Vector Cowl Opening

[Peter in Fairfax, VA]

I have a Vector 40 with a Brodak 40 that I'm looking to fly.  However, the engine looks to me to be too tightly faired in by the cowl / fuselage to adequately cool.  Brodak's instructions call for an 1/8" all around the head.  There is not even 1/16".

Ideas:

1.  Do nothing.  Just go flying when the weather breaks.
2.  Relieve the fuselage to 1/8" around the head and try it.
3.  Relieve 1/8" plus drill a couple of front to back 1/4" holes positioned between the carb and cylinder, spaced 7/8" apart, on centers.
4.  Cut out the entire section between the cylinder and carb.

I've been flying profile a lot lately, so I do not know best practice.  Please advise best course.

thanks,

Peter

[Peter in Fairfax, VA]

Good as-is, does not even need 1/8" clearance?  The exhaust hole in the fuselage (not shown) is huge.

[Dave Harmon]

Hi Peter.....
I also have a Brodak Vector ARF with the B-40 engine.
I cut out the entire section behind the venturi....ie....a straight line from the o/d of the venturi to the o/d of the head.
No cooling problems...the engine runs great,
In fact...I think the B-40 runs better inverted than on a profile.
Home brew GMA fuel, K&B idlebar plug, RSM chip muffler to get the c/g back where it belongs without adding tail weight, 58' lines, Brodak 4oz uniflow tank KIT, no pressure, 11x4.5 Cyclone prop.....nice 4-2-4 break.

[Howard Rush]

Hot air expands, thus the ratio...

For air to expand to three times its volume at 70 degrees F, it would need to be heated to 1130 degrees F, which would hurt the paint.

[Brett Buck]

Good as-is, does not even need 1/8" clearance?  The exhaust hole in the fuselage (not shown) is huge.

  Clearance around the fins will make it worse, not better. I would, however, make the opening come down closer to the venturi, so you get air to the entire cylinder. I am pretty sure you would be OK as-is in the air, but it might start heating up on the ground, which would probably have the effect of slowly speeding up on the ground, then slowing back down in a few laps in the air.  Its OK that way, but it's also an indication that the cooling is marginal.

    Brett

[Peter Grabenstein]

Not a Brodak .40 but a Brodak Vector 40 ARF.

 
Peter

[Peter in Fairfax, VA]

Question and comment.

Question:  Why would allowing 1/8" around the head be a bad thing? What's the theory there?  Some turbulence concern?

Comment:  This exercise has made me realize that the last time I had a cowled engine was 45 years ago when a Navy officer passing through Northern Virginia in 1975 or so gifted me a Nobler with a Fox .35, crafted in a Navy motif.  So, maybe Ty is right, that too much concern about cowling airflow restrictions is not helping my efforts.

There are several distinct memories of that rig. 

One is the large number of broken cranks it endured.  I recall boxing up four to send back to Duke Fox, who had some sort of rebuild / replace program.  The plane had a prop shaft extension which likely contributed.

Another is how the goal for someone starting in stunt at the time was competing against yourself for a score, as there were no skill classes.  I entered one stunt contest.  After that point, I would mostly just use the pattern to solo practice flying fast combat ships.  I'm amused that the NJ guys have somewhat formalized that, as they are people I competed against often back in the day.

The last memory of that plane is demolishing it on pavement when its kinked .012 cables parted.  Flew well on those lines and I just might try solids if I can find them.

Peter

[Bill Hummel]

The October 2011 Flying Models has a nice article by Steve Teerlinck on his rework of a Vector arf. Some good cowl shots. Might give you ideas...

[Brett Buck]

Question and comment.

Question:  Why would allowing 1/8" around the head be a bad thing? What's the theory there?  Some turbulence concern?

    It's not that bad, it will be OK. But, generally, if you can force the air to go through the fins, instead of leaving space for air to just go around the fins, it will cool better. The reason it is not that critical is that the air is only part of the cooling system - the oil also carries away heat. My engine just sits behind a huge opening with maybe 3/4" all around - because I have to have air bypass the engine to cool the pipe.

   Note that in the past we have put baffles around the fins, to force air to go through the fins, and in particular, force air around the back side of the cylinder, where, with nothing else, the flow is very poor.

   It's not much of an issue either way, what you have is probably OK, might be overthinking it. Cooling is usually not much of a problem, we are talking about (in the case of something like a PA75) an engine that could probably safely generate 5 HP, and it's being run at 3/4 HP.

     Brett

[Howard Rush]

...the oil also carries away heat.

Another stunt thermodynamic myth. 

[Brett Buck]

Another stunt thermodynamic myth.

     Fascinating, I was under the mistaken impression that the exhaust was hot, and that the substance goobered all over the bottom of my Nobler was oil.

     Brett

[Howard Rush]

I think I ciphered the amount of heat that could go to the oil here awhile back.  I am too lazy to look for it.