Cooling fans and ideas.

[Zeke]

Inspired by the thread on the serpentine belt, I was wondering about fans. Seems right to me that there is a point of enough air being supplied at high RPM almost to the point of what they call cavitating in hydraulics. Anyone ever experiment with a clutch fan? Someone spoke of how much HP it took to turn the fan and it was significant. I would think the fan could be limited to a max RPM thru a clutch to, arbitrarily, 5000. If one is going to change out the pulley, could a clutch be included?

Now I'm thinking that a modified duck tail could be used to catch air at the top edge and funnel/duct it right into the fan via a plenum on the bottom side of the lid. What about this? Has anyone done anything along these lines? It would make up for the air not being suppied by the fan at high RPM or at least provide extra charged air at speed which is when the load increases.

Just musing early on a Sat morning.

[Grady Clay]

Milt,

Great minds ….

I tried an A/C clutch in place of the crank pulley on the dyno 30 years ago. The issue is that the air cooled 911 heats up (overheats) so fast if the fan quits turning that it was counter productive. A fast %on cycle (80% on, 10 sec cycle) killed the clutch. This was in the era when people were marketing the “Power Pulley” that ran the fan slower.

I made a large crank pulley and ran the belt down to where it was slightly touching the shims at the fan and gained net power. Light bulb.

As another experiment I ran a 36 horse crank pulley on one of my old 40 horse (53 HP) FVee engines. Huge gain.

If you run a 911 on pure methanol, it hardly needs a fan at all.

This set of experiments lead to the "Rubbermaid Solution."

The Factory solution came out in ’75 with the 930 fan set-up. The 1.82:1 fan ratio (245 mm, 11-blades) is a no brainer. On even a mild engine (2.2T) it produces a significant net HP gain under normal conditions. I find it hard to believe that this isn’t standard fair for every 911 engine. We put this on every car from ’74 on.

There are additional benefits: The defrosters work better without the (heavy) additional fans. The A/C works better. Most important, the engine heat is expelled at the source and protects the expensive machinery. This is an absolute savior for 2.7 and later.

The interesting solution today is the availability of variable ratio pulleys. I would try one on a dyno. The serious disadvantage is the high rotational inertia.

Even with the 1.82:1 ratio, the fan doesn’t cavitate or go supersonic at 8000 RPM crankshaft speed (14,560 RPM fan.) Just like an airplane propeller, the sound changes dramatically when it goes supersonic. Hydraulics have the issue of forming bubbles (cavitation) when pushed beyond the limit.

Aah, Saturday morning musing – the spice of life. I think I’ll get my helmet and go out to the track.

Best,
Grady

[Wil Ferch]

I don't think we're anywhere near cavitation....see here..

Feeler: Electric fan

and here....
Engine Fan Drive Ratio


Wil

Summary:
I said...
....it's telling that the racing and street applications limited the tip speed of the various fans to about the same number.....350 mph ( 450-510 fps). I would think either stall or sonic flow would be far away from 350 mph, and we should have some more room to grow. However, HP will be greatly increased and the internal blade stresses might become too big and the fan starts breaking up...I guess).

[kach22i]

I don't know computer rendering - tried a little Saturday morning, went to post it and the site crashed.

Took me a while to get a good Duck tail picture, found them here:
show me your rs clones

I had the same idea Zeke had a while back when I found a wing in which the leading edge was a sloted opening. That is to say the wing (993 aftermarket) was hollow and chambered the "shotgun" air flow into the fan/engine bay. Hey, why not the top edge of a Duck tail I thought to myself - but went no further with it.

Great minds think alike.

Actually, I don't know if the slot would upset the pocket of pressurized air which allows the fan to suck/draw air down through the factory grille.

Drawing air which is more in the air flow at the top of the Duck might render the factory opening null - or worse reverse the flow in a wicked loop.

Build it, test it...........let me know how it works out.

[Jim Garfield]

Isn't this what Ruf has done with the RGT/RS? Although I think that the air intake is at the bottom of the ducktail.

Nice drawing btw.

[Jim Garfield]

Here's one I saw at RRII.

[kach22i]

Originally posted by Jim Garfield
Isn't this what Ruf has done with the RGT/RS?

Well......the picture I modified Saturday morning I copied from the M.A. Shaw website, hope they don't mind. I'm home now so I can finally post it.

As you can see I don't do the computer stuff as well.

[Eagledriver]

Grady,

Wouldn't you want to slow the fan down to increase HP? I would think you'd lose power on a 2.2T by driving the fan faster with the 1.8 ratio. What am I missing?

-Andy

[buster73]

How about an electric drive for the fan, and the fan speed controlled by cylinderhead or oil temperature ("fan on demand"). Would need a big electric motor though.

[Grady Clay]

Andy,

That was the original idea. As it turned out it was just the opposite from what was expected. Yes, the fan takes more power but the additional cooling of the head and cylinder allows the engine to produce more power and there is a net gain at the flywheel.

A 2.2T on premium fuel probably isn’t in detonation mode so I have always ascribed this to increased mass air flow because of the lower temperature in the intake and combustion chamber.

This was done at 4500 RPM, WOT and I swapped the pulley – fan arrangement back & forth perhaps a dozen times. Rennenhaus was at 5300’ altitude which makes the engine much more sensitive to temperature because of the reduced cooling air – barometer is about 24” of Hg.

All this lead to the “Rubbermaid Solution” where you use a VDO windshield washer pump, a collapsible water container, and a single nozzle to spray water in the fan at the rate of about 10 gal/hr. Very cost effective for DE events in hot weather.

Best,
Grady

[kach22i]

Originally posted by buster73
How about an electric drive for the fan,

Do a search on this website, been up and down the block on that one. Kind of came down to "not the best idea" catagory............but was a good learning tool/idea to toss around for a while.

What concerns me about the "Cool Duck" concept is this: every now and when my engine hood pops up an inch or so while driving. Researching I found people saying this is bad because the engine fan is designed to draw air from the grille opening above. Allowing this slot inlet at the back of the car upsets the "static pressure" in the bay. The pressurized areas inside and outside of the car are carefuly balanced - the message sent was Don't Mess With it .

The fact that the Cool Duck would be drawing air in at air stream pressures, or perhaps from the pressure built up from the air dam effect caused by the Ducks "sail shape" might indeed be "Extra Air and not Stolen Air from the original pressurizes zones.

I think the concept is sound and deserves to be built and tested.

I should note that just cutting a slot in the duck tail is not exactly what I drew. I topped off or capped the duck with an extention which helps form a top lip. Also the design would benifit from internal curved webs to keep the air at high velocity. If there is no internal shaping, the velocity is halted and builds up into static pressure. This slower but pressurized air can still be used by the engine, but you loose the "shot-gun" or "Ram air" effect.

I think the thing to figure out is what velocity, pressure and volume the set up will work best at, and design it to meet those goals.


EDIT: Perhaps we can add "Riblets" to the system while we are at it? Thinking it could hold down more air so more air ends up where we want it. Link below.

http://aerodyn.org/Drag/riblets.html

Or...............also known as the Shark Fin thread:
Question for the Aero Experts

[Wil Ferch]

Actually,...this is nothing really new....

If you research the separate books by Frere, Anderson, and Ludwigsen....you'll note that Porsche "discovered" the aero benefits of the duck tail ( first) by raising the engine lid and noting that the lift figures improved dramatically at little/no extra drag. historically, old rear engine NSU cars , Fiats and hotrodded VW's all got improved cooling too with a propped-open lid, too

Back to Porsche.....once the duck was fitted and installed, it helped manage the air flow "into" the grill better than before....and it is reported that operating oil temps while underway drop by about 10 degrees F when using a duck vs. no duck, aside from the overall aero advantage.

The ideas presented here simply take the duck characteristics and go "one step further".

Wil

[kach22i]

Originally posted by Wil Ferch
The ideas presented here simply take the duck characteristics and go "one step further".

Wil

Does that mean we are going in the right direction?

Seriously, while walking into work I figured out that the slot opening in the duck tail will be built-up with static pressure. The tiny turning radius drawn into the top of the duck does little. The radius would have to be two or three feet to "shot-gun" the airflow down with high velocity.

What I'm saying this based on the reverse buckets of one of my favorite hovercraft, and they get only about 40% of forward thrust turned around for braking.

[Wil Ferch]

It would be built up with a combination of both static and dynamic pressure.....

Wil

[kach22i]

Originally posted by Wil Ferch
It would be built up with a combination of both static and dynamic pressure.....

Wil

True, they (the air flow) is made up of both, it's not black and white.

Here is what we have to do, get some "pressure sensitive" paint, some "pilot tubes" and measure the gizzmo up. Have to get them little tab flags waving all over the surface - big attention getter.

Or just build it and watch the temperture gauge.

Not kidding about the paint, here is the link:

http://www.aerospaceweb.org/design/psp/

[kach22i]

Originally posted by kach22i
I had the same idea Zeke had a while back when I found a wing in which the leading edge was a sloted opening.

Found it - from a different site, check out this link (has it's own Pelican thread already):
http://www.m5board.com/articles.php?id=28&page=4

[randywebb]

that's pitot tubes....

the width shown for the slot is a problem

a lip is liable to mess up the lift and drag characteristics

this is a very complex area you guys are talking about.... you really need to do "some research" before trying to build something. and "some" means 2-3 years of fluid flow courses...

[kach22i]

Do duck tails hold water?

They really do look like a puddle would form.

Just re-thinking the whole thing, perhaps adding a 3rd tail light would be in order.

[kach22i]

Alternate on the slot location....pending some real field or lab testing.

Blue RUF 996 came from:
http://www.instant-g.com/Events/Deutschland04/RUF/

[randywebb]

That -looks- better to me.

Realize tho, that any air that goes down that slot will not being going down the original grill - and the duck increases the latter.

How about fat open sides on the duck? Sure it's ugly... but that might be a way to duct some more air in.

You'll need to measure air flow thru the grill and the slot... I would use small vane anemometers hooked into a datalogger.

This may not ever be worthwhile, but it sure has the potential to keep you out of trouble for a few years....