CG with tank FULL or EMPTY
 

I just completed an Andrews MiniMaster .25 powered sport flier. Great kit... fun to build!!
The suggested CG location is shown on the plans, but it doesn't say if that is for fuel tank full or empty.
With a 4 oz. tank, that can be almost 1/2" difference (3 1/3# all-up weight), or 5% chord!!I
I guess for the initial T/O, it would be best to use the more forward CG location (balanced with empty tank).
Balanced with a full tank might result in a stability problem with the fuel burned off.....
I just don't know what Lou Andrews assumed with his CG location.

Anybody have any ideas? .... similar experience??

http://www.rcuniverse.com/forum/atta...mentid=1972319

Always set c.of g with tank empty ... rather have a nose heavy than tail heavy plane.. unless im prop hanging hahaha

I set the Centre of the tank on the cg, this way the plane flies with the proper cg through all tank levels. I use a Bladder tank so the the blader does not allow fuel to slosh about or foam up.

How about balance the plane for the worse case scenario. Gear up, gear down, tank at the cg, tank behind the cg, pusher, puller, Gopro on the tail, Gopro on the nose, etc.

Kurt

If you are unable to set up with the tank on the cg, move it as close to CG as reasonable and measure cg with tank empty as Marzy69 suggested nose heavy is more predictable than tail heavy.

wkevinm

"close to the CG" = just ahead of the CG?
"close to the CG" = just behind of the CG?

Kurt

On the very rare occasions when the fuel tank is at the rear of the model, you go for the worst case (aft-most CG). You would balance with a rear mounted tank full, UNLESS (as in the case of the Northstar) the plan gives a CG for an empty tank, in which case an allowance has been made for the CG shift when it is filled.

Just a note of caution regarding tank on/near the CG. Your .25 might not like having to draw fuel that far and may have tuning issues. Generally if your engine has a fuel pump like most (all?) petrol engines and some (usually competition type) glow engines, then fuel draw is far less an issue.

You may never have a problem with drawing fuel that far with an unpumped .25 depeding on how you intend to fly, but if you do have tuning issues be prepared to move the tank as close to the firewall as possible as a possible fix...

The correct answer was in your question. :)

For proper pitch and yaw stability, the aerodynamic center of the airplane must be aft the point in which the weight is concentrated (just like in any arrow).

As the fuel is consumed and the nose gets lighter, the center of mass or weight relocates closer to the aerodynamic center, but it will not reach it if you follow the designer recommendations.

I personally do not put that much confidence in the "designer" nor their "recommendations", but rather tried and true methods.:cool:

Kurt

You balance for landing. This implies that the tank will be empty in most cases.
The real exception is an electric powered model, since the balance does not change. (Unless the battery or something else
is moving around.) Obviously, it's better to be slightly nose heavy than tail heavy.
I've flown in full size A/C that required fuel management by using tanks in an order that prevented tail heavy or excessive nose heavy
conditions. Even some light A/C require that you periodically switch tanks to maintain reasonable balance, usually side to side.
There is a "sanity check" that says that the balance point should be about 25% of the cord with straight wings.

I assume you are a solo-qualified RC flier. In that vein I strongly suggest two items for your model.
1. Balance your model at 25% (+/- 2%) of the CG using the Mean Aerodynamic Chord with dry tanks. Once you are well sure of the model, then adjust to whatever you wish to do with the model.

2. Do one of two things: Trim those ailerons to ZERO at the wing tip from 2-3" from their tip. Best yet is to cut the ailerons at 2-3" and reglue the short tips UP about 3-5 degrees. Makes for one "L" of a better TO and Landing on initial TO and short-final.

Full that way it won't be prone to stalling on Take-off.You don't want to be Tail Heavy!

Right??? OTOH, With a proper set-up C-G, the model will not be prone to stalling at all. If one balances a model with a full tank (what size is that tank??) then the
C.G. will be slowly moving back as the fuel is burned off. IMO you need to stick with your quad-copters. :rolleyes:

Sounds like he does'nt know where the proper c.g. is? I was just getting him in the Ball Park? I suppose you know where the chord line is on a Plane you've never seen?That matters too? I don't do Quad copters They work on Thrust alone!They use c.g. about as much as a Frisbe

Yes Charlie, I can find the chord line on any wing plan-form. After all 41 years of driving airplanes, 13 military, and 28 UAL, with 5 in the Strategic Air Command's B-47 Strato-Jet, well if I had a problem with locating CGs I would be dead now. The B-47 was very mean on keeping fuel balance.

Now if you don't do quad-copters, well SIR, you are my kind of man. :o Thanks for the reply.

Your airplane appears to be very clean, well built, and nicely covered, and according to your profile you have been building and operating for 47 years and from the picture of your airplane I believe you. With that said I do have a question; is this really the first time you have encountered a balance scenario like this? Just curious... Anyway, I do hope you find the answer you seek.

Bob

I am 61 and have been Building Models since I was a child! In most cases the Tank is slightly forward.Making it slightly Nose Heavy with a Full Tank.Balancing out as it burns off Fuel.The c.g. is going to move around a Bit in Climbing or Nose down situations.I have never noticed any differance in Control with these Minor changes.If it made a Differance in controlling the Plane.They would probably have put baffles in the Tanks?I'm not saying it's not Important (it is)But it will change slightly when Fuel Moves around.Good Luck getting through your next Tank Full!!

That might work for an engine with a fuel pump or with a pressure bladder. But for direct draw or the usual muffler pressure tank you can`t do that. The tank has to be as close as practical/possible to the needle valve. This generally means right up against the firewall with the top of the tank just at or maybe as much as 1/5 of the height above the needle valve.

Dksnyder, as some have said you ALWAYS set the balance with an empty tank. You simply live with a slightly more nose heavy trim when it's full.

As you gain more flights on the model you can consider moving the CG back in small increments and test flying to see where you hit the spot you like.

As for the first flight CG position Lew Andrews, who designed and kitted this model, was a long time flyer and excellent designer. He's got a fleet of great flying models to his credit that made it as kits and made many of us happy. So if he says "put it here" then you do what it says.

I know the rest of you are trying to be helpful but when the plan shows a CG location it's pure folly to second guess this and try something else. Any designer of a model like this is going to show a range of CG locations or a single spot which produces a safe first flight.

When it comes to CG, I think this point is key - " Any designer of a model like this is going to show a range of CG locations or a single spot which produces a safe first flight."

I think many people forget that the directions are talking about "first flight" and believe they are locked in to this location for the life of the plane - or else!

There is another point that anyone that has built several models really does no longer need -- a "Designer". It doesn't take much after doing a few kits to be able to grab some balsa, and a bit of plywood, and make a model airplane. I have done a number of them from 12 years old until my current 78. I have hoarded kits for years. About 90 or so on the rack in my barn workshop at this time. There are a number that show the "designer's" CG at aft of 30%, usually around 33%. Sad!
Some years ago I acquired a 100" wing-span Eindecker kit. (SP ?) The plans showed a CG point at 33%+ for the "balance point". I knew that was wrong so I set it up to use just aft of 30% thinking that maybe I was "...loosing it!" Well on the first test flight I was doing 3-D and I can't even spell 3-D! I kept it going for a bit then shut down and went back to workshop. At 30% of MAC it flew much better but still a handful. Then up to 28% and even better, so I finally did the standard 25%. For some years now the Eindecker makes several big bird meets a year. Very easy to bore holes in the sky and Take-Offs and Landings are very easy with some rudder help along with the ailerons being about 1/4" raised. My documents are for a rebuild that had real ailerons which I liked.
I have taken rubber power kits and plans in the 50" +/- w/s size and made very nice RC models using 25-35 engines. Use the designer in your heads guys. Lots of good modeling there if you will use it.

Yep, the idea of moving the CG to suit the flyer is a good one. Some like a little more positive pitch stability and others find they like a more minimal amount. Shifting the CG and doing the associated elevator re-trim is a great way to set the amount of pitch stability to suit the taste of the pilot. So I'm going to go along with ahicks on this.

Hossfly, the Eindecker in scale version has such a small horizontal tail that even with the long tail length I'm shocked that the company would suggest that far back a CG location. I guess that not all of them get things right. If I were to run the measurements for a proper small scale size tail through some of the CG calculators I'd fully expect them to come back to me with a CG location at something like 23 to 25%.

But for some other designs with a large horizontal tail on a good length fuselage a 33% position isn't out of the question at all. It's just a shame that they messed up that Eindecker plan with the wrong position.

Still, you and the model survived..... :D How was it to fly once you got things dialed in? And just how faithfully scale is it?

dksnyder... That is great looking plane with long moments so I think it will be quite forgiving on where you park the CG. I fly my Carl Goldberg Tiger 2 with the CG at 38%, tank empty. If I use up the tank, I get some slight porpoising on final if I let it get too slow but otherwise it is still quite tame. I tend to avoid forward CG positions as it makes slowing the airplane down for landing a lot more difficult.

If this is the SR Batteries Eindecker kit, the tail isn't scale. It was enlarged to improve the flying characteristics. Which is not to deny that some manufacturers can get the CG very wrong. Never hurts to check, and with all the online CG checkers it's easy.

Balance with empty tank. You will be nose heavy on the first takeoff because you have added the weight of the fuel forward of the CG. As you fly the first flight and burn off fuel there is no way the CG can shift aft into a tail heavy situation. If you balance with a full tank you will not be tail heavy on takeoff but as you fly around you will burn off fuel making the first landing in a tail heavy condition. Balancing with an empty tank means that you can never get into a tail heavy condition.