incidence design
 

Just getting ready to finish up a model that I modified from the original plans and was starting to question the wing and horizontal stabilizer incidences. The plane is the seamaster that originally had a pod mounted engine. The engine thrust line was 5.24 inches above the wing centerline. The wing is semisemetrical and has 1 deg positive incidence to the engine thrust line. The tail is a T-tail design and has the horiz. stab at 2 deg negative incidence to the engine thrust line.

Here is the modifications I made. I pitched the engine pod and made the plane a twin with the engine thrust line at about 1.7 inches above the CL of the wing. I did not change the incidences of wing or stab yet. I did not change the engine thrust angle either.

I am thinking that with the original plan with the engine so high above the wing, the stab neg. incidence was added to counter act the moment the engine thrust would have above the wing. Should I take out the 2 deg neg. incidence from the stab. before I finish it up?

Please, any suggestions would be appreaciated.

RE: incidence design
 

here is a picture of what I am talking about.

RE: incidence design
 

It's an unusual configuration...the high thrustline will have a definite nose down couple, dependent on power. Some engine up thrust would be the better way to control this tendency. Stabiliser to wing difference controls the 'S&L trimmed speed' but being also immersed in the prop slipstream will mean that it will have a definite 'power on/power off' trim change as the tailplane load (lift) changes with the local airspeed. Perhaps reducing the stabiliser to slipstream angular difference will help reduce this, but maintaining some small angular difference to the wing to maintain a bit of speed dependant trim for easy flying would be the way to go.
Evan, WB #12.

RE: incidence design
 

The Sea Master was designed by Ken Willard. Ken was smart enough and experienced enough to know that the model had to still fly well at reduced power and idle settings. So he did not set the wing to tail angles such that they would compensate for thrust effects. Instead he would have angled the thrust line to counter act any thrust related issues that would occur at full throttle. That way the model would not suffer from radical flight trim changes from high to low throttle.

So keep the wing to stabilizer angle as it was shown in the original model. Alter the thrust lines as required to make it fly well at high throttle to low throttle.

RE: incidence design
 

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RE: incidence design
 

I flew the plane today and the plane jumped off the water at about a 45 degree angle. I had to adjust center on the elevator about 5 degree down to get the plane to fly level. Is it just trial and error to get the down thrust in the engines right so that the elevator can be parrellell to the horizontal stabilizer?

RE: incidence design
 

Looking at it another way, there are three degrees negative in the stab and one degree down thrust. You had to trim considerable down elevator for powered flight. I think before I messed with down thrust, I'd reduce the 3 deg decalage disparity and observe. Three degrees is a lot.

Where is the power off trim?

RE: incidence design
 

AA5BY, I only had 2 flights after adjusting the elevator. I did not pay much attention to power off trim at the time, I guess when i was comming down for landing I just flair in up elevator as it slows down to land. I will pay more attention to off power trim the next time I get out, hopefully by tuesday evening. I know I did not have to put any down elevator input in with power off, so I would say it is either even or just a little up elevator.

I think your right in taking the 3 degree neg incidence out of the mix. I guess I need to figure out how to cut the horizontal stab off the tail.

RE: incidence design
 

Can the wing be shimmed to raise the trailing edge? That would be basically the same as changing the horizontal slab. If so, make changes in small increments no more than 1/32 at a time. Decalage is the relationship between the incidence on the wing and stab... so it isn't always important which gets changed though the flight pitch attitude might not be what is desired.

Pitch coupling unbalance can be corrected by up/down thrust IMO only if it is minor.

RE: incidence design
 

That might be a lot easier. I could just shim the back of the wing up by small amounts. Since it is a seaplane, I have used silicone to make a rubber gasket. I wonder if silicone alone would be enough to hold the rear of the wing up. Just put in some silicone rubber and cinch the wing bolts down to allow the amount of shim, and let dry.

RE: incidence design
 

You actually want to play with the CG to reduce this tendency first. A strongly forward CG combined with the higher amount of wing to stab decalage will respond like you describe.

Do a search for "dive test" to find a write up on how to determine a good CG placement. Once you set the CG to suit how you like it to fly THEN play with altering the wing or stabilizer angles and downthrust angle. You'll have a much nicer to fly model if you do the steps in that order.

RE: incidence design
 

It looks like by rough calculations, 1/32" in the rear of the wing of a 12" cord will give about 2 degrees. I think I will just put a wood shim in the rear of the wing saddle, seal the gaps with grease and fly it. I can go more or less this way between flights until I get it right, then I can make a perminant structure to support the wing and cover it.

What exactly am I looking for in flight. I assume that I am looking to get little to no pitch up or down from full throttle to low throttle or vise versa, and to have the elevator close to the neutral position.

RE: incidence design
 

BMatthews, I actually have a rearward CG right now. The CG is supposed to be in the middle of the main spar and I am behind that by 3/8 to 1/2 inch. It flew rather nice after I set the elevator center down. I think I was wrong with the previous statement I made of about 5 degrees down however. After taking off after adjusting the elevator, I had to trim in some up elevator so I am only about 3 degrees down elevator to make the plane fly neutral.

RE: incidence design
 

Seems like a plan to me and I agree that the first step is the CG.

RE: incidence design
 

Tail heavy night be the cause of your needed down trim. A tail heavy plane will also tend to lift off with no or very little elevator and the climb out might be too steep and of course the elevator is also often too sensitive. For sure... put the CG on the recommended position for an established model. As suggested, Ken Willard was a good modeler and I'd give him the benefit of having posted a proper CG unless there was very good reason to think otherwise.

RE: incidence design
 

Ok, I'll add a quarter or two to the nose and see what happens first. That seems like the easiest of all things to get started, just a little duct tape and 2 bits. If that takes care of the problem I will make it permanent. The elevator is not too sensitive for me though. It was just a little different on that first flight, holding in down elevator when landing when you are used to holding in a little up elevator, just seems a little backwards if you know what I mean.

Just a thought though. With 12 oz of fuel in front of the CG, my balance point is where it should be, if not just ahead of it. The original plan is using at most a 8 oz tank. It was fully fueled that I originally had the jump off the water the first flight. So I don't think it is the CG that is my problem. I will check it though.

I understand Ken Willard was a great designer, however I have modified this model quite a bit and it is the modified version of Kens plans that has a different airfoil and wing area, so perhaps the incedences are not correct for my configuration. Thats why I am happy to have the help of all you guys here on rcuniverse. I know how to build and fly but I don't know much about aerodynamics.

RE: incidence design
 

I don't know if these pictures would help you all out in figuring this out or not but these first three picture are on first flight seconds 37, 38, 40. The elevator was set to neutral with the horizontal stab. and I did not put in any up elevator at all in these shots. I did put in down elevator between seconds 38 and 40 though.

RE: incidence design
 

These next two pictures are second takeoff after adding in considerable down elevator into the trim. I did add a little up elevator on this one to get it to break the water, but it climbed out smooth and flat with a little up elevator.

RE: incidence design
 

Nice looking plane. And... no question on your building and flying skills. That you knew what the numbers were showed your savvy.

I was generally aware of the subtleties of incidence but had an experience a few years ago that drove the point home. I'd finished a very old P-51 that had been given to me partially built back in the '70s. On the maiden flight it didn't break ground easily, and required considerable up elevator but otherwise trimmed out with only a small amount of trim. I wondered if it was nose heavy but it flared fine on landing with adequate elevator authority. I only flew once.

On the second outing, expecting once again the need for plenty of elevator to get airborne, I was greatly surprised when it jumped into the air while building speed prior to giving elevator. How could that be? The only thing that had changed was that the plane had been trimmed for flight between the two takeoffs. Then it hit me. The wing configuration was front bolted and there was some albeit slight leeway in torquing the bolts and seating the wing in the saddle. On the first flight I'd inadequately tightened the bolts and on the second flight they were tightened too much. The differences might have ranged only 1/16 to 3/32.

Fortunately, both were within the parameters of air worthiness and that your flights have been successful is affirmation of being in the ball park to wit you are commended. Finally... I doubt that you are seriously tail heavy. Though the lurch into the air could have been caused by tail heavy... consuming fuel often aggravates such condition to a very sensitive elevator to wit you didn't complain. More likely the issue is as your question implied... an incidence issue.

RE: incidence design
 

AA5BY, Thank you for the your response and the valuable information. It doesn't seem like 1/32" would equate to much but it does change things by 2 degrees. As in your case of different torque of the bolt making the difference in the way the plane takes off. I think I may try a minor 1/64" shim and if that helps a little, go to a 1/32" shim. The plane actually flys really well with the down elevator trim, I just thought that if I could get it to fly with the elevator in the neutral position, I would have less drag. I guess I am the person that can't leave good enough alone, I just got to make it better.

Thanks again everbody for all your input.

RE: incidence design
 

By my rough calculations (HP calculator) it will take about 3/8" to get a 2 degree incidence change on a 12" chord. 1/32" will only get you .15 of a degree change. Nothing wrong with making changes in 1/32" increments, just don't be surprised if you're unable to see much difference with the first increment.

Dick

RE: incidence design
 

Unless that Seamaster's a bipe, decalage has nothing to do with it. Decalage is the difference  in wings angles for a biplane:

http://en.wikipedia.org/wiki/Decalage

The wing has an incidence angle, as does a fixed stabilizer. The full-scale Merlin IIIB I flew years ago had a full-flying stab, so it had no angle of incidence.
It's a common mistake, even well-known writers like Peter Garrison get it wrong:

http://www.flyingmag.com/pilots-plac...dence?page=0,2

RE: incidence design
 

I agree with Dick.  My CAD system says it will take 3/8 of an inch to give 2 degrees on a 12" chord.  You might want to try it 1/8" at a time.

Jim

RE: incidence design
 

Eddie:

You didn't read far enough into the definition of Decalage on Wiki. The third paragraph says:<span style="color: #0000ff">Decalage angle can also refer to the difference in angle of the chord line of the wing and the chord line of the horizontal stabilizer.</span> And of course the definingdrawings show a monoplane.

So we are all correct.

Jim</p>

RE: incidence design
 

Jrfpac,
Thanx. I was aware, but am just old school. Wing/tail incidence being called decalage seemed to occur in the early eighties. Prior to that, no one I talked to ever confused the two. I build bipes, so use it for those.