I have a 25% Sukhoi from Sig and it has come time to mount the engine however I have never flown a plane with a cowl so I have one quick question. How far should the trailing edge of the prop be from the cowl? The previous owner put in an engine with a very weird mounting pattern so I'm going to have to put some ply over the firewall and then use some spacers I guess but I don't know how far the prop needs to be forward. This is my first giantish scale plane and my first gasser but there is no giant scale gas general nooby forum so I thought I'd ask here. Also can I make spacers or what kind should I use? I'm using a CRRC GF40i kit engine and it uses an aluminum nitro style engine mount. I think around 1-1/2" to 2-1/2" is the ballpark for the spacer length need. Thanks all.

See how it looks with about 1/4 - 3/8 inch out from back of prop to cowl. There is no set rule....look at a lot of airplanes here on RCU! Capt,n

Up to an inch between the back of the prop and the outside lip of the cowl. That is very close to scale and permits more effective use of propeller thrust. I have a 2.6m (35%) Yak with 1-5/8" between the prop and the cowl. It looks and acts right.

Woot, 200th post, lol. Thanks guy, gonna set it like 1/2-1" I guess it's better not to be too close. I never heard anyone mention anything about the spacing before, I guess it's not really that critical.

The norm is 1/16" from back of the spinner back plate to front of the cowl for 25% scale. 1/8" or little bit more is reasonable to balance out a heavy tail situation.

Well the Sukhoi has kind of a yak type cowl so the middle of the blades are nearest the cowl. Also I may use different shaped props in the future so a lil extra room is probably good.

Also I've heard that some spacers crack and split and throw your engine off the plane... So what kind of spacers are good and where to get them? Thanks.

Move the engine, not the prop. Depending on your engine, simple 3/4" square hardwood dowel works extremely well. PSP manufacturing, Horizon, MVVS, SWB, and quite a few other places make aluminum spacers.

Congrats on #200.

Nonstoprc. It's a round cowl, not aerodynamic type like an Extra.

I don't know what you mean about moving the prop, I fully intend to move the engine and thank you sir for the information. I'll get some aluminum ones and probably end up making my own anyway to get it just right.

No, the Yak has a Sukhoi type cowl.

With such a big round cowl you need some space between prop and cowl to get the max thrust effect from your prop and I would suggest using 3/4" as a minimum clearance.

Karol

<hr />Karol is right on. I would add that a big round cowl the largest dia prop with a lower pitch works good too. The cowl shape makes the space needed different. A pylon racer with a stream lined cowl kinda blends into the spinner...not much gap needed. Capt,n

Yeah I did some mock up mounting and it seems to look good with a good inch of space. I'll need 2" or 2-1/2" standoffs and that's pretty big. I think I'll make another firewall out of ply and mount that an inch forward from the original and then I'll mount the engine on that. I'm not really sure what forum this should be in but people who know gas engines generally know how to mount them.

Seems you need a set of instructions to help you out. Don't need a new firewall. Here's a few of pics that might lead you in a direction that easily meets strength and rigidity needs. A little larger engine though. The "stand offs" are over 3" deep. The method has been used on a great many planes over the years, most of them much larger than what you are dealing with. You do not need to go to great expense for machined metal parts or endure a bunch of trial fitting of parts to construct a secondary firewall.

The picture of the cowl/prop hub is for separation references. The picture of the engine is to assure you the method will function well for what I consider "small" engines since it handles much larger ones so well.

There's always a choice between working smart and working hard.

Make a six sided box with epoxy glued angle stock gussets along all the joined edges. Make it as deep as you need it to mount your engine. Epoxy and bolt that box to your original firewall and then bolt your engine to the front of the new box. Cut access and/or lightening holes in the sides of the box as needed. I would use 3/8 inch thick aircraft ply for the box, NOT light ply. Good luck.

Regards,

Richard

TOM I guess I have to ask a stupid question. You said that the stand-offs in the picture were over 3 inchs long. Using wood with that large an engine you don't worry about torque on the wood. Like I said probably a dumb question but I am getting ready to get into the bigger planes and I would have never thought to use wood for so large a gap.

Ok, now I built a secondary firewall out of 1/4 ply and I spaced the engine about a half inch from it with about 2 million washers and then I cut out 24 1" squares of 1/4 ply and they will stack up to make 4 1-1/2" stand offs from the old firewall to the new one. The plane is pretty old and beat up so this was the only way to go to keep the structure strong. Hard to explain but if you looked at it you'd come to the same conclusion probably. I'll post some pics when I'm done in the next few days. The old firewall was made of 1/4" light ply it looks like and it's anchored nicely to the fuse with thick aluminum angle stock. I think it is turning out quite nicely. Only thing is at my hardware store you have to get really big bolts to get the length I need... I'll need to get some heavy 1/4-20 or 28's to make the big gap. Maybe even bigger ones lol.

Tired old man, thank you sir for the pics, I was thinking about doing it that way but I don't trust my 60 year old drill press to drill straight through anything so I have to do it in segments. I really think this is going to look pretty good when I'm done (though nobody will look at my engine mount probably). Pics coming soon.

Take another look at the way the stand offs are constructed. There are two "beams" acting as one side of an engine mount. Each beam consists of two "legs" joined by a top and bottom "bridge". Each beam is a solid piece of wood with the central core removed for weight reduction. In this case two pieces of 3/4"x2" hardwood stock laminated together. Beams with considerably more layering are used to bridge long spans and carry heavy loads in home construction. The concrete bridges you drive your car over only have 3 sides, similar to the concept used with the mounts. The fourth side is the ground under the bridge, or in this case the engine and existing. firewall.

Each beam is extremely strong and provides tremendous resistance to twisting loads imposed by engine torque. They also mitigate engine vibration far better than 4 spindly aluminum stand offs. Weight differential is minimal. The upper and lower bridges tie each pair of legs together, and distribute thrust loads over a wide area. Turning the pair of beams (if desired) into a full box mount structure would only require the addition of light ply between the beams, essentially turning the beams into another firewall. There is no need to tie the beams together, making a box (actually a cube at that point) since the engine does that for you.

Another nice feature is the cost. In this case the cost was nothing financially since the wood came from a dumpster behind a cabinet shop. It only needed to be assembled into the desired size and shape. It took a couple hours to size, shape, glue, drill, and thin coat with epoxy to seal the wood. A little time and sweat but no time was spent combing vendor sites looking for the right size, and there was no waiting for the mailman or a delivery charge from the brown truck.

Try looking at a plane thinking about how one part of the plane can addsstrength to another part of a plane without generating excess weight. Composite planes depend on that concept. Much like a tree. Each part flows to the next, with each part lending support to the next section in line. Viewed that way you can build lighter and stronger than the other guy. Every time. In this case the engine becomes the structural tie at the front of the beams. The existing firewall becomes the aft tie. The pair of beams combined with the firewall and engine backplate provides torque resistance for the engine, which is further resisted by the existing firewall and fuselage structure.

Independant metal stand offs are seriously inferior to the mounts used in the photos. All I do is large and larger planes anymore and these mounts will work on any engine.

I didn't have bits long enough so I just drilled from both sides. The holes don't have to be perfect since clamping force holds it all together, just like a propeller. Nobody looks at the engine mounts unless the cowl is off. Who wants to sit at the field showing off engine mounts everyday? Never have figured out the "bling" value in polished or anodized aluminum stabd offs. The things are buried inside the cowl for criminy sakes.

You can order long cap head screws from McMaster-Carr, MSC Direct, Fastenal, or several other hardware vendors. Better than stuff found in the average local hardware store and a lot cheaper. Get the good, long T handle hex keys from the same places

Another way to do it is to build a stack of plywood plates of square shape and drill the mount holes at four corners. Cut the middle area off to save weight.

Some of the AW arfs use such stand-off blocks.

Anybody know what forum this should be in? The gas engine forum seems ok but I don't know. Anyway I got this all figured now so I'll post pics soon and it's coming nicely. Still need to fuel proof with some epoxy/thinner mix.

You're mounting a gasser so you're right at home.

I just posted all the pics and explanations so a few others could have a semi educational pictorial.

Regarding tha Aeroworks ply stacks. They now also provide a bag of 1/4" thick aluminum spacers for the same purpose.

Ok, I just got back from the hardware store and I got 1/4-20 bolts lock nuts etc and I got my engine all mounted then took some pics then test mounted the cowl and it's a good space. The bolts were a bit small so I had to take 1/4" off my standoffs. The prop would have been like 3/4" from the cowl but now it turned out about a half inch away. Got a bunch of pics so I'll post like 5 or 10.