Changes Due To Aligning Engine to Cowl?

[Jim Meade]

Specifically for Tom Baker.  You noted you reshimmed the engine so that the cowl and the E-PROPS spinner aligned correctly.  

My question is did you note any flight changes of any kind resulting from changing the angle of the engine to the firewall?

[Tom Baker]

I hadn't flown the airplanes enough before the installation to note any differences. After the installation I didn't note any adverse effects while flying.

[Anticept]

The firewall repair instructions that I have mention that improper shimming can draw the aircraft over to one side or the other, requiring a lot of rudder trim to correct. One of the instructions is to place it in a skid in both directions and it should return to center.

It also shouldn't require rudder trim changes at cruise with changes in cruise power.

[Madhatter]

I increased airspeed 4 kts with centering engine within the cowling using the E-Props. This increase is after several years of using the V12 version  of the E-Props which was not centered as the short spinner allowed it. The V20 version must be centered as it's spinner is designed to overlap the cowl. This is a consistent airspeed increase. 

[GrassStripFlyBoy]

For us SW owners to reference, I'm curious what is the count of washers behind the mount by location - top / middle / bottom and left to right?

I was studying this a week back, my bird is qty 4 washers everywhere, and spinner is needing about 1/4" move up, and bit back to the right as viewed from sitting in seat.  

Very rough spitball, I probably need 6 on bottom and 2 on top instead of the 4 everywhere.

[Jim Meade]

Is there a reference line and measuring standard to use in choosing how many shims one uses in each mount?  Surely that would be an alternat5ive to "lining it up with the cowl" which I'm sure is useful but doesn't sound like an engineered solution.

I'd have to look, but it is not an even number of shims on each mount.

 

[Anticept]

[Madhatter]

You just do what shimming it takes to center the spinner over the cowl, it takes a bit of work but not too bad. Mine overlaps the cowl by about 1/8 inch.

[Tom Baker]

These airplanes are not as precisely built as everyone likes to think. I remember that when I did my composite training they said that when they were laying up a wing or fuselage there were three women who were mixing epoxy by hand, and several people involved with the lay up. They had at least three different molds for fuselages, and they don't match each other. For example there are also three different shapes of baggage doors. I suspect cowlings are hand matched to the fuselage, maybe after the engine alignment is set. Also motor mounts are not all the same. There are to many variables to say that there are "X" number of shims at any given point.

Also for airplanes that are in service it is possible for a mount to ne slightly bent, or during previous maintenance the proper shims were not put back in place. Also if engine mount rubbers are not replaced during the recommended 5 year rubber replacement the engine can sag in the mount.

[Jim Meade]

1 hour ago, Madhatter said:

You just do what shimming it takes to center the spinner over the cowl, it takes a bit of work but not too bad. Mine overlaps the cowl by about 1/8 inch.

Since I modified my cowl to accept a 40A alternator, that approach is not possible for me.  I had to add a 10mm spacer to my E_PROPS setup to clear the cowl.

[Madhatter]

I think that centering the spinner is the important issue, overlap is nice but I would not figure it being a significant factor in performance. 

[Jim Meade]

Corey,

Do you know what is used as the reference plane for the engine in your drawing above?

[Anticept]

The holes for the wing pins. You just need a consistent reference, you could slip something down between the fuselage and wing to get a reference off of them without dismounting the wing.

The instructions say that the tunnel could also be used but its less accurate

[Jim Meade]

I didn't ask my question well.  I understand about the wing root and tunnel.  I mean where on the engine does one lay a level or protractor?  This afternoon I was looking at my plane and decided maybe one could go off the vertical back side of the gearbox.

[Anticept]

Prop shaft, use the face of the flange.

The engine cylinders are another place you could use, but that assumes they are put on correctly.

[Jim Meade]

Is this a Rotax definition, if so where do you find it, or your supposition?

[Tom Baker]

One could assume that the centerline of the crankshaft would be the zero line for the engine. Since there is no way to put a level on the centerline of the crankshaft you need to find somethin to measure from the is a known angle in relation to the centerline. The face of the propeller flange would be the most obvious in that it should be 90° from the centerline, making it a good reference. with a propeller installed that is not a useable surface. I think the machined surface on the crankcase behind the gearbox would also be a source that would be well within the tolerances permitted by the published drawing. Measuring the down thrust will be fairly simple. Measuring the right thrust will require a little more ingenuity. 

[Anticept]

Thrust angle is an industry standard. It is based on the theoretical thrust direction generated by the prop blades, and those blades rotate around a central point. The central point is that prop shaft. The mounting flange on rotax engines are 90 degrees to the prop shaft, so you can do the math from that if the prop is dismounted. Or you can check the prop hub to see if you can get 90 degrees off it somewhere, and do a runout to verify something's not off, and consider that your standard, that's fine too. It's very standard as well for clockwise rotating props (from the pilot perspective) have a thrust angle to the right and down slightly.

The prop shaft and the crank shaft are parallel as well. They have to be with the way the teeth are cut in the gearbox or they wouldn't last long at all because they're not beveled gears, they're involute spur gears, which by design must mate at one contact point for each pair of teeth, across the entire face, to properly support the load.

I do agree my cylinder claim is supposition. I will provide supporting arguments: cylinders are perpendicular to the crankshaft, and when installed, we use a machined straight block to align them during torquing and installation so that the intake manifolds properly seal (i bought the rotax one), which itself have the ports that mate to the cylinders aligned flat, both per rotax training and my own experience placing them on a surface plate to verify.

In addition, per the rotax illustrated parts catalog, the tappets are the same, the rods are the same, the rod housings are the same, the cylinders and pistons are interchangeable**, cylinder heads come in two variants for left exhaust vs right exhaust and are interchangeable respectively, the only differences I can find are the intake and exhaust valves have different diameters and the rockers are mirrored so there is a left and a right. However, these valves and rockers are interchangeable between forward and aft cylinders, therefore they must be the same otherwise this would cause problems as the position of the rockers would reverse relative to the position of the intake and exhaust valves, and so we would have separate part numbers for fore and aft rockers or valves and instructions in their handling (we don't).

Therefore, I find this very very very strong evidence that the cylinders, properly installed, will also be aligned to the thrust angle of the prop, so you just need to use the same point on the fore and aft cylinder to create a "line" to use as the thrust angle. You could remove the intake and use that surface, or pop off the valve covers and place barstock across the machined surfaces that hold the valves and use that for your reference...

But honestly, this probably doesn't need to be this absurdly precise! You'll be flying and testing anyways and tweaking to your liking, the manual for the thrust angle states to set this initially and fine tune it from there, with no limitations on how far to go!

** there is actually a honda style A & B cylinder, red and green piston, which are designed with certain tolerances and are meant to be matched to each other, but outside of keeping those together, there's no difference in position.

[Roger Lee]

The difference in the engine angle if you re-align it with our cowl isn't enough to make a big difference and isn't anything wrong with doing it.

[Jim Meade]

Interesting discussion.

If the prop flange is good for pitch, it should be good for yaw, as well.  The question would be what is the fuselage base reference?  Back to the wing pins or could one go off the engine mount?  Maybe the cylinder heads would provide a reference point for yaw on the engine.

The reason for my question was a discussion I had with a man who has tweaked a Swearengin SX so it is possibly the fasted normally aspirated single engine aircraft on the planet.  It has typically won the Oshkosh races.  He encouraged me to first address leakage around the cowling intakes.  I contemplated some carbon fiber work on the cowling, so naturally I wanted the engine in it's optimal spot first.

Today, I talked with him again.  I told him I'd measured the down angle and it seemed to be 2.7° versus the desired 2.2°.  He said it was miniscule and only a perfectionist would deal with it.  From looking at the cowling, I'm pretty close to right side-to-side.  His parting advice was to simply use flexible baffling to minimize air leaks on the cowling intake.  Then address cowling exits if indicated.

I guess I'll leave the engine mounts as is and refresh and maybe modify the baffling.

Thanks for the discussion.

[Tom Baker]

I would think that to measure right thrust you could mount a steel plate to the back of the gearbox, and attach a magnetic base laser pointer to the plate. 

https://www.zoro.com/bosch-laser-level-gll-1-p-line-and-point-gll-1-p/i/G0708876/?utm_source=google&utm_medium=surfaces&utm_campaign=shopping feed&utm_content=free google shopping clicks&gclid=Cj0KCQiApb2bBhDYARIsAChHC9tdUZst9k21zJvDXqMeaH6D_mvS4j7HBeGEVeoKRcggmQVQ91KYi9MaArscEALw_wcB 

Then measure the horizontal distance from the pointer to the wing tips and do the math to determine the angle.

[Jim Meade]

Sounds good.  Maybe pick a point like the tie-down ring or something else a little more definitive than "wing tip" but I get the drift and think you have a good point.  Thanks.

[Big Flapper]

I just finished aligning the engine to the cowl. It was offset to the right (co-pilots side) The spinner is now centered (left to right) on the cowling. My first test flight there was considerable right rudder needed. It required full right rudder trim to neutralize at straight & level flight. 

Any suggestions?

Thanks

[Tom Baker]

There are consequences to shimming the lower part of the motor mount. When you move the lower mount out it loosens the rudder cables. By adding shims on the right side to move the spinner to the left you reduced the right rudder cable tension. This would make the airplane pull to the left, and require right rudder trim. if you didn't, you will need to adjust the steering rod to re-tension the rudder cables. Especially on the right side.

Just a word of caution. If you make big adjustments with shims you can loosen the cables enough that the rudder could be loose enough that flutter is possible on a test flight.

[Madhatter]

Tom is correct

 

This may or may not be also relevant but when I bought my sw I noticed that the right flap was about 3/8 inch lower than neutral when at 0 position. I reset everything back to factory settings but had a hard roll to the right in flight. Nothing I did would correct the issue. I have re trimmed many aircraft in the past but was not able to to do it on the sw without compensating with the flaps. Flight Design aircraft are hand built and sometimes are not aerodynamically perfect which sometimes requires adjustment of flaps or other components such as engine position. It doesn't take much of a slight distortion or twist in the wing to significantly affect the aircraft. I achieved a significant airspeed increase by making some minor changes, but it takes a lot of work. Years ago there was a company that guaranteed a 20 mph increase on a stock Bonanza by readjusting wings ,ailerons, ruddervators,  etc but was very expensive.