This blog is for entertainment purposes only, and is not meant to teach you how to build anything. The author is not responsible for any accident, injury, or loss that occurs as a result of reading this blog. Read this blog at your own risk.

Tuesday, April 10, 2018

Ch 23 - Oil cooler fan - part #3

Ground testing

The principal goal of this post will be establishing clear safe operating parameters to be used in testing the oil cooling fan installation. Minimizing and managing the risks involved in flight testing is going to be priority number one.

Let me just be upfront by saying that I’m not an expert in fan performance, and what follows might prove to be mildly upsetting to a lot of fan engineers out there. 

So... Now that both fan and shroud are mounted to the plane, we could just jump in and go fly. 

Completed oil cooler fan installation (for ground use only)

But should we? Let’s think about it for a minute…

For one thing this is an automotive type fan, and I bet it was never intended to see speeds of 200 kts (230 mph or 370 kph) across its blades. Still, while that’s an achievable airspeed in a Long EZ (shallow dive with some added power), it doesn’t necessarily means that the air flowing through the oil cooler will move that fast. 

“How fast does the air go through the oil cooler then?” you might ask.

The answer is… “I have no idea!”  Nor does anyone else I have talked to, for that matter.

The main reason why we don’t know is that, due to its own compressibility, air changes speed and pressure as it moves through variable size passages. 

Clear as mud so far? I thought so.

Case in point, the Long EZ’s main air intake, which is a NACA shaped trough carved into the belly of the plane (actually grafted onto the plane, but it  does look carved).

NACA inlet duct (bottom engine cowling removed)

The peculiar shape of this passage is such that it allows the high speed air to slow down as it enters the engine compartment, while at the same time increasing its static pressure. 

The NACA inlet duct thus pressurizes the bottom half of the engine cowl above the outside static air pressure, and does so with very low drag, as opposed to a less efficient scoop type inlet. This is why NACA ducts are still the preferred inlet method for bottom/up engine cooling schemes.

This high pressure/low speed air at the bottom of the engine compartment is in an ideal state for being channelled through the contorted engine baffles and cylinders cooling fins. 

The differential pressure that forms between below and above the engine keeps air flowing in the right direction, and its slower speed allows tight bends in the airflow to be achieved with less separation, turbulence, loss of efficiency, and internal drag.

This air flow eventually exits the rear of the plane from the top cowling, above the engine

Air flow through the cylinders cooling fins

Of course, some of the incoming air is also diverted and used up by other items, such as carburetor, oil cooler, heater muff, and possibly a few more pieces of equipment depending on the installation. 

Air flow from the NACA inlet

The slower air also spends more time in contact with the hot items in the engine compartment, and expands as it heats up. Then, it picks up speed and expands even further as a result of the converging aft cowling geometry (Bernoulli's principle). This shape promotes acceleration of the airmass back to its original speed (theoretically), before it is ejected out of the engine compartment, smoothly and efficiently rejoining the fast air above and below the cowling (again…theoretically)

That’s roughly the big idea.

Top cowling hot air exhaust

Wow! I think I lost track of this post's subject for a minute. Sorry about that! Let’s get back to testing.

Because we don’t know for sure how fast the air goes through the oil cooler, we also do not know how fast the fan will be windmilling, so we cannot be sure whether the fan bearings will be able to hold up to constant inflight spinning .

Unfortunately for us, the fan does not have a fan-rpm indicator, so if we just went up and flew the plane right now we wouldn’t know this very important detail to the life expectancy of the fan. 

If the fan spun too fast for too long the bearing could seize (most benign failure mode), or disintegrate altogether (worst case scenario) rattling inside the bottom cowling until something got damaged, eventually making it past the engine, and ejecting onto the wooden propeller. 

Neither option is very appealing, but the second one could certainly be disastrous.

The fan manufacturer publishes only limited engineering data, however what data we do have is at 13.5 voltage, representative of engine running and alternator charging the battery. 

Almost by definition then, whatever rpm the fan is able to reach at 13.5v (normal bus voltage in a 12v system), must represent the designed normal rpm, and will constitute our "Never Exceed" rpm value for flight testing. Notwithstanding the safety margin possibly engineered into this fan, I will consider safe an rpm envelope that is at or below this value.

Maradyne's fan data

The static pressure Maradyne charts here represents the resistance to airflow (friction) caused by the air moving through pipes, ducts, hoses, filters, hood slots, air control dampers or louvers. 

Clearly from the chart above, increasing the static pressure a fan must operate against reduces the amount of airflow delivered by fan, and looking at the data you can see that the fan blades are in a stalled condition at 1.2” H2O, and that the current jumps to 6.3a. When a fan stalls like this it becomes incapable of moving air, all it does is make noise, produce vibrations, and drain the battery. 

In our situation we will be operating in synch with the direction the air naturally wants to move because of the already higher pressure inside the cowl, so we’ll only have the oil cooler resistance to contend with, so fan stalling should not become a factor.

So, how do we find out the normal range fan's rpm? This value is easily measured using a hand held tachometer, and a variable power supply.

Gathering data for table #1 (fan at 4635 rpm at 14.4v and 5.9a)

It turns out that at 13.5v this fan likes to spin at 4300 rpm, so we will use this number as our maximum safe value. 

Next, I moved on to testing the fan in a windmilling scenario, but before doing that I tried pushing the fan to the maximum voltage my bench power supply could muster (20v) in order to highlight any obvious flaws, like weird sounds or vibrations at higher rpm, all indicative of substandard or deteriorating bearings.

Since I'm not sure yet what would happen to the fan in flight, I wanted to ascertain its behavior in a worst case scenario, way above the designed rpm range. 

6796 rpm at 21.8v and 10a 

I found that the fan spun flawlessly at almost 7000 rpm, no weird sounds, no rattling, no unusual vibrations, though it was a bit scary to be so close to.

Table #1 - fan being driven by the variable voltage DC power supply

Obviously an electric motor turns when supplied with electricity, but you might or might not know that it can also produce electricity if it were to be mechanically turned, as in an air driven turbine.

Fan acting as an air driven generator

This creates a new set of concerns, like how to handle the reverse current flow in case the Ground Cooling Fan switch is inadvertently left ON during takeoff.

There are a few ways of doing this, but in the case of low currents, the easiest way to prevent reverse flow is to insert a diode in line with the load. 

Ground Cooling Fan wiring diagram

At higher currents different strategies might be necessary, so it is imperative that the circuit be properly sized to handle the actual current (forward and reverse) that will flow through it. 

important M073K fan specs

The reverse current will depend on the yet to be known windmilling fan-rpm, which in turn depends on the aircraft speed, and the particulars of the individual installation. 

This data can only be gathered during flight testing.

As for ground testing, I knew I’d be limited in scope by my equipment since I don't own anything that can spin super fast, but I could get up to 5000 rpm using my converted belt driven Mini-Mill. So, I attached the fan (blades removed) to the mill, ran it at different rpm, and charted the results.

Fan motor being spun by the mill (multimeter off camera)

Ungracefully, amperage testing abruptly ended when I blew the internal fuse in my 10A multimeter at about 22 amps (“Doh!”), hence the lack of windmilling data (red NA) above 4200 rpm.

Table #2 - fan generating electricity while being spun by the mill

No direct fan-rpm reading is possible once airborne, but windmilling voltage could be made available to the pilot during flight testing, and a fan-rpm inference may be drawn from this voltage using the experimental data of table #2. 

Depending on what rpm the fan ends up being driven to in flight, this installation might or might not show to be acceptable, as far as fan bearing integrity is concerned. 

With our self-imposed structural fan-rpm limit set at 4300 rpm, the induced voltage limit is -10.8v (from the chart above). Note though that at this fan-rpm the motor is producing over 22.2 amps. Ideally, I'd like to keep this maximum back current to about -15a or less, which means about -5.5v, at a paltry 2000 fan-rpm.

If flight testing showed high windmilling rpm (i.e. high reverse flow current), this would make the fan switch even more of a critical item, to be absolutely checked in the OFF position before every takeoff. This might pose an unacceptable hazard to the electrical system.

Flight testing the windmilling fan at all usable airspeeds is a definite must, and it's going to be the key to unlocking the suitability of this installation to the safety of flight.

From our newly acquired experimental knowledge (recorded in table #2) we now know that 10.8v corresponds to an acceptable inflight fan-rpm regime, at least as far as its physical integrity is concerned. 

Given the unknowable nature of the actual fan-rpm, a small risk will have be taken in the quest to gather this data. However, using table #2 we can now design a flight-test schedule that addresses our concerns, and minimizes the dangers. 

Next time I will talk about flight profiles, as well as emergency scenarios appropriate to different phases of flight. Then, I will assume responsibility for the risk I have been trying to minimize and manage all along, and go flying. 

All that could have been done has been done at this point, and there is only one thing left to do to put this case to rest  once and for all, one way or the other...

"Cash me outside... howbow dah!"

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