engine mount

[prkaye]

I'm still working on the empennage, so this question is totally premature, but I'm curious about how engine positioning works. This occurs to me because I'm trying to sort out what all would be involved in putting a non-standard engine in my plane.

Suppose I wanted to use a nonstandard engine that would fit under Vans stock cowel (e.g. a Mazda Renesis). The cowel is shaped to surround the engine, and has an opening at the front for the propellor (right?). So the point at which the propellor attaches to the crankshaft must be at some specific position (i.e. specific horizontal distance from the firewall, and the vertical position so the propellor ends up where it should).

1) Is this correct positioning achieved by designing a custom engine mount with dimensions specifically to accomodate the cowel?

2) Can such a mount be designed without physically having the cowel constructed (i.e. do the plans show you the measurements of the exact position of things like where the propellor attach point should sit?)

3) Is designing an engine mount around an existing cowel a backwards way of doing things?

Forgive me if these are very naive or uninformed questions

[rv6ejguy]

You need to have measurements for the reduction gear, engine and propeller/ spinner in hand first. The stock Van's cowling can be used in most cases. Ideally, you should suspend the powerplant package and determine its C of G and weight and position the new engine to replicate the Lycoming installation. Certain components such as the battery and radiators can be moved to correct the C of G after the fact.

Careful consideration of the redrive and propeller needs to be made before proceeding so there are lots of chicken and egg things happening on an engine swap deal. Spinner back spacing and redrive length determines where the engine will be placed. The offset between the crankshaft (eccentric shaft) and the prop shaft determines the height of the engine and cowling clearance to the intake system. Be sure to measure lots. There have been many cases with people buying a planetary drive with zero offset and finding out that the intake manifold is outside the cowling, forcing them to graft on "blisters or fabbing a lower profile manifold.

You should determine radiator and oil cooler placement as these may impact the mount design. You can't have a tube passing through the rad you mount later. On the Wankel, consider space and weight of a silencing system as these are very loud in naturally aspirated form.

If you have the enlarged 3 view, you can determine the thrust line and transfer to the fuselage to start measuring. Note that most Van's designs offset the engine to the right around 1.2-1.5 degrees and that the cowling shape has extra material on the left side so that the spinner sits square. The prop flange is in the center of the cowling, the rear of the engine is offset to the left to get this angle. You should check this angle and also check if the thrust line is offset up or down also.

The mount pretty well has to pick up from the stock 6 locations on the firewall. I'd study other designs from people who have previously mounted Wankels to RVs for ideas. Be aware that the rubber mounts will deflect a bit when the engine weight is put on them some you might want some way to compensate for this when the engine is in place.

In most cases the Van's cowling must be trimmed at the aft end 1-2.5 inches to fit the fuselage. There is therefore some latitude in placing the engine fore and aft.

The basic mount pickups on your block need to be bolted to the engine (having already preplanned the basic tube layout of course).

When jigging up the engine, I use an engine stand which is modified to hold the engine by the prop flange. The engine and fuselage must be level if there is zero up or down angle on the engine. You then need to project a centerline forward from the fuselage and position the engine with the correct offset. Using plumb bobs and electronic levels, you can shim the engine stand to get everything perfect, then you are ready to start tacking tubes.

Alternately the "Ross method" can be used where the engine is suspended vertically over wooden or metal template of the firewall with the 6 pickup holes duplicated.

Careful consideration of the tension and compression loads should be made on your mount layout and be sure to triangulate in the lateral direction also to take up gyroscopic loads from the propeller and whirl modes where simultaneous vertical and lateral G loadings could causing high torsional and bending loads.

Lots to consider but many people have done it.

[jhutch]

Phil,
Do a search for Real World Solutions and Conversion Concepts Inc.

Jason Hutchison
RV-6A
3rd Gen Mazda Rotary

[cobra]

The mount design I prefer far for the Mazda engines is the Schertz beam arrangement- it is detailed in Paul Lamars website http://www.rotaryeng.net/

I don't care for the unnecessary extra weight nor the increased chancd of leaks from of the "sandwiched" plate nor the "bent cantilevered" support that the CCI mount uses. The S-Beam leaves a lot of space under the engine for the radiator and ducting, but it does require a new oil pan, which you should replace with a larger sump anyway on the Renesis.

PLamar recently designed a neat looking cowl for water cooled RVs which includes space where needed and is a bit more streamlined than the OEM Vans cowl. Additional info should be available soon. My second choice is probably the P-40 influenced cowl from Sam James.