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Missing in my builders manual is the FWF section, which I assume discusses the engine angle with reference to the firewall.
I'm mounting the Yamaha RX-1 with a C box, which turns a Left hand prop as does a 2 stroke Rotax.
Best reference I have so far is zero angle up/down and 4 degrees to the left. Anyone have better info?
Thanks,
Brett
The FWF section comes with the FWF kit that you get. I don't recall anything about engine offsets in the Lycoming FWF manual for my Series 5. Doesn't look like there is any offset by looking at it. Best bet is probably to call Kitfox and ask.
Thanks Jeff, yeah visually looking at the original motor (subaru) I could not detect any angle either.
My model IV had a heck of a pull to the left so my right foot was falling asleep on long flights. (no trim tab) I added four spacer washers to the engine to offset the pull and had no effect. Maybe that trick works on bigger engines and props but in my experience the 912 is not affected by it.
I'm just musing on theory here and don't know if this will work but where did you add the spacer washers? That's quite a gyroscope on the front of the engine, so I'd have thought you'd need to take into account precession when you're adding a 'force' by offsetting the thrust line.
You want the engine to stop pulling to the left and so, effectively, you want the thrust to go to the right a bit. Since the 912 rotates clockwise (when viewed from the rear) I think you need to add the washers to the bottom mounts of the engine. This has the effect of 'pushing' at the bottom of the prop but then it precesses through 90 degrees to the left (in the direction on the prop) and so the force acts on the left side of the prop (viewed from behind) and takes away that boot full of right rudder.
As I say, that's the theory but I'd be very interested to see if it works in practise and I apologise if I'm trying to teach anyone to suck eggs :)
(PS: It's freehand circle, so no wonder it didn't join up properly. That and I can't draw)
Interesting discussion. But may I counter your theory here a little? (Although if that truly is a freehand circle, it is an impressive one indeed, and one that I could never match! So maybe I should keep my mouth shut! :)
I agree that applying that force to the bottom would have an instantaneous effect like you diagram, but only in a dynamic sense (during the small amount of time when that force is applied). Except for momentary changes in pitch and yaw, we hopefully won't be changing engine angle "on the fly" (in the air), so I would respectfully argue that this line of reason wouldn't apply.
I think that an angle adjustment to the right (or left for my ccw yamaha) is sometimes done for two reasons. 1. to vector thrust in the opposite direction that the engine torque is pulling (and creating Wheels right foot to fall asleep). And 2. Maybe more importantly to counter "P factor". Not the one we all get a couple hours into a x-counrty flight, but the one where the descending blade grabs more relative pitch in a high angle climb where the relative wind is at a negative angle to the line of thrust of the engine. That blade pulls harder and yaws the plane the opposite direction.
Anyhow, all in good fun here! My recollections are from some college engineering and physics classes, so I won't claim there aren't some holes in the ideas described by me here!
:D
This might be apples and oranges, but I built my engine mount for my currently flying Model IV. Doing some preliminary research, I was told by a trusted source - the who part escapes me now - that there was no offset. I had the typical right rudder needed effect which rudder trim took care of.
It is all good fun and, as I said, just me trying to expound some theory :)
I understand the P Factor argument and absolutely agree that the best way to deal with this would to be to add rudder trim or at least a trim tab and set it for cruise conditions. Likewise, I get the dynamic thinking and, again, agree this solution would not suit all flight regimes; it would really only suit one combination of RPM and speed (which would account for torque and P Factor) but I would suggest that be set for the cruise where the problem seems to lie.
In that way I agree with the dynamic theory BUT I would suggest the permanent 'offset' would work for a particular RPM and speed mentioned above and postulate the next bit of theory:
Imagine you're in you're taildragger (as all light aircraft should be :) ) and there is an infinitely long runway. You add your take-off power and then ease forward on the stick. Just for the sake of argument, let's 'freeze' those dynamics at 40kts because things will clearly change as we accelerate but I'm suggesting a solution only for a cruise speed set up, so am taking just one speed on the take-off. As we all know that force we apply to the stick translates to a forward force on the top of the propellor arc which then precesses through 90 degrees to the right (on our 912), causing the aircraft to yaw to the left (just like Wheels cruise set up). At 40kts we'd still need to maintain the same forward force on the stick to keep the tail level and so everything is balanced as we whizz down our infinitely long runway.
Now imagine that we've fitted a device which can push the engine mount bolts forward and move the mount itself. I reckon that if the bottom bolts were pushed out there would come a time when the 'force' of the redefined thrust line and the precession that I've described in my initial post would equal the force of the stick being pushed forward and, correspondingly, reduce the rudder required to counteract it and adding washers to the bottom mounts would have the same affect.
I suppose this would, in addition, always help counter the torque of a clockwise-spinning engine but you'd better be ready with the left rudder when you take off the power to land :D
Anyway, blah, blah, blah, theory is (sometimes) fun but fit a rudder trim tab ;)
Yep, no one engine angle will solve all situations! I'll need to add a rudder trim tab!!
But fwiw I did put a couple degrees left engine angle in while fabricating the yamaha mount... just in case my theory is right, er I mean left... :D
If that doesn't work maybe ill mount a 912 up there with the yamaha. More power and the counter rotating thing should balance everything out LOL!
That was FUN!!! To answer the question, We added the washers (of no effect) to counter the thrust vector (to no effect) on the upper upper left and lower left of the engine (as viewed from aft.) I"m a farmer. lucky I can fly at all.
That would, of course, be the thinking of any right-minded person. I suspect my right-mindedness, hence my wondering if the washers should be on the bottom :D
Let me know if you ever decide to experiment, it would be very interesting to see the results:)
Id glue pigs ears all over the surfaces of the plane to get it flying right before removing that engine mount again for what I had to go through to get that thing installed.
I am currently replacing the old (1999) rotax UL with a brand new one. If I am going to do anything, now would be the time, but I'm in the pigs ears club myself. There are about 18 minor mods to the engine to make it fit the model IV cowl.
1.swap manifods, carbs, change water coolant exit elbows, move the spider, cut down the oil reservoir etc. etc. I am adding an EMS 10 to monitor engine stuff. I could have BUILT a plane by now. I"m making a list so the next poor dog has a clue.
Geeze Wheels, you'd think that switching to the (pretty much) same motor would be a piece of cake. Dang!
If it makes you feels any better, I am facing the cowl problem as well. My answer is fiberglass mods though... or "hood bubbles" as I call them. I'll be cutting the cowl and letting engine stuff hang out, then I'll bring it to the fiberglass shop and have them do some magic when I'm done hacking it up!
I moved the CDI off the engine as well. that required fabrication of a mount to the backside of the engine mount (fuselage side) and re-routing of plug wires etc. Its no small job to put in an "out of the box" 912. I"m just glad I have a semi warm hangar and an understanding wife.