Hi all, It has been about 30 years since I built a plane and the mind is a bit cloudy so bear with me. I am building a Sig 4-Star 60 and am starting to work on the firewall. It says to mount the engine mounts using the lines drawn on the firewall from the plans. I assume that means to line up the top rails of the mounts (where the engine sits) on the thrust line marked on the plans and also center them on the centerline at the width of my engine correct? Also it says I will need 2 degrees of downthrust. Do I get that by adding washers behind the mounts? Also I am going to use a Saito 82A 4 stroke for power, will that be enough and does anyone have any comments good or bad about the Saito engines.
You'll need to select a motor mount, e.g., a Great Planes, and then follow its instructions (you find the instructions on a PDF at their site). Generally, there will be a template with the mount that you'll line up with vertical and horizontal center lines drawn on your firewall; however, this is the simplest case. If you're in doubt and can't fathom the instructions, you might contact the mfg of your model (Sig?).
For the two degrees of down thrust, you can build a plywood plate that is epoxied over your original fire wall, or you can build a new firewall. I don't like the idea of washers, but it too is a solution. I'm surprised you have to build in the down thrust. On many models, this is already built into the firewall.
I can't comment about Saito engines, but I suspect they are good quality. Frankly I prefer OS; however, you should follow the engine size, either 2 or 4-stroke, recommended by the model maker. An overpowered model is often difficult to trim out.
Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:
Yes, that is correct. I suggest either a mount designed especially for the Saito 72/82 or a Great Planes adjustable mount. If you get the GP mount be sure you get the one with the long mounting beams since a 4 stroke sits further forward from the firewall than a 2 stroke.
Also it says I will need 2 degrees of downthrust. Do I get that by
There are several ways of doing this.
You can make a shim and epoxy it behind the motor mount.
Redo the firewall in the plane (I would not suggest this since it is too easy to screw it up and then you will have bigger problems).
Use washers. I suggest the washers and there are 2 ways to do them.
If you look in the fastener section of your local hardware store you will find a variety of washers in different thicknesses. You can mount them behind the motor mount (between the motor mount and the firewall). If you decide to go this way get the largest diameter washer in the thickness you need.
The last method (suggested method ) is to put washers between the rear mounting lugs (beams attached to the engine ) and the motor mount itself. This way if you need to adjust the amount of down thrust it is a simple matter to change the washers.
Before mounting the motor mount give the firewall and surrounding areas a coat of thinned epoxy (I thin it about 10% by volume with acetone ). This will fuel proof the area and also harden the firewall somewhat so the washers will be less likely to imbed themselves into the wood.
Also I am going to use a Saito 82A 4
Saitos are awsome engines. The 82 will have plenty of power to haul the plane.
A couple of things with 4 strokes - USE A TACH TO SET IT UP ( there are VERY FEW people that can accurately set a 4 stroke by 'ear' ), another thing is that if the engine is not producing smoke at all speeds it is running lean, and the last thing is that Saitos can easily handle higher nitro fuel (30%+ nitro ) so you don't need to be afraid a high nitro fuel. I suggest 15% nitro. In any case be sure you have a minimum of
16% oil. Many people say to only run synthetic oils. I run a synthetic/castor mix. Yes, the castor does leave some staining on the muffler, but I prefer the knowledge that IF I get a lean run, the castor is going to give me some protection.
My Saito 72 is very happy with 15% nitro and produces excellent power. At present I have it in a 60 size Bingo (early 4 Star clone ) and it will do just about anything I want along with a decent vertical (not unlimited, but decent ). I am running an APC 13x6 prop on the engine, and normally fly at 1/2 - 2/3 throttle.
With a Saito (most 4 strokes for that matter ) you will either need to get a linkage for the throttle OR reverse the throttle servo in the transmitter since the linkage works backward from a 2 stroke.
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