Dihedral differences

[stranex]

Hi,

Not sure if this is the correct forum, if not please feel free to move somewhere more appropriate!

Anyway, can anyone tell me what difference in dihedral is "allowable" between one wing and the other?

I am just building a H9 Spit and purchased a digital level which I thought would help me check the angles were all square. Only problem is, it is SO accurate that I keep adjusting, rechecking, sanding etc but finding it IMPOSSIBLE to get each wing to the exact same angle! Will 1 degree difference cause me any problems or am I just being unneccesarily fussy?

[MCarlton]

Are you sure it is the dihedral? I don't know the model, but what are you using as a reference level to measure the dihedral?

Make sure you are not trying to compensate for an out of square fus

[stranex]

I'm pretty sure it's dihedral? Basically it's the angle the wings make with ground when viewed from head on?

The fuse is a little out of square with the vertical stab so I thought that if I make sure all the flying surfaces (ie vertical stab which is already attached and can't be adjusted, wings and tailplane) are square then I should be ok?

What I've been doing is to place the a/c so the vertical stab is at 90 degrees and then measure the angle of the wings by placing the level at the same point on each wing and reading the level. Obviously the angle differs depending on where exactly on the wing you place the level but by comparing at several places the angle on one wing is about 0.6-1 degree greater than the other (around about 4.5 degrees at the mid point root on one side and 5.2 degrees on the other side)

[MarkusN]

If you get differences of one degree I'd hazard a guess that your level "fakes" its precision with its digital display and isn't really that precise at all. A difference of 1° left to righ would result in quite a pronounced roll input and needing corresponding aileron trim.

[stranex]

Thanks Marcus, but why do you think my level isn't that precise?! As it's not flown yet I obvioulsy don't know how much roll it will produce hence my measuring now.

How do you guys normally test for this? Mk1 eyeball? If so, surely a 1/2 to 1 degree difference is VERY difficult to spot anyway?!

[stranex]

Re-reading my original post I think I may have confused some people so just to clarify.....

If my wings are "cantered" ever so slightly so that the angle between the vertical stab and one wing is about 0.5 degrees greater than the other, will the produce a noticeable rolling?

Also, how do all you builders out there (and there must be thousands!) ensure that the vertical stab and wings are square such that the angles mentioned above are equal?

[mnowell129]

I think you are overthinking it. "noticeable" ? probably not. If it's hard
to see with the naked eye, this might mean one click of rudder trim to
correct. You may have more than this due to other factors.
Don't worry about it, go fly it.

[stranex]

Thanks! That was I was hoping to hear! Just that after a 4 month conversion to electric, air retracts, battery hatch etc etc on only my 2nd ever model and LOTS of money, I'm trying my best to make the 1st flight as smooth as possible!

[MCarlton]

The fuse is a little out of square with the vertical stab

If that is the case, your wing will therefore be out of square with the vertical stab, thus, when you put the vertical stab upright, the wing appears to have odd dihedral. My guess is that it isn't the wing that is the problem, my guess would be (without looking at it in the flesh) that your vertical stabiliser is not truly vertical.

Here are some checks to make.

Measure the angle between horizontal stabiliser and vertical stabiliser (should be 90 degrees on a spit)

Hold model with wings level and squint from the front, do the tips of the horizontal stabiliser "meet" the wing surface at the same point on each side?

how do all you builders out there (and there must be thousands!) ensure that the vertical stab and wings are square

Mainly by dry fitting, measuring, sanding, dry fitting, measuring, sanding etc little by little until it all measures out ok. Include in that, distances from each wing tip to the corresponding tailplane tip. Wing tip to fin tip, wing tip to fus centre line at firewall, then the checks I said about earlier.

All I would say is that if you can notice it now, experience tells me that will drive you round the bend, because you will "see" it all the time.

[Texas Buzzard]

Re-reading my original post I think I may have confused some people so just to clarify.....

If my wings are "cantered" ever so slightly so that the angle between the vertical stab and one wing is about 0.5 degrees greater than the other, will the produce a noticeable rolling?

Also, how do all you builders out there (and there must be thousands!) ensure that the vertical stab and wings are square such that the angles mentioned above are equal?
.................................................. ....................................
I don't know if your plane is a low winger or a high winger. If it's a Spitfire ( WW II) it's a low winger.
A high winger is not a sensitive to misalignment as a low winger is.

BUT if you are within 1 degree of having the same dihedral you are A-OK.
Normally we measure the dihedral of the wing before we mount the wing on the fusalage. I never use a protractor - I measure the distance between the wingtip and the building board. I use a little more dihedral for a low wing plane than for a high wing plane. Don't worry. As said before your trims will make it fly. :)

[stranex]

If that is the case, your wing will therefore be out of square with the vertical stab, thus, when you put the vertical stab upright, the wing appears to have odd dihedral. My guess is that it isn't the wing that is the problem, my guess would be (without looking at it in the flesh) that your vertical stabiliser is not truly vertical.

Exactly what I've been trying to explain! Trouble is, the vertical stab is already attached (it's an ARTF) and so cannot be moved without some major surgery. Beacuse of this, like you say, the wing has a SLIGHTLY odd dihedral which I have almost corrected by sanding one side of the wing seat and enlarging the fixing bolt holes.

Measure the angle between horizontal stabiliser and vertical stabiliser (should be 90 degrees on a spit)

Hold model with wings level and squint from the front, do the tips of the horizontal stabiliser "meet" the wing surface at the same point on each side?

Again, this is what I've been trying to explain! I am doing this to ensure all the flying surfaces are square but by "squinting" you surely cannot be 100% accurate, same as with my digital level being 1/2 degree out? The fact that the fuse will be ever so slightly out of alignment with the flying surfaces doesn't concern me as much as the aerodynamic effect of the fuse is negligble

Mainly by dry fitting, measuring, sanding, dry fitting, measuring, sanding etc little by little until it all measures out ok. Include in that, distances from each wing tip to the corresponding tailplane tip. Wing tip to fin tip, wing tip to fus centre line at firewall, then the checks I said about earlier.

As mentioned above, I have been sanding trying to get it accurate but whenever I change something, another measurement becomes "out". That's why I wanted to know what margin of error I could work with.

I think I'm going to quit trying to get it within 0.1 of a degree before I go insane and go fly the thing! Will let you all know how it goes....

[JetPlaneFlyer]

If it looks right to inspection by the naked eye then it's more than good enough to fly fine.

Perhaps if you were building a state of the art contest model the final fraction of a degree of precision may be worth worrying about (though even then I doubt it makes any measurable difference to flying performance).. But for a fun/sport model what looks right is right, don’t worry about it, just fly it.