Just finished a BUSA 1/4 Nieuport 28 and the top wing came out +3 and the bottom +1. It flys pretty good except I run out of down elevater trim! I have the down thrust the plans call for, but cannot change the wing incidence without rebuilding the wings and struts so I have decided to install positive in the stabilizer which came out about 1/2 degree negative.
How much positive do you think it would take to make it fly level?

Sparky,

Here's the way I would do it. It's not going to be exact, but it's pretty close. First, if your radio has a "sub-trim" feature, use that to move the servo into a postion that will allow you to trim the plane for level flight. Then, assuming the stabilizer and elevator have about the same surface area, measure the angle of deflection of the elevator used for level flight. Reposition the stabilizer to 1/2 the elevator deflection. This should allow you to fly level with the elevator and stabilizer in trim. Good luck.

Paul

With the upper wing at a higher angle than the lower one, I'm surprised you are satisfied with the flight characteristics. My experience with many large bipes (2 Phaeton 90's, three Phaeton 40's, seven 60 sized Aeromasters, one Giant Airomaster and about 5 other 1/4 scale) all flew best when the upper wing was about 1.5 degrees less angle of attack than the lower wing was. Especially on the Aeromasters, when the upper wing approached the same angle as the lower wing, the plane tended to porpoise a great deal when you tried for level smooth flight. Of course CG is also very important as is thrust line when you tune for best flying characteristics. Now, if you want maximum endurance and always fly at one speed, you can trim it out with the upper wing at a positive angle relative to the lower one but you pay a big price in maneuverability and constant trim at all speeds.

Thanks Paul, I am afraid that would give the stab an incidence greater than the top wing, which seems excessive, what do you think? Maybe I am too far out of kilder to correct it that way.
Rod, the BUSA plans call for 0 on the bottom wing and + 1 1/2 on the top, must be some other desighn features in play.

on the model unless it is a woefully underpowered lead sled- set both wings the same and be prepared to adjust the horizontal stab angle
that is the full scale approach on many bipes and is also practical and the most simple approach
your downthrust is not an issue.
back in WW1 the old planes were woefully underpowered and relatively heavy -tho they made up for that in being easily flown to pieces.

Can you put some washers/shims under the trailing edge of the top wing?
A friend built one of these and we did that...put a couple washers between the wing and the rear cabane tabs.

Might not "cure" the problem, but will help.

shimming the top wing did not work well, it only decreases the incidence at the center of the wing and leaves the tips washed up, I tried shimming the struts along with it but did not work, would have to rebuild the struts. I have disasembled the tail feathers and will raise the stab 2 degrees, let you know how it comes out.

Thats the leading edge of the stab?
Right?

I believe that is not a good idea, Sparky.

Excuse me the question: are you sure it is not just nose heavy?

we all on the same page here?

Yes Dick, that is the leading edge of the stab, I am raising it 1/4" which will make it 2 degees positive. Acording to what I have read, having positive in the stab along with positive in the wing provides longitudinal stability for the airplane, (sometimes refered to as longitudinal dihedral), which will be a plus.
The airplane is NOT nose heavy, that is the first consideration, I had to add 1 1/4 lbs of lead and it is ballanced perfectly!

Sparky,

I believe something may be wrong by having both wings and stabilizer with positive angle, especially after you stated that the plane flies pretty good except with down elevator trim.

The stab is supposed to counteract the nose pitching effect of the cambered wings, for single or dual wings.

Although obvious, I have asked about the balance because the elevator trim is an indicator of unbalance sometimes.

Please refer to these articles:

http://findarticles.com/p/articles/m...2/ai_n8799373/

http://findarticles.com/p/articles/m...g=content;col1

http://findarticles.com/p/articles/m.../ai_n15640837/

Regards!

Sparky,

Is there any way you could shorten the forward attach points of the upper wing struts? If you could get the top wing to a slightly less positive incidence than the bottom wing, you might solve much of the problem. I built a RTC Giant Aeromaster that had the struts and attach points pre-set by the manufacturer. All incidences checked per the instructions, but it required a lot of nose down trim to fly. I shimmed the upper wing to reduce the incidence by 1 degree and solved the problem. If all else fails, try drooping all ailerons a couple of degrees. This will move the center of lift back a little on the wings and help lower the nose. Good luck.

Paul

balance at about 25% of chord and adjust stab for minimum elevator trim at desired cruise speed. and -rig both wings as closely as possible to same true operating angles

Thanks for the tip Paul, I did not think of drooping the ailerons but sure it would help. Glue is almost dry for the new stab position, will test fly tomorrow, weather permitting.

Sucess! Raising the incidence in the stab pretty much solved the requirement for excessive down trim, however, the airplane is still very sensitive to throttle setting, i.e., it climbs with a very small increase in throttle and looses altitude with a small reduction in throttle. I am hoping additional downthrust will help eleviate this.

If you have not solved the problem yet, I'd suggest you use the stab as a reference, set the lower wing to the same as the stab and then set the upper wing at -1.5 degrees with respect to the stab. You will need about 1.5 to 3 degrees downthrust and 1.5 to 3 degrees right thrust. make all incidence measurements relative to the stabilizer.

It's a WWI biplane...with high lift airfoil wings...
You're going to have substantial trim changes with changes in the power setting / speed...

JMO, but, It is what it is...

The stab incidence change helped (possably a lot ) but what I'm getting at is that you're going to have substantial changes in trim / attitude when you make substantial changes in throttle...just the nature of the beast.

You could re-design the whole airplane...with fully symetrical airfoil(s ) and everything 0-0-0 but then it wouldn't be a BUSA Nieuport any more

My friend's had so much down and right thrust that the engine's (G23 ) shaft comming out at such an angle (or compound angle? ) that it just looked goofy...and it still had a strong tendancy to climb with increase in speed...and decend when power was reduced...much like a trainer...he just learned to live with it.

I think I would have shimmed the T.E. of the top wing, and modified the interplane struts.

A couple of degrees positive pitch on the horizontal stab is commonly found on bipes...owing to the massive downwash from the combined mainplanes.

BUSA's solution is to add more down thrust, I have done so and will make more test flights. I have built and flown 6 other WW1 biplanes and never had this problem before.

If this is a really strong tendency then it implies that you are running with a degree of pitch stability. To confirm this I would try setting it into a trimmed slow speed level cruise that does not require any back pressure on the stick. Then push it into a 30 degree dive and let the elevator stick return to neutral. If it srongly noses back up then you are flying with a high degree of pitch stability. If it pulls back up but slowly then that is about right. For a model of this sort it should not want to keep going straight or try to tuck to a steeper dive.

If it pulls up strongly and you want to cut down on this then the correction is to move the CG back a little, retrim the elevator and try this same test again. Keep fudging it back and re-trimming until it has a reliable but slow pullout on it's own. A good setting for a model of this size and type would be for it to lift the nose to level from a 30 to 40 degree dive in about 80 to 100 feet of altitude from the time you neutralize the elevator stick. From there you can counter some of the tendency to climb and dive due to throttle by adding some more downthrust. But it's best to get the CG correct first and then use only enough downthrust to make the climb and dive tendency managable. You really don't want to get rid of all of this since the tendency to climb with increased power is very normal and very scale like. You just don't want to have to deal with too much of it.


As for positive incidence in the stabilizer this means nothing at all. It's only positive to some imaginary center line. What really counts is the angle between the wing and the tail. As long as the wing is positive compared to the stabilizer all is right with the world. The rest is just smoke and mirrors.

Thanks for the scoop Mat, sounds like you know what you are talking about! I will try your routine, the top wing still has more + than the stab. My dexterity and reactions are not what they used to be and I need to slow the airplane down and get a realy good reading on it's response, will try a lower pitch prop to help.

Sparky,

If you able, try mixing your throttle to the elevator. I fly a quarter scale Gee Bee that I've had to mix the throttle to the elevator and rudder. Really helped tame the beast on take off and go around's.

Paul

Mmmmm..... programmed in "downthrust" er.... you know that's what we call "CHEATING" don't you?

While this trick would work for power on to power off it could backfire on you if you don't know the side effects. For example suppose you go into a full throttle dive and for some reason want to reduce to idle while in the dive. The sudden shift in elevator trim will make the nose want to rise sharply instead of maintaining the dive. Similarly if you're floating along nose high and stuff the throttle to full the initial tendency will be to dive a little until the speed rises enough.

I'd still suggest sneaking up on a best pitch stability CG for each model and pilot and then fudge in some mixing at the Tx if you want more.