cooling fans

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raymondbird

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Anyone think a ~4000 CFM puller fan and matching large radiator could cool a 6L LS engine ok? With no intake scoop I mean, just cockpit air with open canopy and exiting through louvers in the belly behind the wing.

I'm building a scale replica and don't want to spoil the outline with a ventral belly scoop. Original (ME109) had rads in the wing but with a big gear well cutout ahead of spar, if I go cutting another big hole for a scoop right behind that, wouldn't the torsional strength of the wing skin be severely compromised? I believe so and there is so much room for a larger rad in the back.

Thanks for any thoughts, yay or nay!
 

wsimpso1

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Torsional stiffness with skin cut outs can always be made. Not as light as continuous skins and more work to analyze, but will work fine. Doublers around the cutouts, fancy ribs and spanwise stiffeners, etc.

Trying to get enough air to cool an engine at in-flight power results in needing a bunch of air and significant inlets and outlets. Work out the fan power to move enough air through suitable heat exchangers, and a fan only makes sense in near stationary applications.

I say use heat exchangers and gear in the conventional positions and be happy.

Billski
 

raymondbird

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Torsional stiffness with skin cut outs can always be made. Not as light as continuous skins and more work to analyze, but will work fine. Doublers around the cutouts, fancy ribs and spanwise stiffeners, etc.

Trying to get enough air to cool an engine at in-flight power results in needing a bunch of air and significant inlets and outlets. Work out the fan power to move enough air through suitable heat exchangers, and a fan only makes sense in near stationary applications.

I say use heat exchangers and gear in the conventional positions and be happy.

Billski
Think you are right and many thanks again. BTW, just found another site actually too where a fellow was trying to cool his airboat V8 with a horizontal rad and fans and it just wouldn't cool enough at high power settings he concluded. Too bad as it sure would save some weight as you say and be easier.

But, you do see 800hp trophy trucks racing in the hot desert with a rad buried in the back. Seems to work for them. Massive rad and fans I guess . . . ?
 

wsimpso1

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Think you are right and many thanks again. BTW, just found another site actually too where a fellow was trying to cool his airboat V8 with a horizontal rad and fans and it just wouldn't cool enough at high power settings he concluded. Too bad as it sure would save some weight as you say and be easier.

But, you do see 800hp trophy trucks racing in the hot desert with a rad buried in the back. Seems to work for them. Massive rad and fans I guess . . . ?
Most ground racing equipment has around 50% duty cycle, which lets you get away with smaller heat exchangers. At low speeds power is down because they are traction limited in the lower gears, then there is braking (nearly zero duty cycle) and cornering (modest power). F1 is supposed to be around 40% duty cycle - at tractive limits until around 100 mph. About the only race cars running higher duty cycle on power are on super speedways and at places like LeMans before they added the chicanes, but they have a lot of q to make the radiators work.

We have to do (at minimum) 5 minutes at 100% power, then hours at 75% power. We have to cool for 100% or higher.

Billski
 

WonderousMountain

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Looking at your Me109 for reprise,

It looks like the lower surface closes out the top one with a 45ish diagonal. The radiator is back 15-16? inches, it's not on the lip. My thought is you will get fair torsion out of the original structure. Solving a problem that doesn't exist?
 
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pfarber

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How big is your inlet? Sucking a vacuum will not cool anything.

Give some rad sizes and inlet out et numbers fan diameter etc.
 

raymondbird

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How big is your inlet? Sucking a vacuum will not cool anything.

Give some rad sizes and inlet out et numbers fan diameter etc.
Sure, inlet would be open cockpit air. You can laugh but the high and wide turtle deck is open and would be a huge plenum chamber. Small scoops could be added as well, as long as they don't spoil the scale profile too much. Would not be "sucking a vacuum" for sure.

The 109 had an inverted engine so the fuselage is still quite wide at the bottom even behind the wing. Room for a 20" wide radiator and more than 3' long if needed.

Lots of great ~4000 cfm fans available these days, Ford Taurus/Lincoln or aftermarket and that should cool more than 300hp.
 

Vigilant1

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Sure, inlet would be open cockpit air. You can laugh but the high and wide turtle deck is open and would be a huge plenum chamber. Small scoops could be added as well, as long as they don't spoil the scale profile too much. Would not be "sucking a vacuum" for sure.

The 109 had an inverted engine so the fuselage is still quite wide at the bottom even behind the wing. Room for a 20" wide radiator and more than 3' long if needed.

Lots of great ~4000 cfm fans available these days, Ford Taurus/Lincoln or aftermarket and that should cool more than 300hp.
If we suck/push 4000 cfm through the cockpit area and we have an effective cross sectional area of about 3 SQ ft, (free area not blocked by the pilot, seat back, instrument panel, etc) then the air will be moving through the cockpit at a speed of around 15 mph. That would get annoying, and not fun in winter.
 

wsimpso1

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300 hp engine and 4000 cfm fan? Sounds like cooling will be inadequate to me. Let's just go through a first principles check of this. Changing to SI now...
  • 300 hp engine will have waste heat going out the coolant HX of about 300 hp;
  • 300 hp is 224kW of cooling - hdot;
  • 4000 cfm/(3.28ft/m)^3 is 113 m^3/min or 1.89 m^3/s or 2.31 kg^3/s - mdot;
  • Air specific heat is about 1.005kJ/(kg*K) - csubp;
We learned way back that h = m*csubp*dT which becomes hdot = mdot*csubp*dT. Solve for delta T

delta T = hdot/(mdot*csubp) = (224000 J/s)/(2.31kg/s*1.005kJ/(kg*K)) = (224 kJ/s)/(2.31kg/s*1.005kJ/(kg*K)) = 96.5 degrees C

Back to Fahrenheit - delta T = 174 F.

You would have to raise the air temperature going through that HX by about 175 F... Sounding inadequate to me, but we press on.

If the HX fully takes the cooling air to coolant in temperature (no real mobile cooler can actually get near that) and max coolant temperature is say 225 F, the highest ambient temperature we can operate at with a 100% effective HX is 225-175 = 50 F. I suspect that you are more likely to need it to work well up to about 100 F, so you would need to get delta T to around 125 F or less.

Most coolers will achieve more like 50 to 75% of the possible delta T's between them. 1/.5 = 2 to 1/.75 = 1.3 for cooler multiplier.

This 300 hp engine will need more like 7500 to 11200 cfm to even have a chance at cooling a 300 hp engine at full trot, like we do in airplanes for the first five minutes of every flight. Better go 12,000 cfm with room to go higher still.

What this also says is that a truck running 300 hp with a 4000 cfm fan and no dynamic driven airflow is good for no more about a 36 - 53% duty cycle. I suspect that there is some ram air assist for the radiators, even stuck behind the cab.

Billski
 
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wsimpso1

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Oh, and the air flowing through the cockpit is now up around 50 mph… Maybe the original style rads with underwing scoops and pressure recovery ducts are looking better and better….
 

raymondbird

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300 hp engine and 4000 cfm fan? Sounds like cooling will be inadequate to me. Let's just go through a first principles check of this. Changing to SI now...
  • 300 hp engine will have waste heat going out the coolant HX of about 300 hp;
  • 300 hp is 224kW of cooling - hdot;
  • 4000 cfm/(3.28ft/m)^3 is 113 m^3/min or 1.89 m^3/s or 2.31 kg^3/s - mdot;
  • Air specific heat is about 1.005kJ/(kg*K) - csubp;
We learned way back that h = m*csubp*dT which becomes hdot = mdot*csubp*dT. Solve for delta T

delta T = hdot/(mdot*csubp) = (224000 J/s)/(2.31kg/s*1.005kJ/(kg*K)) = (224 kJ/s)/(2.31kg/s*1.005kJ/(kg*K)) = 96.5 degrees C

Back to Fahrenheit - delta T = 174 F.

You would have to raise the air temperature going through that HX by about 175 F... Sounding inadequate to me, but we press on.

If the HX fully takes the cooling air to coolant in temperature (no real mobile cooler can actually get near that) and max coolant temperature is say 225 F, the highest ambient temperature we can operate at with a 100% effective HX is 225-175 = 50 F. I suspect that you are more likely to need it to work well up to about 100 F, so you would need to get delta T to around 125 F or less.

Most coolers will achieve more like 50 to 75% of the possible delta T's between them. 1/.5 = 2 to 1/.75 = 1.3 for cooler multiplier.

This 300 hp engine will need more like 7500 to 11200 cfm to even have a chance at cooling a 300 hp engine at full trot, like we do in airplanes for the first five minutes of every flight. Better go 12,000 cfm with room to go higher still.

What this also says is that a truck running 300 hp with a 4000 cfm fan and no dynamic driven airflow is good for no more about a 36 - 53% duty cycle. I suspect that there is some ram air assist for the radiators, even stuck behind the cab.

Billski
Wow, you make me feel so guilty - should be paying for answers like this. Thanks so much again!

I should do the math before posting as I never appreciated before how much air flow it takes. Here is a 400+hp machine and you are saying more than 12,000 cfm is going through those 2 little holes . . . ? Will have to work that out for myself to believe it.
 

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Voidhawk9

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I should do the math before posting as I never appreciated before how much air flow it takes. Here is a 400+hp machine and you are saying more than 12,000 cfm is going through those 2 little holes . . . ? Will have to work that out for myself to believe it.
The racers are going mad fast = more mass flow through a given inlet. Plus, they use water/methanol injection AND sometimes spray-bars for evaporative cooling as well! More than just air flow to be considered.
 

wsimpso1

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Wow, you make me feel so guilty - should be paying for answers like this. Thanks so much again!

I should do the math before posting as I never appreciated before how much air flow it takes. Here is a 400+hp machine and you are saying more than 12,000 cfm is going through those 2 little holes . . . ? Will have to work that out for myself to believe it.

There are two things going on in that racer:
  • The velocity through those two holes is only slightly lower than the airspeed of that airplane;
  • Delta T in air cooled engines is much higher than in water cooled engines.
    • In liquid cooled engines, the coolant can not be allowed to reach boiling point, which limits delta T to about 125 F;
    • In air cooled engines, it is routine to run CHT's in the 350-400 range with very long lived cylinders. When one is willing to accept shorter cylinder lives, even higher CHT's are accepted. delta T of long lived air cooled engines is thus 250 F and for short lived racers can be 325 F;
The combination of much higher input velocity and higher delta T's allow it to be successful with small openings.

This is all just some basic algebra applied to an equation we saw (or should have seen) in high school chemistry:
  • h = m*csubp*dT where h is the energy involved, m is the mass changing temperature, csubp is the specific heat of the mass we are watching, and dT is the chang in temperature.
  • Make it a rate by changing to hdot = mdot*csubp*dT and now we deal in flow of heat and mass...
Run some numbers. V is on the order of 400 ft/s, inlet area is about 26in^2 or about 0.18 ft^2, you have about 74 ft^3 per second, which is already more than the example worked earlier. Then they have three times the delta T to remove heat from the heads. Looks doable to me to remove 500 hp worth of heat from an engine that a long life is not expected from when running at that high power. Oh, and 90 years of air cooled racers have shown us that little openings on the stagnation point on fast aircooled engines works.

Examples of water cooled fast airplanes abound. P-51, Spitfire, Hurricane, Bf-109, Mosquito, even the Lancaster. Small openings receiving full dynamic pressure, well shaped pressure recovery ducts giving low velocity high pressure air through the HX, then velocity recovery exits. The Mustang is one of the best of the ones on WWII tactical aircraft.

Look at the modern variants in racers to see even more development... At speed the inlets and outlets are tiny, the ducts are long and very carefully configured to minimize losses, and the outlets are adjustable to allow enough cooling at lower airspeeds. Then they also spray water on the HX when running at speed, which greatly increases the amount of heat that can be transferred...

Billski
 
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Vigilant1

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  • Delta T in air cooled engines is much higher than in water cooled engines.
Yes, due to the higher deltaT each square inch of heat exchange area in a (direct) air cooled engine can exchange more heat than in a "liquid cooled" engine using a radiator. The air can be made hotter, so a given amount of cooling air can remove more heat.
Which, at this level of analysis, indicates an air cooled engine would have an advantage in racing (if less cooling air required = less cooling drag). But, of course, it is a lot easier to design an effective pressure recovery ducting system for a liquid cooled engine, and that's how total cooling drag for a liquid cooled setup can be reduced below that of an air cooled installation (despite the higher volume of air that may be required).
Still, without that well designed ducting system, a liquid cooled aircraft may have higher cooling drag than a similar air cooled aircraft.
 

raymondbird

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The racers are going mad fast = more mass flow through a given inlet. Plus, they use water/methanol injection AND sometimes spray-bars for evaporative cooling as well! More than just air flow to be considered.
OK, but they don't climb out that mad fast and they're not all racers, yet use the same small holes. A lot less hp of course.
 

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wsimpso1

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Wow, you make me feel so guilty

Only feel guilty for not looking up and doing your own math. It is good for you. Grin. But we can help with that.

For me, that pass through the math was kind of recreational. I had to look up (took about ten seconds each on the internet) the specific heat and density of air, and I worked in metric because I already knew that 1 hp is 746 Watts. To work in Brit units for energy is not as cumbersome as say for MMOI, which can really get you tangled up in your underwear.

Billski
 

wsimpso1

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OK, but they don't climb out that mad fast and they're not all racers, yet use the same small holes. A lot less hp of course.
A careful look at the racer cowls and you will see the outlets are much bigger than stock. Inlets are usually bigger too. Get some dimensions and run the math. I think you will find that the usual cowlings keep the engines adequately cooled for the life expectations at each power and speed.

Look up posts by RV6EJGUY on radiator size and ducting. Ross is the proprietor at SDS, has a long history of running EJ22 Subie on his RV6, and has really useful advice for anyone doing liquid cooling. Russel is one of his customers, runs a 6 cylinder Subie Glasair and wins all sorts of races with it. He has an almost ridiculously small looking inlet for his HX's. Lots of good guidelines on rad area and rad volume sizing.

Billski
 

raymondbird

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Yes, due to the higher deltaT each square inch of heat exchange area in a (direct) air cooled engine can exchange more heat than in a "liquid cooled" engine using a radiator. The air can be made hotter, so a given amount of cooling air can remove more heat.
Which, at this level of analysis, indicates an air cooled engine would have an advantage in racing (if less cooling air required = less cooling drag). But, of course, it is a lot easier to design an effective pressure recovery ducting system for a liquid cooled engine, and that's how total cooling drag for a liquid cooled setup can be reduced below that of an air cooled installation (despite the higher volume of air that may be required).
Still, without that well designed ducting system, a liquid cooled aircraft may have higher cooling drag than a similar air cooled aircraft.
Hey thanks and that's what I always thought but didn't quite understand, it's less drag, yet there's more airflow. Still seems counter intuitive but Bill sure explains it well with all the math. Remember Ross (rv6ejguy) pointing out too all the examples of the same airplanes being faster on less HP when using liquid cooled engines. Very many examples of that.
 
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