Repairs, Upgrades and things to look for.

The Smithsonian National Air and Space Museum credits the Laser 200 for bringing about a new generation of aerobatic aircraft
davevon
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Location: Vero Beach, FL

Re: Repairs, Upgrades and things to look for.

Post by davevon » 20 Jan 2019 13:34

I don't understand why you are so certain that the airplane is not nose heavy. The heavy elevator on landing is sure sign of a forward CG. Try landing a Cherokee 6 solo and then put two people in the back seats. Solo you may not be able to flair without adding nose up trim.

Sevro tabs move the opposite direction of the elevator when you move it up and down. Trim tabs only move via the trim lever and stay stationary to elevator as it moves. With servo tabs you can change their linkage ratio and increase or decrease their boost. You can't determine the CG by where the elevator is in flight. WIng and horizontal stabilizer incidence angles will affect what you see.

How much stick force is required to hold the elevator neutral on the ground when stationary and the engine is off? That is the maximum affect the weight of the elevator has unless your pulling Gs. Landings are usually very close to a 1 G maneuver, the roundout to flair will have some slight increase in G. Just the prop blast will reduce the amount of stick force to hold the elevator in neutral. Try it on run up. The counter balance weight on the RV elevator is there for flutter.

Another factor in the control feel equation is the stick length. I've flown RV's with shortened sticks and it ruined the RV feel. With the Laser a shorter stick would affect both aileron and elevator feel in a negative way.

The Laser ailerons became unique because of the Leon Tolve report. I suggest going back and rereading all the info I've posted on Laser ailerons and flutter. In essence because of what Leon computed the Laser ailerons needed a massive amount of weight on the spade arm to prevent flutter. All that mass and the plywood skins on your ailerons give you the inertia feedback you feel in the stick. It's there whenever you start or stop moving the stick. By comparison, a Pitts's ailerons are lighter because they're smaller and and don't have all the mass balance. My first flight with my Laser I didn't try a point roll because of the aileron feel... It may be possible to improve the feel with spades, but it's more of a factor of getting use to them.

I was somewhat surprised I wasn't able to develop spades for the Laser as easily as I was for my S-2B, about the same aileron airfoil and interaction with the wing. I now attribute it to the Laser's greater wing span and higher speeds. My current setup works great at Pitts speeds. At 170+ mph the feel is dependant on control input quickness. Slow inputs and they get quite heavy, quick inputs like sequence flying and they're single hand light. I'm not done with spade testing. I'm going to try the inverse Zimmerman planform to try and reduce the vortex lift effect I think I'm seeing now.

Mine had more play in several hinges than what I was happy with. I used beer can shims in the hinges to take out the play. Another area of concern with the hinges is because of their large diameter bearing surface they are more sensitive to friction. Even more so with the ailerons. I found that poor lubrication can increase control friction in flight because of the added loads on the hinges. I've been using STP for my hinges with good results, there may be better out there.

Here's some videos on the ailerons and balance


Tuft test


If you're going to get into spade testing, nibble at it. Small changes at a time and only change one thing at a time. I wrote a short synopsis on spade development for the Acro list forum. If you're not on the list I'll see if I can dig it up.

Dave

davevon
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Re: Repairs, Upgrades and things to look for.

Post by davevon » 20 Jan 2019 14:23

Here's a paste from the Acro List forum:

Spade voodoo.

First I have to say I’m not an expert if there is such a thing, but I have done a few, dug into about all I could find and quizzed those who have done more than I.

The objective of spades is to get a mechanical feel by modifying the hinge moment of the aileron with the hinge moment of the spade. Pretty simple, the voodoo comes in from all the variables and how they interact and there are a lot of variables!

#1 Flutter! Because of the aero elasticity of the wing and ailerons they’re usually more prone to flutter than the other control surfaces. Flutter onset can be extremely sudden and catastrophic. Be very mindful with the mass of the spades if they are part of the flutter mass solution.

Starting with the wing and ailerons:

The ailerons float in trail, meaning they are going to seek their own neutral position. Wing airfoil, aileron airfoil, wing to aileron interface and any mechanical bias will have an effect how and where the ailerons will find their happy place. With a symmetrical wing/aileron setup and no mechanical bias changing the linkage typically won’t change the roll trim. It will most likely just move the ailerons up or down together, move the control stick center and not change the roll trim. A quick sidestep on mechanical bias, what the ailerons do while sitting on the ground. An out of balance from side to side of the ailerons will cause a bias as well as linkage that binds or is causing the ailerons to bias right or left. This bias will affect roll trim and centering. The slave strut geometry on the Pitts S-2’s I’ve worked on cause a centering bias. Which incidentally allows a higher spade hinge moment and still retain a good center feel.

The aileron hinge moment is determined by the wing airfoil, aileron airfoil, size of the aileron, position of the hinge along the aileron chord line, the fit of the aileron to the wing, the fit of the aileron to the wing when deflected and the relative airspeed over the wing. I’m sure there’s more I’ve overlooked. For most higher performance aerobats the two main camps are early Pitts symmetrical ailerons with the hinge center in the center of the L.E. radius and the later aerodynamically boosted (Super Stinker) style with the setback hinges and L.E. profile that works with the wing T.E. It should be noted that Pitts type biplanes use less spade than a comparable monoplane using the same style of aileron. The earlier style of aileron has a higher hinge moment requiring a larger spade and more attention to other spade variables. The boosted ailerons have a much different hinge moment and depending on the airfoil and it’s fit to the wing will affect how the hinge moment changes with deflection.

Besides the aileron and spade hinge moments the aileron feel will also be influenced by wing mass rolling inertia and aileron system mass-inertia. Wing mass is starting and stopping roll, aileron system mass is starting and stopping the control stick. There’s also the possibility of detached airflow and bubbles on and around the ailerons that will affect their effectiveness and feel.

The Spades:

Just about all the spade variables interact and overlap with other variables, this is where the voodoo starts to come in. What works on one airplane may not work on another airplane of the same type. When spade testing I always heed what a wise man once told me, nibble. Only do a little bit at a time. Also try to do only one change at a time. Because the variables overlap so much if you change more than one thing you won’t know which thing you change made the difference for better or worse, or if the two changes counteracted each other to some degree. You also don’t want to get yourself into an unwanted aileron snatch setup.

Obviously the spade hinge moment is a force x distance. Moving the spade further from the hinge centerline increases their moment. Which way you move it will affect the moment around center deflection vs. higher deflection angles. Because typically the spade is mounted forward of the hinge line they don’t behave symmetrically through their total travel. The down going spade is doing most of the work, it’s in cleaner air not tucked up to the wing.

Spade generalities:

The spade thickness seems to have an effect on response around center. The thicker spade has less effect at center. L.E. sharpness will also have an effect around center. The planform of the spade will have an effect on how the thickness affects the spades response around center. The thicker spades also seem to produce less vortex lift.

The angle of the spades will affect aileron trim, feel around center and at higher deflections. With a Pitts the spades are not used for trimming roll. All the roll trim is done with wing rigging, the ailerons are adjusted to match the wings. The spades are angled to trim each other for no roll trim. Monoplanes are not so simple, you’re not going to re-rig the wing so it’s up to the ailerons. Three basic options: trim tab (P-strip/gurney flap) on the aileron, mismatched spades and spring bias on the controls. I haven’t tested each method with both boosted and non-boosted ailerons, but from what I’ve seen with non-boosted ailerons aileron trim tab vs. mismatched there is definitely a difference in feel, so they’re not an equal option and would be personal preference.

The spade neutral angle ie. the angle of the spade when the aileron is at neutral. There are several thoughts on this: spades centered with the center line of the airplane, spades centered with the flight axis of the airplane and the spades centered with the local flow around the wing. The angle has an effect on center effectiveness as well as higher deflection angles. Raising the spade L.E. reduces the down traveling spade effectiveness. It can also cause a snatch feel because of the reduced effectiveness around center. The spade thickness, L.E. shape and planform will interact with the spade angle.

The spade planform or shape can be one of the variables with the most voodoo. Because it’s very low aspect ratio, low Reynolds numbers and at times high AOA the shape can have a pronounced affect on feel. Besides flat plate lift generated by its local AOA it can also produce significant vortex lift. Its shape, thickness, L.E. shape, aspect ratio and local AOA will all have an effect on the production of vortex lift. The control input rate can have an effect on the vortex lift generation because you can produce higher AOAs for a short period of time. The feel can be of higher stick forces with slow aileron input while rapid inputs have light stick forces at the same airspeed.

Spade tuft test:

Spade shapes: https://pdfs.semanticscholar.org/f266/8 ... f6fe9f.pdf

https://archive.org/details/DTIC_ADA397533

Some keys to spade testing:

· Don’t do anything that might upset the flutter cart, error on the side of caution.

· With the ailerons in their neutral flying position start with spades set at the same angle to each other, digital level comes in handy.

· I would suggest start with the spade L.E. down at least 2° to the fuse centerline. I realize there’s probability some wing incidence.

· Change only one thing at a time.

· Nibble, make only small changes.

· Take notes.

· When things get way out of whack have a setup you can go back to for starting over.



Be safe and have fun!

Dave

davevon
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Posts: 59
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Location: Vero Beach, FL

Re: Repairs, Upgrades and things to look for.

Post by davevon » 20 Jan 2019 14:31

More from the Acro List forum:

My motivation for revising the spades on my B in the video was from all the test flying of the S-1S's and T's that were coming through my shop for rigging. Hopping in the B after flying one of those and it felt like steering around a truck :-) The setup I came up with lowered the spade for wing clearance and increased moment arm, more spade nose down, more area and planform change for more vortex lift. The spacers you can see in the picture incorporated the angle for the nose down and the differential angle side to side due to the spade arms not being the same. With the spacers no washers were needed, it flew hands off with and without the spades on. The aileron feel was now between a S-1T and a S-2A with great centering. The bent up tips actually reduced the space effectiveness, but they gave them more of a stock appearance...
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With the revised spade setup it threw the control harmony off. The elevator now felt heavier. I first noticed the effect of it doing 4 points, my muscle memory was allowing the airplane to settle while inverted. When I bought my B the elevator servo tab horn had an extra hole to increase sensitivity, to me it felt like the ailerons and elevator were out of harmony so I put it back in the stock position. After the spade tweak I moved the servo tab pushrod back up to the second hole and it was in harmony again and all the maneuvers locked in again just with less effort :-)

A small digital level is almost a must for setting up the spades. It makes it real easy to reference the spade angle to something on the airframe, and comparing the spade angle from side to side. Don't be surprised if the spade arms themselves are not set at the same angle.

Dave

davevon
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Posts: 59
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Location: Vero Beach, FL

Re: Repairs, Upgrades and things to look for.

Post by davevon » 20 Jan 2019 16:13

The last tidbit from the Acro List forum:

The bent up ends were just to give the appearance of a stock setup to the untrained eye :-) They actually diminished the spade effectiveness.
Here's a rectangular spade deflection sim. This is higher aspect ratio than the Pitts spade and you can see most of the deflection is spanwise.
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Here's a 5mm solid carbon, 90° bias unidirectional interior and twill surface, spade tested on the Laser. 5mm was a suggestion and it gave me the mass I was looking for. Proved to be too thick for my setup, adding a radiused L.E. helped, but not enough... The stick loads around center definitely got higher as compared to the thinner aluminum. Sharpening the aluminum spade L.E. made it even better on my airplane. The thicker spades may be a better setup for the aerodynamically boosted L.E. ailerons.
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The Laser spade development has been a real challenge and not what I expected. They're not yet exactly what I want, but I've taken a pause to do a reset. I'm going away from the high vortex generation planform and looking at the inverse Zimmerman shape. It's been working on the aerodynamically boosted ailerons, but may not work on my simple radiused L.E. ailerons...

Dave
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boosted180
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Re: Repairs, Upgrades and things to look for.

Post by boosted180 » 29 Jan 2019 18:36

Thanks for all the great information Dave. I'll spend some time to go through everything. I guess I'm not certain that my CG is not too far forward. In fact my first time flying her home, that's what I thought to myself. But after seeing how heavy the elevator was, I guess I just made myself think that that is the reason for the heavy stick feel and uptrim needed to maintain level flight. I guess the only way to really find out is to start measuring to figure MAC and the actual CG.

BTW, you're in Florida? Where? I'll be in Ft Lauderdale next week. If you happen to be near and want to meet up, let me know. I tried to PM you, but I guess you're not set up to receive PMs?

boosted180
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Re: Repairs, Upgrades and things to look for.

Post by boosted180 » 29 Jan 2019 19:13

To answer your question of how much stick force to hold the elevator in neutral on the ground - it's heavy. Much more effort than in flight with the air holding up the elevator. In flight, it feels pretty light and as you would expect.

Stick length is as long as it's physically possible without hitting the bottom of the panel. My RV had standard stick length, but first thing I did was added as much length as possible to have a light stick feel. Near center the RV feels great. Roll gets exponentially heavy as you move to extremes (as you would expect b/c of the small ailerons and no spades and the big fat square wing). Elevator feels light and communicative throughout range of motion.

The Laser by contrast feels heavy and resistant to moving at center for the ailerons, but as you increase travel towards the extremes, it gets lighter. At extremes, the roll on the Laser feels light and fast. The elevator feels light throughout range of motion, but heavy only as the airspeed decreases (starts to feel heavy at around 110mph). On the ground, it feels like a work out to hold the stick back. (I'm being sarcastic, but you get the point)

davevon
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Posts: 59
Joined: 01 Sep 2016 18:27
Location: Vero Beach, FL

Re: Repairs, Upgrades and things to look for.

Post by davevon » 29 Jan 2019 19:25

I'm a couple of hours north of Ft Lauderdale, Vero Beach, FL74.

I've got my Laser apart doing a condition inspection, its a couple of months past due...

Let me know when you might be able to make it up. Maybe you can make some comparisons between my Laser and yours.

Dave

boosted180
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Re: Repairs, Upgrades and things to look for.

Post by boosted180 » 29 Jan 2019 19:30

From the photos, can you tell if they elevator size is standard? According to the notes from the previous owner, it was enlarged by 30% over plans. Same with rudder. The ailerons were also lengthened. Can't remember exact number but I think they're 98" or somewhere in that neighborhood.

davevon
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Posts: 59
Joined: 01 Sep 2016 18:27
Location: Vero Beach, FL

Re: Repairs, Upgrades and things to look for.

Post by davevon » 30 Jan 2019 11:40

Your rudder and elevator do look larger. Here's a tape measure on mine for comparison.
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My ailerons are also about 98" long, one bay longer than the original.

Dave

davevon
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Posts: 59
Joined: 01 Sep 2016 18:27
Location: Vero Beach, FL

Re: Repairs, Upgrades and things to look for.

Post by davevon » 30 Jan 2019 13:37

I've been having issue with multiple pictures inserted in my replies. The spade response took several tries to get them inserted....

Here's the missing picture of the elevator chord:
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Dave

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