How does a plane that is tail heavy behave compared to one that is nose heavy?

A very tail heavy plane can be dangerous because it may get off the ground and not be controllable. A very nose heavy plane may not get off the ground so danger does not present itself.

Right, and depending on the plane, it can go from good CG to dangerous tail heavy with a small shift in weight. A 3D plane is a good example, Short length and huge surfaces with a lot of throw. That last 1/4"hasto be done in steps or you might not bring it back.

Don

A nose heavy plane is very sluggish, making you feel you don't have enough elevator authority. A tail heavy plane will feel too responsive, making you think you have too much elevator throw. I tail heavy plane will also be difficult to trim (pitch). The nose will hunt. One moment you think you need a bit more nose down trim, the next moment you think you need more nose up trim.

A tail heavy plane is hard to land because of pitch sensitivity. A nose heavy plane is hard to land because you run out of elevator and the plane comes in like a rock.

Simply speaking,
Kurt

Good thing about the small shift threat, is for a small shift to cause dangerously tail heavy, the CG has to be aft (and close to dangerously aft) already. It'll be far enough aft already that anyone would have already known the sucker was close to dangerous.

Small shifts of CG won't suddenly wake a sleeping monster until the CG is already aft. Since most CG shifts come as a response to how a model is flying, when the monster is aroused it shouldn't be a surprise for the guys who're wanting more response to an already responsive airplane, and shifting it aft a little to see if there was more response to be had.

If you're working with a model and want to know how close the CG is to "dangerously aft", a good tip is to check elevator throw.

If very little elevator deflection gets lots of response, your CG is aft, bet on it. And if it takes little elevator movement to stall the wing, same thing.

in very general tems a nose heavy airplane will be more stable, as the cg move back the airplane becomes less stable. Keep going aft and it will become so unstable that it is uncontrollable. Unstable is good for maneuverability as long as you don't go too far aft. Too stable (cg too far forward) can be bad as you will loose control effectiveness. Speed also increases as CG goes aft.

[8|Thanx for all the input. I maidened my nitro seawind yesterday , now i have read all the threads on this plane and most problems are getting of the water. But there are some questions regarding CG. I have the CG as per instructions but it seems that these are no good. this is what happened. I straight away needed a lot of up elevator to stop her diving badly.I have built and maidened many planes but never before was the trim so far out that I could not trim it in flight without help. I could not let go of the stick and trim fast enough,she would dive straight down,but if i tried to climb she would shoot straight up. it was as you say as if elevator was to sensitive but wildly so . But the tendency to dive confuses me[:-][:-]
I managed to land with great difficulty but she did seem better with engine cut and flaps down. The factory CG is 90 mm, I need to get it correct before I try again as I dont think I will be as lucky a second time.
Suggestions please...[&o]

At the beginning of the flight, the model should be nose heavy respect to the ideal recommended CG position, due to the fuel weight.

Therefore, it seems to me that the problem may be in the incidence angle of the wing.

I believe so, because the tendency to tuck down that has to be compensated with strong elevator may indicate the wing positioned by the stab slightly negative and close to zero AOA.

I suggest trying increasing the AOA a little bit or verifying the current decalage with an incidence meter.

Want to know where the CG should be for flight? Get a yardstick and make the 9 measurements this application ( http://www.geistware.com/rcmodeling/cg_super_calc.htm ) takes for input. Plug the measurements into the blanks and then fill in 5% for Static Margin and run the app. Record the CG returned. Then change the 5% to 15% and run the app again. The CG returned, combined with the 1st, is the reliably safe CG range for that airplane.

ALO, you are describing the symptoms of a tail heavy plane. It's surprising, because manufacturers usually specify a very conservative balance point, so make sure you are measuring it correctly.

Tail heavy and a diving tendency are not contradictory at all. A tail heavy plane is unstable in pitch, so it is almost impossible to trim the elevator to a happy medium that will result in level flight. A little down, and the flight path will deviate down more and more; a little up, and it is the opposite. It makes a plane pretty hair raising and you have to fight it every moment.

The quickest way to check for this is simply to move the balance point forward a little. It probably won't take much to make a big difference.

Jim

Thanx for all the advice.
I have plugged the values into the blanks as such,measured in inches
"A"= 10 "AA"=6 "B"=10 "BB"=6 "S"=0 "SS=0 "Y"=32 "YY"=12 "D"=28
It gives me a wing aerodynamic center of 2.5 inchs that is 6,35cm a lot forward of the 9cm cg of the manual.
Would the location of the engine make a difference? Engine on this plane behind the cg mounted on the tail.

"Wing aerodynamic center" isn't what you're looking for. The application computes a CG location. You want that value when the Static Margin is 5% and the value when the SM is 15%. That defines the CG range that'd be reliably safe for your model. The idea is to run the application with 5% and then 15% and get the CG range.

If you look down the page below the number you're looking at, with the 9 measurements you list, the "Ideal CG aft of the root LE" is shown as 3.31" for 15% SM and 4.31" for 5% SM. The application is showing a CG range of 3.3" to 4.3" measured at the wing root from the wing leading edge.

You balance the model to discover where it's CG is located. Devices are sold that do that job, but you can also use your two index fingers. You find out where the CG is that way. You find out where it should be by reading the mfg's suggestion in his manual, or using an application like geistware's, or working out the formulas yourself.

Thanx I see it now,that would make the factory CG correct. I have measured the CG again on a device I built and have used on all my planes at it is at factory spec. But it is almost impossible to get the plane to balance the slightest movement back or front will cause it to tilt . This must be caused by the engine location (On the top of the tail). If I balance the aircraft upside down I get the same reading but it is far less sensitive. All threads on this aircraft show discrepecies with the CG but no one has an answer yet. It would seem to me that CG on this plane is very critical as shown buy the difficulty of balancing it. I am still not sure how much if at all to move it forward.
Thanx for getting back to me so soon.

Is this your model?

You can not balance it straight up, position for which it is unstable; it is not that the CG is too sensitive.

I suspect improper thrust angle here.
Did you note any difference in behavior between full power and idle?
Are flaps active? If so, did you note any difference when using them?

Yes thats the model,to be honest things happened a bit to fast to say. I was to busy to notice but i do think it was a bit better with throttle closed and flaps down. Although a hard landing was still caused by the "porposing"affect of the elevators.

The symptoms are very clear. Even when slowing for a landing, the plane is porpoising. The CG is too far to the rear. I use a formula on my planes too, but airplane flight is complex and the ultimate guide to CG is how the plane behaves in the air.

Now that I see the design, it is obvious that this plane will be sensitive to CG, because the tail arm is very short and the stab is very small. That means the stabilizing force is smaller than it is on most airplanes.

At this point you have already flown the plane and it is exhibiting exactly the symptoms of a CG that is too far back, so there is no mistaking what to do. I would move the CG forward. How much? It's a guess until you try something and fly it again. I would try about a half inch forward of where it is now. It will be much more flyable. Moving the CG forward is quite safe within reasonable limits. You will need more up trim with a more forward CG, but it won't be unstable as it is now.

I cut two thin strips of masking tape and place them parallel to the wing at the balance point that I want. Then I just use my fingers. Sounds like this one wants to be upside down for CG measurement, so put the tapes on the top of the wing and turn it over.

Jim

ALO 111,

If the fuel tank is located behind the CG, the model should be balanced with the tank being full, which is the tail-heaviest condition.
While the fuel is used, the CG moves forward, which is safer.

This site you can find video, review, manual and specifications of a model that is very similar to yours:

http://www3.towerhobbies.com/cgi-bin...&I=LXKEW6&P=ML

Nitroplanes Seawind Seaplane 60
Wing span: 70 in / 1780mm
Wing area: 703 sq in / 45.5 sq dm
Flying weight: 9.1lb / 4130g
Fuselage length: 57 in / 1450mm
Engine Required: 2c 0.61 cu in
4c 0.91 cu in

Great Planes Seawind Amphibian
Wingspan: 71" (1805mm)
Wing Area: 676 sq in (43.6 sq dm)
Weight: 10.25 - 12.25lb (4620 - 5555g)
Length: 56" (1410mm)
Engine: .60 cu in (10cc) 2-stroke
.70-.91 cu in (11.5-15cc) 4-stroke

Center of Gravity: 1-1/32" (26mm) Back from the wing's leading edge.
This aircraft does not have a CG range, do not move the CG forward
or aft of the above figure.
Control Throws Low Rate High Rate
Ailerons: Up & Down 3/8" (10mm) ½" (13mm)
Elevator: Up & Down 7/16" (11mm) 5/8" (16mm)
Rudder: Right & Left 1-1/8" (29mm) 2" (51mm)
Flaps: ½" (13mm) 1-1/8" (29mm)

http://www.greatplanes.com/techsupport/technotes.html

However, in your post #8 you stated that “The factory CG is 90 mm”, which shows a great discrepancy between two equivalent models of 64 mm (2”-1/2)!!!!!!!!!!!!!

Here you can find a RCUniverse review of the Great Planes Seawind (sorry, couldn’t find a review for Nitro Planes’):

http://www.rcuniverse.com/magazine/a...article_id=723

Finally, you are not alone.
This thread illustrates how others have faced similar problem with that model:

http://www.rcuniverse.com/forum/m_4780929/tm.htm

Copied from post #6 there:
“My NP Seawind had the CG set @ 3.54" behind the wing LE. It went totally out of control. Since the plane fairly screams "TAIL-HEAVY," I set the CG to 1" behind the wing LE (following the Great Planes instructions instead). The Seawind took off (without a water rudder), flew beautifully and landed very smoothly. It required 3lbs of dead weight in the nose to balance it. It now weighs 11lbs with 3 lbs of useless weight. I am thinking of installing an electric motor and battery in the nose with a l-o-n-g shaft to a steerable water propeller in the tail. Might as well make the weight useful.
It is unfortunate that, as useless as the NP Seawind manual is, one of the few solid pieces of information (the CG point) is disasterously AND INEXCUSABLY WRONG, WRONG, WRONG! Maybe they meant 3.54 centimeters”.

I went out one day with a trimmed out freeflight model and a roll of solder. I added solder to the tail between flights. It went from a near vertical climb (under power) to a less steep climb to level fight and I even got it into a shallow dive by moving the CG back more and more. I did some calculation of free flight CG, tail volume, etc and came to the conclusion that there is about a 10% range of Mean Average Chord for Cg position ranging from marginally stable to really stable.

I cannot thank all who have offered there help and advice enough,you guys are the greatest. Everyone came with a positive constructive attitude and lots of years of experience.
I am going forward ..lots forward with the cg and I will let you all know what the result is . Once again thanks to all.

Next, for precise location of any CG, you may want to consider building one of these excellent rigs:

http://home.mindspring.com/~the-plum...%20Machine.htm

http://www.rcuniverse.com/forum/m_70...tm.htm#5534736

Best luck with your model!

Hi.
Here's a link to a video that I made of the seawind. Its in a thread under seaplanes.
Hope you like it<a href="http://www.youtube.com/watch?v=VTlG0KspxZU">

http://www.youtube.com/watch?v=VTlG0KspxZU</a>