View Full Version : thrust line mod

rocket 204
05-20-2010, 01:38 AM
has anyone experamented with this mod on the 22 20 s when you look at these aircraft it looks like the engine prop combo is trying to pull the airplane to the ground rocket 204

Steve Pierce
05-20-2010, 06:36 AM
Haven't tried it but the engine is pointed down 4 degrees and to the right 2 degrees. Not sure if it will work because of the angle of incidence of the wing on our short wings. Still want to experiment with it.

05-20-2010, 10:10 AM

How much different is the Pacer angle of incidence compared to a cub?


Steve Pierce
05-20-2010, 12:26 PM
The Super Cub is 1" higher at the leading edge in the 30" between the two spars than the PA12 or the Short Wing Pipers.

Zac Weidner
05-20-2010, 02:09 PM
Where did you find the 4 degrees? I was looking for that measurement because we are going to have to shim the engine mount rubbers to obtain the correct angle. I am assuming that this is 4 degrees at the front edge of the prop with the airplane leveled at the door frame area.

Steve Pierce
05-20-2010, 02:39 PM
Zac, it is all in the engine mount. I have the drawing posted in the Members section. What have you checked on your mount? I would be suspicious of the cowling or fuselage before Kosola's mount.

05-20-2010, 04:23 PM
All of the experienced Pacer pilots that I've talked to think it would be a really bad idea. Even with SC tail feathers on, the tail still drops out on you first. So imagine having to do a go around when your are loaded...

Zac Weidner
05-20-2010, 06:14 PM
Well, we also had it touching the steering rod, which told us that it was the mount and that the firewall was not distorted because if that was the case, it would have moved the nose gear proportionately. We got the cowlings from 3 different airplanes and they fit together perfectly on the airplane with no engine, which would be a highly unlikely coincidence for all of them to be proportionately wrong. Today, we used two washers on the lower left corner of the mount and one on both the upper left and lower right to move the engine up and to the right about one inch at the nose bowl. It lines up perfectly now with this modification. Kosola said that it was acceptable to shim it in this way. It took all day, but it is perfect now.

Steve Pierce
05-20-2010, 09:27 PM
Zac, I'd suspect the cowling. Firewall is flat and engine should be 4 and 2 degrees like posted earlier. Shimming the engine to meet the cowl is going to change the rigging of the airplane. Seen Cessnas that couldn't trim out that were caused from dynafocal mounts installed in the wrong order those changing the thrustline of the engine.

Zac Weidner
05-20-2010, 09:58 PM
We don't know how to measure the 2 degrees right, but the engine was about 4.5 degrees down at the prop before the shimming. We will check it again as soon as we level the airplane again.

Steve Pierce
05-21-2010, 06:51 AM
Put a straight edge across the prop flange and measure to the firewall and do a little geometry.

Zac Weidner
05-21-2010, 07:20 AM
We will probably do some of that. Would we be correct to deduce that since the static ground line is 4 degrees from the airplane being leveled, the prop would be vertical with a 4 degree angle of attack at the butt ribs? Just out of curiosity.

05-21-2010, 10:29 AM
Too many variables (are your tires the exact same diameter as the ones Piper used when they came up with that 4 (x2) degrees off the HRL? I don't know, either! Is your nose strut EXACTLY the right inflation"? Dunno that, either). Since the firewall is SUPPOSED to be "vertical" when the airplane is plumbed, if you must "not do the plumb bob trick", then you can "shoot" your prop circle (like Steve said, with a straightedge flat across the prop hub) from off the firewall, irrespective of the angle of the dangle of the whole airplane. If you are going to "just" use a digital protractor (or any other "method" for that matter), then I would say leveling the airplane is your best bet (or maybe even "zeroing" your D.P. off the firewall, but leveling the airplane takes any and all "production tolerances" -and any "frame twangs"- out of the equation).

Doing it any way except levelling is really not good enough for Engineering Work, but that'd prob'ly get you by on a government job. Ever hear the old phrase that says "Measure with a micrometer, mark with chalk and cut with an axe"? 8=)

There IS a note on the engine installation drawings about "shaving (grinding?) the washers" to get the proper offsets, but the "effect" must be minimal because there is also a thickness limit. If you have a DRASTIC offset error at issue, "adding washers" does not sound like a good practice to me. If the mount was repaired and it is not "to the letter" of the drawing within reasonable limits, then that mount would be REJECTED, if it were going on MY airplane. Rejected for "improper repair procedures". Washers!?! Never seen THAT ONE recommended before! This ain't no J-V basketball game, this is airplane work!

[All this said, I'm still thinking you are going to find your cowlings are the problem; you ARE aware that the bottom edge of the right side cowl -where it"latches against the bottom cowl"- is APPROXIMATELY one inch longer than the left side, right? Boy, that would be the FIRST thing I'd do before I got a brain hernia trying to figure out my offset]