I've done with my P-40
https://www.youtube.com/watch?v=y0EDKtlMYbE

[ryramZ]

RE: FMS 1400MM P40B

Glad to see you here Brandon!

[Brandon Moon]

RE: FMS 1400MM P40B

Glad to see you here Brandon!

Thanks, glad to be here :).... time to join the team :)

[Konrad]

RE: FMS 1400MM P40B

Were the gear failures consistent? Or were there more than one failure mode? Like one had a bad control board and the other was subjected to a hard landing.

All the best,
Konrad

[Brandon Moon]

RE: FMS 1400MM P40B

Were the gear failures consistent? Or were there more than one failure mode? Like one had a bad control board and the other was subjected to a hard landing.

All the best,
Konrad

Not sure why they failed. First one was never working out of the box and the second replacement just simply quit working after 20 cycles...and no..haven't even flown the plane yet

[Konrad]

RE: FMS 1400MM P40B

Ouch, that's starting to look like a quality issue with FMS or their supplier.

All the best,
Konrad

[Brandon Moon]

RE: FMS 1400MM P40B

Ouch, that's starting to look like a quality issue with Freewing or there supplier.

All the best,
Konrad

I think I just got two bad ones...hopefully third times a charm  ;)