V-Tail Incidence
 

Since I don't have much experience setting up V tailed airplanes, I thought I'd check here to see if my procedure is correct.

I bought a NIB Lanier Predator ARF at a swap meet this weekend - cheap, and I thought I'd put it together for 424. From my research, the version I have is the up-dated version of the V1 Predator 500. It has the glassed 3/8" firewall, 1/4" ply wing hold-down plates, 4-40 torque rods.

I'm getting ready to glue the tail on, and set my incidence meter on the wing and zero'ed. I put a bubble level on the V-tail saddle and it's way off (high in the back) which would require down trim to correct (assuming it's off). I have read that the Predator and the Viper both have negative incidence in the tail.

I just want to verify my methodology for checking the incidence before I make any adjustments.

RE: V-Tail Incidence
 

Sounds like you have done your research correctly.  I am putting a Viper RTC together and I haven't used the incidence meter yet.  I need to order some and get'er done.<div>
</div><div>Will you make it a legal 424 plane and come to the race at Old Julian airport the first weekend in May??</div><div>
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RE: V-Tail Incidence
 

I'd love to make the Julian race, but I'll be out of town that weekend. I was really looking forward to showing Vic how it's done - agian. I'll have plenty of practice by the next race in Sept.

RE: V-Tail Incidence
 

The setting up of a vee tail on any model can pose problems for those not familiar with geometry. Simply put the dihedral joint and the centreline of the fuselage( plan view ) is where you should check the incidence....not the fuselage sides! The block sitting between the top surfaces of the tailplane is not a good reference point- although logically you would think it should be. Ideally the tailplane and wing should be 0,0 and if you lift the trailing edge of the tailplane about 1-1.5mm then you will be very close to the correct position...noting that because the fuselage in plan view is tapered that the tailplane will look wrong (when viewed from the side) is in fact correct.
Sometimes a nose heavy model will need a similar adjustment but that's another subject.
FWIW if you measure the centreline of the leading edge of the tailplane to the infill block and similarly the trailing edge you will find about 1.5mm ( on memory) variation..
Hope this helps, Trevor H

RE: V-Tail Incidence
 

I would check your bubble level and incidence meter, see if they are in
agreement with each other. In other words, make sure that zero
degrees on the incidence meter is also indicating as being centered
on the bubble level.
Many times they are not. I have 2 incidence meters, they are about
1/2 degree out from each other.

RE: V-Tail Incidence
 

the next 424 race at OJA after may will be the first of nov. the sept race this year will be 422 only.
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RE: V-Tail Incidence
 

Use a Viper Dog fuse and it will take care of it self. It will be 0,0,0
http://www.matneymodels.com/q500.html

RE: V-Tail Incidence
 

If the vtail saddle is parallel to the wing, you will have a negative incidence at the tail (assuming your fuselage tapers.) The vtail saddle should look like it has a noticeable amount of positive incidence. You have to account for the v-tail falling deeper into the fuselage as it widens. It is like the old toy where you slowly moved the steel rods apart to get the ball to roll uphill.

Kurt

RE: V-Tail Incidence
 

The v tail saddle in molded in for you, With the 0 incidence. And the vtail saddle is parallel to the wing

RE: V-Tail Incidence
 

Kevin, the vee tail seat being parallel to the wing can only work if the fuselage under the tailplane is non tapered....I have to agree with the previous correspondent.If you draw a cross section through the fuselage/ tailplane at both leading and trailing edges you'll see why regards trevor H

RE: V-Tail Incidence
 

I bet Kevin means the bottom of the vtail is parallel to the wing - he knows what he is talking about and has built about a bazillion q500 fuselages. But just guessing.

Kurt

RE: V-Tail Incidence
 

I posted this about 5 years ago, but it is how the figure where the leading edge and trailing edge meet the fuselage when the wing is at zero-zero.

First, measure the width of the fuselage where the leading edge and trailing edge of the V-tail meets the fuselage. Subtract the TE width from the LE width and divide by two. I try to measure this to a 1/32" accuracy. Take the Tangent of the angle to horizontal (180 – {V-tail angle})/2. For example on a 120 degree V-tail, it’s 30 degrees. So working out the trailing edge position is : [(LE width– TE width)/2] x tan (30). This distance is the amount that the TE of the V-Tail is lower on the side of the fuselage than the LE for zero incidence. Place the fuselage with the wing mounted so that the leading and trailing edges are blocked up an equal amount (zero-zero) on the obligatory flat surface.

Now a fully race trimmed quickie has almost no control throw, and if you miss getting the tail incidence right, you can end up with an airplane that is nearly unflyable. However, if you move the CG forward from race trim, you can increase the control throw (50% to 100% more travel) and a slight error is easier to deal with.

RE: V-Tail Incidence
 

HP,
Thanks for that info. According to your calculations, the V saddle will always be higher at the LE and lower at the TE to the thrust line. (assuming the saddle tapers in width from front to back)

On my Predator, when the wing and firewall are @ 0*, the sides of the saddle are also at 0*. But, since the saddle tapers in width the V tail sits low in front. If the Predator had a conventional tail, the Stab would at 0*.

I feel more confident that the saddle needs to be adjusted.

Thanks!

RE: V-Tail Incidence
 

I think I got it.

Thanks everyone!

RE: V-Tail Incidence
 

Hi everyone,
Just thought I would chime in and thrown in my 2 cents worth as I have just redesigned a viper and cut it out so i could make improvements to the fuslage. I also ran into problems getting the correct incidence between the wing and the tail. According to my research the viper wing has 1 to 2 degrees up incidence in relation to the thrust line. That is up and down thrust not left or right. I had a guy explain to me that the thrust line is the same line as the camber line in the viper. So I measured the centerline of the firewall and the centerline of the tail and drew a line down the side of the fuslage. This line is the thrust or camber line. Then I measured down from the LE of the wing to the line and measured down from the TE of the wing to the line and I have a wing that is tipped upwards from the line by 2 MM. It was explained to me that the designer of that airfoil did that to get the optimim performance out of the wing. With that being said, where do we get our best performance, matching the incidence of the wing to our tail or do we match the incidence of the tail to our thrust line. It seems to me if we match the tail incidence to the trust line we would be more in line with what the designer of the airfoil intended. I dont know and a series of tests would probably indicate what works the best. Has anyone out there experimented with this?

Lenny

RE: V-Tail Incidence
 

When I did the Viper Dog I set it up 0,0,0. And all fly with NO trim. I have a smart level that goes to the tenth of 1 degree. It will read a 1/64" ply wood shim.