I have a plane that I am too far along to change what I have done. I have a large twin that when I was installed the horizontal I have added 1-1.5 degrees more positive incidence to the tail compared to the wing. I did not catch it until it was too late. I know it will make the tail come up fast and I will have to counter it with some up trim for level flight.
Should I be really worried about this? Will this make my CG more critical? Will I have to keep the speed up more on landing? Would It tend to stall easier?

Thanks In advance

Ty

It won't do any of those things because you'll have to compensate for this by using that much more up elevator trim which will cancel out the bad stabilizer trim. The model will fly as per normal.

Depending on how much negative incidence the stab had to the wing on the plans having 1 or 1 1/2 less negative isn't neccessarily a bad thing. But if the stab is actually at +1 to 1 1/2 compared to the WING's angle, so the stab has more positive angle than the wing, then I strongly recomend that you cut into the model and correct the issue. A stab that is more +'ve than the wing will require a bucket load of up elevator trim and won't look very nice.

I built this plane from scratch. I have always zeroed the tail to the wing in the past. I fly mostly Meister scale planes and used the same airfoil on the twin. That airfoil is mostly flat bottom. I rechecked the stab to the wing and I believe it has a 1 degree more positive that the wing sets right now.
I built a 3 engine plane and set it up the same way and when I flew it I had to trim it with about 1/8" up trim when i fly it straight and level. You really can't tell or see the trim that I put in it. The tail does come up fast and I do have to add up elevator to make it not tip over.
I am worried about this new model becuase it is a much larger model and the weight is double. If I am not worried about the look of the trim am I OK?

Ty

The incidence in the tail will have most effect on takeoff, making the takeoff run longer, is about all you'll notice.
It's easily trimmed out in flight anyway.
Shouldn't be any problem as long as you're aware of it.

You'll definetley need a bit more up trim this time but that should be the only real effect. To help compensate I'd be sure you go for a bit of up trim right off the bat just to help reduce the workload. Start with the same ANGLE as your other models. The angle is what is important and not the actual measurement at the trailing edge.

Good luck with it.

PS: Since flight and static effects can sometimes differ you may find that the extra incidence on the ground may tend to want to lift the tail when taxing with little bursts of throttle. I say this because the raised leading edge may create a little more of a hovercraft or Ikranoplane type effect as the prop blast packs air between the higher incidence stab and the ground. Paul, is that sort of what you were implying? This may also show up when you first advance the throttle for takeoff. Just be ready with a bit more up elevator than normal and be sure to release it as the speed, and the tail, comes up. Of course if it is a larger model and the stab is mounted higher or it's got trike gear then this probably won't be noticable.

Thanks your your advise. I just needed some comfort. I have been working on this plane on and off for 4 years. Had too many other projects. It should be a real floater even at 45lbs with the airfoil I put on it. It has a 126" wing span and 2005 sq inches of wing area. I plan to have it were I can add alot of up trim if needed.

Thanks Ty

All I can say is....

WOW ! ! !

Just a thought. Are you sure that you're using the actual peak of the leading edge to the trailing edge as your wing incidence line to compare the tail to? It may be an illusion of the picture but it seems like the wing has lots of positive incidence compared to the tail.

Of course nothing will alter the difference since you've been using the same setup on a number of models. There WILL be a slight change compared to the others.

.
A buddy built an ARF Cessna which took forever to get off the ground, but once it did, it popped off and flew way too fast.
I flew it once, and when I landed it, measured the horizontal incidence relative to the wing... serious leading edge up on the horizontal, which forced the nose wheel into the ground until it got enough airspeed to rotate and fly.
One of the things we do in the Kadet Team is add about 1/4" leading edge up to the horizontal when building the fuselage.
Or raise the wing trailing edge about the same amount. The result is the upper speed limit on the plane increases, with not too much degradation of the low-speed stuff.
Ty's problem will be a bit more serious on takeoffs, since he has no nose wheel, and might he be prepared to have some up-trim in at takeoff, if the incidence is really as he says.
Hold up elevator though is an invitation to a tip-stall at liftoff.

Oh, for sure.

I was thinking only of the first part of the takeoff to help reduce the nose over tendency. As soon as the speed is up a little and you know the elevator has authourity then it should be tapered off quite quickly. I fly tail draggers exclusively (only two trike geared models in all my time) and for me it's automatic to hold full up for the run up and until the model has a bit of speed. It's usually back to neutral by a running pace unless it's an old timer in which case it's neutral by a jogging pace.

All I can say is...

Roll up those paper towels, man! Have some pride!!!

All kidding aside, it's a simply gorgeous model! I wish it were mine, and I hope your tail incidence works out fine.

-David

The pic i put here is decieving. The wing is flat along the bottom of the fuse. It is a very thick airfoil. I can pull off the wing just outside the the nacelles. I put a straight edge down the center line of the airfoil, from the tip of the trailing edge through the center of the leading edge. That is what I used to set the incidence. I have always done it this way without problems. I just got a little more on the horizontal than I planned. This set up will make a flat bottom airfoil fly straighter with less climbing with greater speed if they are both Zero. Again my concern was 1 degree too much..

Thanks for the comments on the plane. I love to built unsual projects....


Ty

Hal Debolt has used an incidence setup for pattern for a number of years that hat the tail about 3/4 to 1 degree more positive than the the wing. This is with symmetrical wing and tail airfoils. His comments were that it grooved much better than a standard 0 - 0 setup.

I wouldn't expect anything different with your - somehow "airplane" as a term just doesn't do it any justice. Nice work. You can send it to my home address anytime.

Hands off there Ben, I saw it first...

Tall Paul,
You sure got a bunch of advise and perhaps some good in it!
However, I did not see any suggestion of the basic aerodynamics envolved. This would be your real answer.
Any airplane> by itself the wing tends to rottate, leading edge
drops and rotation follows, increass with airspeed.
The horizontal tail is used to correct that tendicy and keep the wing flying
as desired. To do this the tail must produce lift.
When the amount of lift of the wing is counteracted by the tail lift on the
end of the tail moment arm there is a lift proportion. The correct proportion will be effective over a wide range of flying speeds.
So, it would seem your situation is OK. Would believe close enough to
allow flight. If in flight you found some up trim needed the next chance
simply move the balance back a bit at a time until the trim is no longer
required.
Do keep us informed about progress!
Good luck!

Hal deBolt

Thank everyone for your responses.
Well I took the Ju-88 out htis last weekend to a fly-in that had a really large run way. I have really been stressed over this horizontal incidence. It was late in the afternoon with only about a 5 mph headwind. I rechecked everything out on the plane. I went ahead and put about 4 clickes of up trim in the elevator to help counter act the incidence. I did several fast taxi runs and the tail came up fine and the plane became light on the wheels without any trouble.
I finally set the plane up on the center line slowly advanced the throttles. The tail came up she tracked straight down the runway and lifted into the air. The plane had plenty of power but it was not fast. Gear came up fine. She trimed out with about 4-5 more clicks of up. It rest of the flight went fine. The plane had no attitude change with the addition of full flap. It come in really nice and slow with full flap. The landing was really gentle but i still had one of the gears fold back. I am using the Robart rotating retracts. The plane was so low that ther was no damage. Overall I am really please with the first flight. I had hoped to have in flight pic's but I have not got the from My friend yet. I'll post the later, If he got them....

Ty

AWESOME!

Congratulations on a successful first flight!
You mentioned adding more up trim...did you end up with a lot of up elevator to maintain level flight?

I have a little up trim to maintain level flight but it is not enough to notice.


Ty

Yes, a hearty congratulations for a successful first flight of a great looking model.

I know the details were set in concrete before you wrote your first post but if we were able to ease your fears and help contribute to a more relaxed first flight then it was all worthwhile.

Pity about the retracts. Hopefully you can arrange for a more positive lock for the next flight.