rotccapt

hi all i am building a top flight 1/7 scale p-51 and i am going to be running a modified saito 125 that was converted to gasoline and spark ignition. so i had an idea to utilize the holes in the cowl for the scale air intake and the rear radiator exhaust to cool the engine. so i made a baffle to run all air that enters the cowl threw the cooling fins on the engine. then i plan on boring a hole in the firewall and making a duct that will connect this hole to the radiator exhaust. would this work as long as my total exit area was bigger than my inlet area? also i had had a thought to add on to this duct the inlet of the radiator to hopefully create a vacuum at the end of the duct. would this idea work? i know i would have to do some math to get the venturi effect to work but if it is a worthwhile project it would be worth the extra work. worst case i open the bottom of the cowl to create my exit

the first pic is my baffleing. the second pic is my radiator exhaust and the last pic is the inside of the fuse and showing the hole for the radiator scoop flow

CGAV8R

My Feedback: (2)

Hey Rotccapt, yes, I think that will work very good.

Aaron D.

BMatthews

I think this has a very good chance of working well. But remember that the head is the primary heat source. The sidewalls are a close second but if you don't control the temp of the head adequitely you're dead before the starting gate falls.

Remember that to avoid the stagnant bubble at the rear of the cylinder fins you want some baffling that conducts the air around that point. And the exit from the cylinder is generally best to be more of a slot that then can transition to a round or rectangular duct.

Likely you won't be able to spend a lot of idling time on the ground. The prop blast may not have enough effect to drive sufficient air through the "carb" intake on it's own. You'll likely require airspeed for this to work well.

rotccapt

the head partial sticks out of the cowl so that should help. i am having a hard time wrapping my head around your reply bmatthews is this what you mean see pic

Ram-bro

My Feedback: (101)

asI understand it you need a 4/1 ratio , for every i" of air inlet you need 4" outlet

R8893

My Feedback: (20)

I second BMatthew's suggestion that you get good airflow over the head. I have several models with a baffle in front of the engine that directs 2/3 or 3/4 of the incoming air over the head. Just finished a Val with similar arrangement as yours, plus the inlet baffle to the head. I have not been able to fly it yet, so can't say that it works. I have a Royal P6E with Super Tigre 90 with air exit out the radiator on the bottom of the fuselage; engine is upright. Not a smooth flow path. I did put a sheet metal lip in front of the exit hole to create a low pressure area. It is hidden inside the radiator. Been flying this model for 3 or 4 years without any indication of a cooling problem. The air inlet at the front of the cowl is a hole about the carb throat diameter over the carb, and an triangle in front of the cylinder. Wide part of triangle in front of head. However, I understand that gasoline engines run hotter than glow. I do run my engines rich too.

BMatthews

Rotccapt, your sketches are exactly what I was describing.

rotccapt

do i really need the duct through the fuse or could i do without it? also what about putting a small say 40mm ducted fan in the duct that would activate at or below say 1/2 throttle to aid in cooling at low air flow times like idling at the pits or on approach it would not run very fast just enough to pull air through the cowl just an idea. i also have not fully decided on running the saito on gas just yet it is all set to go just need to get this cooling thing down. i may do a couple flights with it on gas and see how it goes

BMatthews

Well YOU'RE the one that wanted to run it using the scale exit duct...

One thing to keep in mind is that running on gasoline with spark will result in the operating temperatures being quite a bit hotter than with glow fuel. With that in mind a fan in the duct to help out may not be a bad idea. But when not powered it'll act as blockage..... something else to keep in mind.

Your idea of using the belly intake as a way to generate a suction venturi is a good one but the problem is that you'd need to do a mock up and run some pressure sensing lines to the mocked up ducting to determine the optimum design that generates the most pressure drop and thus the maximum cooling airflow. It's not enough to just stick a venturi in the system and call it good. Oddball things can occur if not tested and suddenly you find that air is trying to BACKFLOW up the duct to the rear of the cylinder. Or more likely that instead of a suction you end up with some pressure that fights the flow of air from front to rear.

Starting to sound like an opening in the cowl isn't that bad a way to go, isn't it....

rotccapt

yea i know.

what i meant in the last statement was do i need the duct that connects the fire wall to the radiator exhaust? or could i just open up the hole in the fire wall and let the air find its way to the back of the plane. this is all just a thought process for me and to throw ideas out and get some feed back on my idea. originally i had though of liquid cooling the head but for sake of complexity and weight i threw out that idea.

the fan could stay on all the time but if it was not drawing enough air it would create back pressure in the system and would not make my problem any better

R8893

My Feedback: (20)

rotccapt, I would opt for the "let the air find it's way to the back" approach (it works for me on the P6E and an H-P O-400). With the 1/7th scale model you really don't have a lot of weight carrying leeway to lift the fan and batteries. I would stick with glow on the Saitolower temps and no batteries to carry.

rotccapt

ok if i go glow would it be benificial to run a remote glow driver to light the plug at below 1/2 throttle to help aid in flame outs at low throttle settings?

this settup will not add too much more weight just one maybe 2 batteries and a switch

R8893

My Feedback: (20)

the onboard glow heat is pretty cheap insurance, especially if you need some nose weight. I have it in a Don Smith Ki-61 in which I have a major emotional debt factor.