After two crashes, one a week ago Friday in the trunk of my car when my flight box turned over on the tail of my trainer, and the second this last Friday, I am forced to spend a bit of time in the shop, not at the field. While I'm putting things back to gethere, again, I want to address some bad habbits my plane has.

I think I know the cause of the last crash, a nose high stance on a trike gear. Just at the point of rotation for take off, the tail would lift just a bit and one wing would drop. I figure I was pushing on the nose wheel and it just fell over. It does bring up the question though of just how to set up the plane.

I've read Dean Pappas's Triming from the ground up series in Model Aviation and he has cleared up a bunch of my questions. One lingers though and he breifly touches on it. My trainer is an ARF, and is sadly lacking in useful information on just about all aspects of setting the plane up. There is no reference to a Datum line and what the incidents of the wing and stablizer should be and what the down and right thrust of the engine should be. My first couple weeks were spent trying to figure out what was screwy with the way it handles. First, the CG was set with a slight nose heavy, but we had to trim in quite a bit of down elevator to get it to fly level at cruise speed. Having just read Dean's first installment, I got out my old machinest protractor and tried to get a set of angles that I could relate to the values in Dean's article. What I discovered was that the wing was about 8 degrees away from the stablizer. Dean shows a 4 degree difference in his example. So, I shimmed the trailing edge up about 3/16" and that did eliminate the down elevator trim issue.

Now, two problems show up. First was very agressive climbing at full throtle and nose over steep glide at idle. I believe this is due to insufficient down thrust on the motor. The second issue was lack of elevator response near touch down on landing. We dialed in all the additional up elevator possible and while it helped some, it didn't resolve the problem.

Along came the crash in the trunk and I had to rebuild the stabilizer and elevator. While I was at it, I gave the elevator an additional 1/4" of cord. That seemed to answer the lack of response, but the additued on landing is still a problem. I cut the throtle back on approach and the plane wants to nose down. I'm fighting to keep it level with the elevator and then it starts getting soft. I've had a couple really hard landings as a result. After Fridays crash took out the firewall, I decided to start from scratch on getting the incidents and down thrust where they belong before going back to the field mid week.

My questions, 1. is there a rule of thumb for incidence angles and down thrust. 2. Is my approach of developing the angles and working on the relationships vs worrying about the datum line the right approach. Last, 3. The wing is flat bottomed, but only from about 1/3 the way back. If I draw a line from the trailing edge to the leading edge, the leading edge is 3/4" higher than the extended flat bottom. I expect I have a semi symetrical airfoil vs the flat bottom trainer wing. Am I correct in using the cord line as my reference for wing incidence vs using the flat bottom ?

Sorry for the long winded question, but I'm kind of grouping in the dark for a light swithc here.

Don

dont go flying on fridays

Not really. Incidence is relative to the specific airfoil and airfoils differ greatly. Down thrust is a trim function that is used for many different reasons and none can be boiled down into just a single reason.

The angles like the incidence angle require that you know the centerline of the fuselage (datum line) so those aren't two different approaches. Do you need to work on the angles? Maybe. But it'd be the angle of the wing relative to the ground you need to sort out.


The centerline of an airfoil is found by doing a couple of things. Find the smallest radius circle that will match up to the forward outline of the wing's leading edge. Do the same thing for the trailing edge. Draw a line between the centers of those two circles and you have the centerline of the airfoil. That is the line used to layout the angle of incidence. Designers do not consider the flat bottom portion of a wing for much of anything.

What model are you talking about?

If it is a trainer, it's design will have been well sorted out if it is a popular trainer.

There is a very long list of possible reasons you crashed on takeoff. About the last ones on the list would be ones that pertain to the incidence of the wing or the downthrust, especially for a trainer.

In the list of things you've mentioned, one does sound like it might have contributed. Ground stance, on the pitch axis, can cause a problem if the airplane rolls with the nose too far down or too far up. Sit the plane on a level surface and look directly at it's side. If the fuselage does not appear to be level to the surface it's sitting on, figure out why. And then worry about how the wing is pitched relative to the ground, and then relative to the fuselage.

Are the gears assembled as designed? Most nose gear can be installed with quite a bit of leeway for length. If the airplane sits nosehigh, loosen the collets and shorten that front gear. Did you retrofit the main gear?

Can you supply a picture of the airplane? Take one from the side if you can. For most of the trainers that're popular in our hobby, if the wing is relatively close to being parallel to the ground when the airplane is taxiing, will take off perfectly.

BTW, a nose high stance would probably not cause the tail to lift at "point of rotation". It most often causes the airplane to lift off before you've decided to rotate. The airplane lifts off as soon as the speed gives the wing the lift that wing is angled up to give. And that is often before the tail is working well enough to hold the pitch. The airplane usually snaps over.

Have you verified the CG? Where is it?

The plane is a Lanier Explorer 40. From the start, I've been unhapppy with the plane. There was a list of issues with it from the get go. All of the hardware with the exception of the main gear has been replaced, including the wheels. The motor mount supplied was the second cousin to a prezzel. The push rods and linkages were unusable, the rudder pushrod hung on a rear bulkhead. I found several spots where the fuselage doubler wasn't glued to the fuselage side. Turns out after the crash that there was a lot more of those that I couldn't get to while there was top and bottom sheeting on the fuselage. In searching RC Universe forums, I've not found any positive comments about the plane's flying ability.

The stock nose gear had the stiffness of a coat hanger, and kept bending. It also didn't have enough winds so the centerline of the wheel was way off to one side and there was no way to get it under the pivot point of the gear. I replaced it with a DUBro front gear. The mains are still stock, however after some hard landings, they would also bend back and out to the sides. I would spring them into place every now and then.

When I get it back together. The front end is nothing but the sides all the way back to the wing right now. I should have it back by tomorrow though. The plane did sit nose high, partly due to my giving it a bit more prop clearance. Couple that with the mains spreading out due to hard landings and I'm guessing it was way to high.

I assumed that there is little effect on the flight due to the aerodynamics of the fuselage, and that the datum line was just an arbitary reference for the rest of the incidents and down thrust. I do know that with the three popsicle sticks under the trailing edge, it handled a bit better. After rebuilding the tail, I forgot the sticks when I put it back in the air and it was back to the down elevator trim that I started with.

Thanks for your input. I sure would like to get this plane flying somwhat well so I can get qualified soon. Rainy season is about to start.

Don

I'd be very tempted to scrap the Explorer and replace it with a different trainer. I don't have anything against the Explorer (I've never seen one), but yours seems to be the first one out of the factory on a really bad Monday.

The Tower trainer is 65 bucks and seems to be a decent trainer
http://www2.towerhobbies.com/cgi-bin...?&I=LXCAS2&P=0