Don't know about how this engine will do in this plane, but as far as Classic legal; as long as the exterior appearance is the same as the original, it's legal. Moving the bulkhead back should effect that.

Thanks for the response guys.

From web search my chosen engine, the one I have, is twice as heavy as that pictured on the plan but produces twice as power.

Fuel usage is still to be determined but I can modify the tank area as necessary.

Thanks
Craig

The plans say TJW Stoker is the designer. I don't think it is anything like the Kenhi Cougar as my wing is only 45".

I may have miss spoken saying Classic. It is a 1958 design with a 1958 engine. I am a novice so perhaps it is Old Time? But then it may be different in Australia as well.

I still have to see if this old engine will run and then hold a needle. I fly the planes I build so if it won't do it I will have to find one that will. Then I can settle on tank size and position.

The plans show a diesel with the all around exhaust. So not rear exhaust.

Craig,

If you are already a diesel kind of guy, this won't be news, so my apologies.

Diesels are known for being quite economical on fuel quantity. (Not necessarily cheaper to run--certainly not in the USA where ether is practically liquid gold.)  That means the space allocated for the tank in your Cougar may not be very big. How many ounces is the tank shown in the plans? You can always run whatever size tank fits in a sport plane--there are no rules--but I find that really short flights are frustrating. I have a Junior Flite Streak with a Mickey .19 Red Head that is a very pleasant flyer--for just 3 minutes or so....

People have often said that using a bigger prop on a diesel than a similar sized glow would comfortably run allows the smaller engine to pull a slower moving sport or stunt plane quite nicely. (Think George Aldrich and his full-fuse Peacemaker.) So you are actually dealing with two separate differences in your design changes:  changing from diesel to glow, and changing from old technology to somewhat newer.

If you can provide the engine weights and the distance from what you estimate the engine CG is to the aircraft balance point on the plans, we can calculate the shift required to achieve the same overall CG, if that is helpful to you. Of course, this all assumes that as drawn, the CG comes out pretty close to the plan, with no/little tail or noseweight required. If the original design was tailheavy...the impact of your engine choice would not be as significant.

Sounds like an interesting project.

A picture of the plane or the plans would help.

Dave

I was a bit skeptical of the 4.5 oz. number for the diesel.  A Fox .35 is 6.4 oz, and that is really light. So I went and looked. If the plans are calling out an Allen-Mercury AM35, that is actually a 3.5 cc engine, or a .21 cu. in. displacement. So maybe the 4.5 oz. reported in Model Engine News is correct.  As a further comparison, the OS .46LA is right at 9.52 oz., or about a quarter oz. more that your Glowchief .29.

Some quick math using your supplied weights and a nominal moment arm of 6-5/16" for the stock engine location (ref. to cylinder centerline), says you'd have to move the engine back 3-1/4" to get back to the same mass moment. That's huge, and does not seem reasonable. (It would be somewhat less since you'd be moving the prop, fuel tank, and some of the nose structure, etc.) Other assumptions as discussed previously.

Instead, if you used the heavier engine with the CGs in about the same place, you would have to add 1.5 oz. of tail weight at an arm of 20" from the airplane's CG. You'll have to decide if you think the plane can carry roughly 6-1/4 oz. more than the plans prototype. If you had a weight target of say, 36 ounces for a stock configuration, you would be adding almost 18% to the flying weight.

You gave the wingspan as 45". What is the wing area? Could you mount tail weight further back that 20" from the airplane's balance point?

Are you anticipating this will be a "goes around in circles" airplane, or that it will do at least the Old Time maneuvers?

Dave

Hello Steve,
Goulburn, really? Did you see the dog on his Tucker Box and the Giant Ram whilst you were there? That really is country New South Wales!

I have heard many others speak kindly of Gordon. A true pioneer of model engines in this country!!

Unfortunately the Glo Chief isn't worth that much, maybe $45US. Now, if it was one of Gordon's Taipan 2.5cc diesels....... In good condition up to $200US. New I think you could write your own amount.

I think you are right about the plane design. I have looked at comparable, more modern, engines and I still come up with 50% more weight than the AM 35!  4.5oz is just really hard to get close to!

If I can get the engine to run decently I will build the plane as is with the Glo Chief for nostalgia sake.

If all it does is go round and round so be it. A bit of fun anyway!

Dave
It hadn't really dawned on me the problems that the 4.5oz engine would create. As you say the LA46 is about the same as the Glo Chief but in a completely different power bracket!!

The furthest I could put tail weight would be 19" from the centre line of the CG. On the rudder.

I calculated the wing area at about 360 square inches, with flaps.

I have the build feature from Aero Modeler it does not give a suggested weight however. It does say to not deviate rearward from the CG as it is set up for competition sensitivity.

It does talk about the more popular, at that time in Britain, Frog 500. I looked its weight up and that states 7.65oz. It was 3.6" from the rounded back part to the drive washer and 2.2" from the back of the cylinder head to the drive washer, so it to would require the former to be moved. But I think we have moved on from that question!!

Just for interest the article says it was built and flown in the 1956 Gold Trophy with a K&B 19 which i looked up and the Web says weighed 6.05oz.

I'm not really sure where that leaves me. As I said I will build it. If you guys come up with a compelling reason to change engines I will. It does not have to stunt but a loop would be nice. If it won't run properly then I have a couple of OS 25's or an Enya 29 I could use. But I still see similar weight issues with those!

Thank you very much for the input.

Craig