Thread Tools
Jul 23, 2004, 10:05 AM
Authorized Self-Portrait?
kitfoxdrvr's Avatar
Thread OP

The Answer: To Cool.


down_shift started a thread a week or so ago asking "To Cool or Not To Cool?". Well, I have decided for me the answer is to cool. I have a Hoot that I have had a slight problem with and want to make sure that heat buildup is not a contributing factor. I do not want to keep buying esc's to feed my hungry Pletti monster, and I don't want any gook coming out of an overheated rcvr pack, either! I am familiar with the use of NACA vents on full scale aircraft for various tyoes of cooling airflow entry, but I wanted to know how you guys addressed this once you decide "To Cool". Any info on size, templates for shape and location for the entry and exit holes would be greatly appreciated!

Thanks, Steve
Sign up now
to remove ads between posts
Jul 25, 2004, 01:51 PM
BANNED!!!
soholingo's Avatar
usually two small triangles in the front on the left and right side of the fuse for air entry. One Large Triangle in the back for exit.

I have one more power setup to try in my mini graphite before I start cutting these holes myself...

j
Jul 25, 2004, 03:17 PM
Landings are not optional
DeuceTrinal's Avatar
I believe a standard rule of thumb is to have the outlet larger than the inlet(s). This promotes air flow and prevents the vents becoming drag inducers.
Jul 25, 2004, 03:48 PM
Senior Member
Inflow would be at itīs highest at such a location where surrounding air pressure would be at itīs highest. According to the Bernoulli it would be where the velocity is at itīs lowest. Definitely not at the fuselage right behind the propeller; thatīs the place with highest velocity and lowest pressure.

So where would we have the highest pressure? Well, maybe some of you have heard about an aircraft called P-51 Mustang It has the intake of itīs hard working radiator right UNDER THE WING...and so do most of the liquid cooled WWII fighters to that matter. Why? Because the pressure around the aircraft is at itīs highest right there! So, the NACA inlets should locate at that part of fuselage which is under the wing of our shoulder-winged models. OTOH I believe that in most cases, all the cheese-holes done to fwd fuselage might generally bring less air inside than the leaking wing saddle! So the suggested inlet arrangement might be at work all the time without us realizing it. Maybe we could let NACA holes alone and just make sure our wing saddles are leaky enough.

What about the outlet? End of the hotliner fuselage has almost never enough cross-sectional area. Separate outlets before the end of the fuse easily create a lot of drag, not to mention that they create a stress riser exactly where you do not want one. Why not arranging so that air could bypass the motor FORWARDS and inside the spinner. Spinner would then act as a centrifugal turbine. Suction created by lowered airpressure right behind the propdisk would also help.
Last edited by Petri Nygren; Jul 25, 2004 at 04:01 PM.
Jul 25, 2004, 09:28 PM
BANNED!!!
soholingo's Avatar
Quote:
Originally Posted by Petri Nygren
What about the outlet? End of the hotliner fuselage has almost never enough cross-sectional area. Separate outlets before the end of the fuse easily create a lot of drag, not to mention that they create a stress riser exactly where you do not want one. Why not arranging so that air could bypass the motor FORWARDS and inside the spinner. Spinner would then act as a centrifugal turbine. Suction created by lowered airpressure right behind the propdisk would also help.
Ok, this is intriguing enough to merit doing. If you have the exit in the nose of the plane, will it not cancel out from the pressure of the air blowing into nose of the plane?

Before I cut my MG, I want all possibilities explored...
Jul 25, 2004, 10:53 PM
Senior Member
Quote:
Originally Posted by soholingo
If you have the exit in the nose of the plane, will it not cancel out from the pressure of the air blowing into nose of the plane?
One would suppose so. But then why have all my brushed gliders been gathering brush dust inside the spinner? Sometimes most mysteriously, without any explicable route through gearbox.

It should be proven by tests, but I suspect that unless spinner is extremely conical at the prop cut-outs, centrifugal forces + suction behind the prop disk keeps the air wanting to get out only. In any case it could be arranged/boosted by forming the spinner cut outs right. Maybe a measure of flour inside the spinner/fuse would tell a tale.
Jul 26, 2004, 12:02 AM
Registered User
rcelectfly's Avatar
I have seen several cases on F5B and F5D planes where the controllers were experiencing heat related shutdowns that were solved by cutting a triangular hole in the top of the nose just forward of the speed control. This was done without any exit other than the opening in the end of the fuse.

It may not be the best technical solution but it is simple, easy to do and it works.

Chuck
Jul 26, 2004, 01:10 AM
Registered User
shaneyee's Avatar
if you check out the plans for the Allegro E lite designed by Dr Mark Drela, the entry point is under the leading edge of the wing and the exit is behind the spinner just as Petri describes. The point of entry is also described as the stagnation point which I think means its the point where the air stream of air splits to go up and over or down and under.

Shane
Jul 26, 2004, 04:47 AM
Spoooooon!
TheTick's Avatar
I once saw a Carbon Sting DS'ing at around 140mph when the entire nosecone ejected forward off the plane for no apparent reason. Can't explain why, but for some reason, air was moving forward in the fuse at such great pressure that it shot off the nosecone against a 140mph headwind.

Later, I saw another experienced DS'er tape his nosecone to the fuse because he's heard of this same phenomenon happen to other people. These nosecones are not easy to remove as it is. What would cause air to move forward in the fuse at this magnitude? I don't get it.
Jul 26, 2004, 06:03 AM
Registered User
Andy W's Avatar
Quote:
Originally Posted by rcelectfly
I have seen several cases on F5B and F5D planes where the controllers were experiencing heat related shutdowns that were solved by cutting a triangular hole in the top of the nose just forward of the speed control. This was done without any exit other than the opening in the end of the fuse.
Did that myself on my Sirius, although I did add a small vent behind the wing TE also (the rear of the fuse is sealed on the Sirius). Controller would come out cooking before, no issues since.
..a
Jul 26, 2004, 06:50 AM
Registered User
Fredrik W's Avatar
I can really recommend Simprops Turbo-spinners. They have a hole in the center and no back plate. So with a few holes in the fire wall there will be plenty of airflow around the motor.

And the aluminium spinner also works as a heat sink for the motor shaft.


// Fredrik Wergeland
Sweden
www.flyingmodels.org
The Great Electric Motor Test
Jul 26, 2004, 07:11 AM
Registered User
Andy W's Avatar
There is no room around the firewall on my models that I've built in the past - the gearbox completely fills the motor mount area.

I may have a little room in my V11, however, as the Kontronik GB is narrow..
..a
Jul 26, 2004, 07:30 AM
Authorized Self-Portrait?
kitfoxdrvr's Avatar
Thread OP
Andy's problem is the same one I have: My Pletti leaves VERY little room around the motor for airflow forward. I have cut holes in the firewall around the gb as best I can, and have about 1/16" between spinner and fuse to allow air movement, however. I am considering, based on the info given here, placing small holes on either side of the fuse under the wing saddle to allow this airflow forward within the fuse, over the batteries, esc, motor and out around the spinner. How big should they be, at what point in the wing chord, and what shape and orientation would I want?
Jul 26, 2004, 08:10 AM
BANNED!!!
soholingo's Avatar
Quote:
Originally Posted by TheTick
I once saw a Carbon Sting DS'ing at around 140mph when the entire nosecone ejected forward off the plane for no apparent reason. Can't explain why, but for some reason, air was moving forward in the fuse at such great pressure that it shot off the nosecone against a 140mph headwind.
Could have been less pessure exhibited on the nose because of the increased velocity of air agianst the nose... Increased pressure behind the firewall would pop the nose off... My question: Why wasn't the nose screwed down?

j
Jul 26, 2004, 08:16 AM
Senior Member
Quote:
Originally Posted by Fredrik W
So with a few holes in the fire wall there will be plenty of airflow around the motor.
Turbo spinners sure look nice, but looking at them I always wonder if anything actually goes in since there usually isnīt equally efficient outlet anywhere in the model! Air warms up inside the fuse so it requires a bigger outlet than inlet, as stated by the old rule of thumb. Turbo spinner also has to fight the overpressure by wing saddle leak.

Drelaīs cooling system seems indeed to be exactly what I was scetching: http://www.charlesriverrc.org/articl...e2m/fuseE1.pdf
So, if designer of that caliber uses it, I guess it must work...So inlet in the fuselage sides, right behind the wing leading edge, close to the bottom surface. Outlet is just air passage for motor. Just watch out all your gear inside does not get blown out in flying direction!


Quick Reply
Message:

Thread Tools