Fan flight testing
I highly encourage you to take a second look at last week’s post, as most of what I will touch upon here is predicated on the ground testing experiment conducted in the shop.
In order to gather the missing data, we will be going flying today. Later we shall analyze the results, and come up with conclusions as to the suitability of this installation to long term flight safety. Then and only then will I feel comfortable leaving the fan permanently attached to the plane as I await the arrival of the first hot summer days.
Should today's test prove successful, when the warm season eventually arrives, we shall conduct the final functional testing of the fan’s actual effectiveness in reducing oil temperatures on the ground.
But, before we actually get airborne, we need to ready the airplane by temporarily wiring it up, so that our “testing equipment” in the cockpit (basic voltmeter) can connect to the fan in the engine compartment.
As you might remember, the reason for using a voltmeter in flight is the correlation between volts as read on the meter, with fan rpm, and fan amperage output. We mapped this mostly-linear relationship in the last post, and we are now ready to use it.
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| 10.8v corresponding to safe fan-rpm (see previous post) |
Because this is a one time occurrence, there is no need to rip half of the airplane apart just to run a couple of wires, and since the fan is right below the oil filler door on the top cowling, I plan on running a few feet of wire outside the plane from the this door to the aft portion of the canopy, and then inside the plane all the way to the front seat.
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| Running wires from the oil filler door to the rear cockpit |
In the airline industry, maintenance occasionally uses what they call “speed-tape” on the outside the airplane as a temporary fix, you may think of it as airline priced duct tape. In the past I have conducted my own tests on duct tape outside the LongEZ, it is crazy strong (you figure that out at removal time), so I have no doubt our arrangement will work just fine for one flight.
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| "Speed-tape" over the wiring |
I chose to use insulated alligator connectors inside the cockpit to allow for a quick disconnect of at least one of the leads, should it become necessary to isolate the fan electrically, and placed them in a position where I cannot accidentally bump into them, but where I can still reach them with my left hand. Keeping the wires ends somewhat staggered prevents the fan from shorting itself should both alligator clamps become disconnected in flight.
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| "Sky-Lab" |
So, what’s the plan?
I envision the flight testing program as a mostly scientific pursuit, with a bit of art and intuition thrown in, needing to foresee hardware malfunctions, as well as obscure remote failure scenarios, and unpredictable odds.
Flight testing not only has to establish ways for the plane to collect the sought after data, but it also requires anticipating everything that could possibly go wrong, and design strategies that address those eventualities. From the look of things, we are well on the way to doing just that with our initial voltmeter and wires placement plan.
Needless to say, the big bugaboo of the flight testing phase is going to be the remote possibility of a self destructing runaway fan on takeoff, or anytime thereafter. Not only would the data collection phase of the flight be compromised, but the safety of flight might be impacted as well.
Aviation safety has come a long way by basically adopting Murphy’s Law that “anything that can go wrong will go wrong” as its mantra, and relies heavily on strategies tailored to minimize and mitigate such eventualities. The point here is that risk cannot be completely avoided but it can and should be managed appropriately.
During our testing I will be using an “action camera” mounted on a chest strap to videotape the flight (airspeed and volts mainly), this way I won’t need a third hand and a spare brain to try to capture the details I need. I did not consider a data-capturing passenger to be an ethical (or legal) option, so this will be a one man flight testing adventure.
From this point on, all airspeeds will be in knots IAS (indicated airspeed), and any voltmeter values outside of the normal range established during ground testing (0v to -10.8v) will be cause for an expedite return to base, and ending the experiment.
For our purposes, I am going to subdivide testing into three separate phases of flight, each with its own challenges and emergency actions. They will be the takeoff ground roll, the initial climb, and maneuvering flight. No need for additional phases, because if the fan makes it all the way to maneuvering, it will be just fine for approach and landing as well.
The takeoff roll phase will begin at power up, and end at rotation. Because this is the safest time to end testing, and involves the easiest emergency procedure by just closing the throttle and applying the brakes, I decided to extend the ground run to a slightly higher airspeed than normal, up to 80kts before taking “my problems” airborne.
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| Ready to go fly |
Ready for takeoff at the end of runway 5, voltmeter reading 0v as it should, I made one last mental review of what to expect. From this point on I’ll be looking closely at my voltmeter for the first indication of voltage, and by correlation fan spin-up.
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| 80 knots, 0 volts |
With rotation speed easily made with no incidents, I entered right into phase two of flight testing, the climb.
The expectations for the climb, as for the rest of the flight, are the same as for the ground roll, less than 10.8v (absolute value) reading on the meter.
The emergency action are quite different however, should that not prove to be the case, and they include wrapping the pattern immediately to the left for a close downwind, slowing down to the lowest voltmeter reading, and landing immediately. I could be back on the ground in about one minute at this point, so the possible danger from the over-speeding fan would be kept to the very minimum.
Of course, by now I have already figured out that 80kts is a safe speed for the fan, and because it is also a flyable speed, my stress level just took a major dump. I now know that at this airspeed, there is not enough airflow through the oil cooler to spin the fan, and should anything bad happen, I can always slow down to 80kts to stop the fan.
Granted, it is possible it might take more energy to start the fan than to keep it running, and perhaps slowing to 80kts might not stop the fan, however we just proved we aren’t remotely close to fan overspeed territory here, and that’s what matters.
After a few seconds climbing at 80kts with 0.000v showing on the meter, I decided to lower the nose, letting the airspeed build up and seeing at what point I’d get initial fan rotation.
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| 100 knots, 0 volts |
I am sure by now you are probably wondering whether the voltmeter is even working, or perhaps it might have gotten disconnected. I had the same exact thought, and at around 500’ above the ground took a quick peak at the leads…
Nope! Still attached.
Well, that is just weird! Where’s the heck is the oil cooler airflow?
Obviously the fan is not spinning, since even turning it with one finger produces enough tension to be read on the instrumentation, but does that necessarily imply that there is no airflow?
Let’s open a quick parentheses here…
I later tested the fan holding it out of the window of my truck with another aircraft builder driving it (thanks Eddie), and quickly discovered it takes around 40kts (wind adjusted with opposite direction runs) to turn the blades over.
The obvious implication is that just because the fan is not spinning, it doesn’t mean there is no airflow through the fan (read oil cooler). There could actually be up to 40kts of airflow through the fan before I can be alerted to it by the voltmeter.
Close parentheses.
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| 110 knots, -1.3 volts |
At 110kts, I knew we had this b#*ch nailed!
Finally some voltage started reading on my meter, confirming not only that the fan had started windmilling, but that the fan-rpm were well within safe parameters at this airspeed.
Much relieved by how things had gone in this one minute flight so far, it was time to enter phase three… maneuvering flight.
The only emergency action at this stage would be to slow the airplane down to a previously identified safe airspeed where the fan would be controllable once again, then test some more, or call it done and go home.
Personally, I needed to see that I had full control of fan-rpm via changes in airspeed. I spent a some time going back and forth from 110kts to160kts with complete predictability, though the voltages were slightly different at individual airspeeds depending on whether I was accelerating or decelerating toward them, but all in the safe sub 10.8v (absolute value) range.
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| 120 knots, -1.7 volts |
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| 130 knots, -1.8 volts |
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| 140 knots, -2.7 volts |
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| 150 knots, -3.2 volts |
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| 160 knots, -3.6 volts |
What a great day of testing!
So far I had managed to safely takeoff, climb, and maneuver to a pretty high cruising airspeed without any incidents or issues.
Because I rarely fly faster that 160kts IAS, I could have just packed it in for the day, and be satisfied with the results. But with an airplane capable of so much more, it wouldn’t have been a complete vetting of the installation had I not pushed it all the way to redline.
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| 170 knots, -4.3 volts |
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| 180 knots, -4.9 volts |
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| 190 knots, -5.5 volts |
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| 195 knots, -5.8 volts |
Ok, so technically I was a couple of knots short there, nevertheless I think we succeeded in proving a major point today…
The oil cooler fan installation is safe!
Whether it works as expected on the ground in reducing summer's high engine oil temperatures remains to be proven, but we can now take flight safety concerns off the “most wanted” list.
Unfortunately the camera was bumped into at some point before takeoff, and not all footage is usable as the speed-tape got cut out of the shot quite a few the times, plenty of it remained however to validate our results.
For those interested in the data, I compiled a graph based on some of the speeds and voltages taken from the inflight chest-cam video footage.
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| Voltage output from the fan at various airspeeds |
From the experimental knowledge we gained during ground testing, it is now possible to correlate actual indicated airspeed to fan-rpm.
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| Correlated fan-rpm at various airspeeds |
As you can see, the highest fan-rpm reached at the fastest airspeed this LongEZ is capable of, is merely 2200 rpm, roughly half of what it normally spins when powered by the battery (4300 rpm), and from the table at the beginning of this post, we know that the maximum induced current would only be -15.5a, which is easily stopped by an appropriately sized diode.
I feel comfortable closing the inflight portion of the oil cooler fan testing now, and while the summer round of ground tests might still disprove our main assumption, we will still have had fun, safely learning and growing in the process.
Isn’t that exactly what the FAA had in mind when they created the amateur-built certification category after all?
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