LS171Malibu

My Feedback: (16)

I scratch built a Quickie for our local Sport 25 racing. For the most part, we must use a 424 airframe with an OS 25 at 52oz min. weight. I have built many planes and scratch built many also, but I messed up the incidence on this one. I made a reference mark from the stab zero to the firewall, which hit the center of the firewall. The LE is 11/64" closer to that line than the TE, which confirms the horrible downward pitch issues during flight. I am setting up to straighten things out and attemp to make this model competitve. I am using a MH17 airfoil. What should I use for wing incidence and thrust line angles? Any suggestions?

Kevin Matney

My Feedback: (1)

Zero-lift angle of attack is -0.15 degrees and Ideal of attack is 1.04 degrees according to CompuFoil 3D.
Find out were you are at like FW to wing then wing to tail, and then work out what is the best way to go is first before you do any thing.

luv to race

My Feedback: (1)

Are you comparing the stab incidence to the firewall? More importantly… what is the stab incidence in relation to the wing? When you block the plane up on your bench, put your incidence meter on the wing and shim the plane so that the wing is zero (or what ever angle the wing should be flying at, which I wouldn''t think it would be more than 1 degree plus/minus). Then check the stab incidence to the wing. I can''t imagine you''d want to be more than 1/4 either way. That obviously depends on the airfoil, which I''m not familiar with.

RB

LS171Malibu

My Feedback: (16)

That deminsion was comparing the wing to the horizontal. Roughly 3/16" lower at the LE. I had made a template out of .063 T3 for cutting the saddle, all I can figure is that it slipped. Oh well, I''ll straighten her out.

LS171Malibu

My Feedback: (16)

Kevin, the cores were beautiful, thank you. If I am reading your post right, I need a positive wing incidence of 1.04 degrees, in relation to the stab?

StanDouglas

Every once in a while I build one that has pitch problems even though I use templates, a router and cut the wings from the same templates and a Feathercut system.

When it has happened I simply loosen the wing bolts and place 1/32" shims under the front bolts (given I want to give it more positive incidence) tighten the bolts and fly. If it''s better, but not great, land and add another shim and so on until it''s right. Of course if its getting worse we may need shims under the trailing edge bolts.

Once I''m happy, I re-pot the wing saddle with epoxy and filler with the shims in place. The shims become part of the saddle. Your going to find that getting the incidence to within .04 deg is a little precise. And anyway the actual incidence required will be slightly different depending on how well you cut the wing core; wire lag, sanding the skin, incidence of the tail and all the rest.

Depending on all the variables you might actually need to alter the engine thrust line as well. Usually Q500''s work well with 0 engine incidence. But some actually require a different set up. You can do all the usual things like for a pattern plane. It''s actually an art form, so it may help to get a local artist help out!

Ciao

LS171Malibu

My Feedback: (16)

I started by doing exactly that, shims under the LE. My only problem was that I did not have enough shim stock to complete the test. lol [:''(] I never would have guessed I needed more than 3/32". I am going to borrow a digital meter and set as close to 1 degree as I can, and go from there. I have the CG at 3.125" on a 10" cord, I figure that should be close.

diggs_74

That may be a little far back on the CG.. Remember, a tail heavy airplane may amplify any pitch issues.. Just a thought..

LS171Malibu

My Feedback: (16)

What is a good method to determine a starting point on CG. I have always used 30% to 33% of the mean aerodynamic chord. BTW On the maiden the CG was at 2.5", and the move to 3.125" made absolutely no noticable difference in flight. Of coarse, if there was not a huge incidence issue I feel the CG change would have changed the flight characteristics.

diggs_74

Wow, the incidence must be off a bunch to not notice a change that large.. I would make sure and go to a more forward CG after fixing the incidence, just to be on the safe side. Maybe 2.75" for starters..

LS171Malibu

My Feedback: (16)

I''ll set @ 2.75" and work back from there. Thanks.

Kevin Matney

My Feedback: (1)

What you need is the Zero lift for all out speed, you get the other in a trun. what that means is how hard you can turn with the most speed

LS171Malibu

My Feedback: (16)

Ah, I get it! THANK YOU!!! Kevin, what is your opinion on the optimum CG?

Kevin Matney

My Feedback: (1)

The best CG is 2.468 From Compufoil Pro

garys

I would think you want more than "zero lift" at speed, as you need enough lift to keep the airplane flying (unless you''re airplanes have no mass to be lifted into the air?). I would also think the "ideal" angle of attack would depend on the given segment of flight (IE..in the straights, turns, ect).

Flying on a race course is a huge compromise. Most, if not all, racing airfoils never reach their "best" angles of attack in either the straights or turns.

LS171Malibu

My Feedback: (16)

Thanks again Kevin. I guess I need to stop using lead shot mixed with epoxy for ballast until I have at least worked the bugs out. lol

LS171Malibu

My Feedback: (16)

I dropped by the LHS and picked up a laser incidence meter yesterday. Boy, this bird didn''t have a chance. After setting the zero ref to the horizontal, I found the wing to be @ -1.75 degrees. That''s not all folks... there was also 2.5 degrees down thrust acting in harmony with the wing, it''s no wonder why she liked outside loops. Looks like I got my tail out of wack, probably when sheeting the top aft of the fuse. Thanks for all of your assistance, I''m gonna shim and fly on friday if the weather is good.

diggs_74

Let us know how it works out..

Bill Vargas

My Feedback: (5)

Before you start hacking away on the saddle,,,

when you zero out the wing, what are the angles on the firewall and the horizontal stab???



BV

StanDouglas

Gary in the above is absolutely aerodynamically correct. In order to fly straight and level with a 3.75 lb airplane you better have 3.75 lb of lift. If if it's zero you have an RC car at best! The incidence is difficult to calculate because you must factor in the actual airfoil data, the speed, the mass and the low Reynold numbers involved. Most of the faster current airfoils are symmetrical and therefore have a no lift at 0.0 deg. incidence, not like the older MH17 airfoil.

When I was messing with Q500 airfoils I used Xfoil for analysis. I never looked too much at Martin's MH series as the NACA 65012 and modifications of it had less drag at Q500 speeds and Reynolds numbers. By the way the MH17 also has a more negative pitching moment that you must over come with the horizontal stab which translates to more drag. But never the less my conclusion was that you needed a smidgen of positive AOA in the wing. If you set up at zero wing and tail you would need to carry a slight amount of up elevator in order to force the AOA for the lift required. One might argue what's the difference in carrying a little up or having incidence in the wing or negative incidence in the tail? I would guess not much except maybe cleaner airflow over the tail if the elevators not deflected all the time. Plus if you force the entire airframe to move with the elevator you're also changing the thrust line of the engine as well as the wing.

It would be interesting to find the actual AOA of a Q500 pulling through a hard turn at pylon one. It would be tough because of the rate of change in relative air flow. It obviously doesn't go past around 12 deg. or it would snap when the wing stalled.

But it sounds like you are well into solving your issues.

By the way, from the photos above it's a great looking airplane.

SD

LS171Malibu

My Feedback: (16)

Thanks for the tip. I am going to shim to 0.0 and tape the gap, set thrust line to 0.0(all relative to the tail) and fly again. Adjust from there.

Thank you SD for the positive comment on the appearance. The main reason I built this plane is because I wanted a racer for Sport 25 racing here in Fort Worth that looked different than all the ARFs.

Kevin Matney

My Feedback: (1)

Here is what you need to set up a new plane $34.99


[link=http://www.sears.com/shc/s/p_10153_12605_00948295000P?vName=Tools&cName=Hand+Tools%2C+Carpentry&sName=Levels+%26+Protractors] Craftsman Digital Torpedo Level[/link]

LS171Malibu

My Feedback: (16)

OK, I've got my ears on... I like it and understand what it does, how is it used in our application?

vicman

My Feedback: (10)

I got two little line levels at Lowe's for a buck each. There is enough scrap balsa around the shop to set the level on in my typical V-tail.

HighPlains

My Feedback: (1)

About 6 degrees. But you have the software, so figure it out when the wing is creating 120 - 160 lbs of lift. It's not a static number anyway.