N844X Test Flight

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So flying N844X is a different type of flying than I have ever done, and uses a rather large amount of fuel (25 gallons per hour of fuel at IDLE, 30 in low cruise, 40 in high cruise, 50 in climb).
Because the fuel consumption is so large, I want to at least get every bit of info I can out of each minute flown.
To that end, I come up with a pretty careful suite of checklists and documents before each flight, and decide pretty precisely what I will do with each minute in the air.

Here is a checklist for a recon flight, where the goals were to tune the fuel-flow indicator, build some low-altitude performance charts, calibrate the stall warning and gear warning system, and see if the glide ratio of the airplane changes with altitude (if so, then Xavion needs to know about it!)

So, here is the checklist:

starting fuel fuel added (truck, and enter as well!) gas guage
102 – 102


 alt       rpm     trq            itt        ng       ff         ktas
7500   1900  400 _______________________________________
7500   1900  500 _______________________________________
7500   1900  600 _______________________________________
7500   1900  700 _______________________________________
7500   1900  800 _______________________________________
7500   1900  900 _______________________________________


 8500   1900  400 _______________________________________
8500   1900  500 _______________________________________
8500   1900  600 _______________________________________
8500   1900  700 _______________________________________
8500   1900  800 _______________________________________
8500   1900  900 _______________________________________

go to 12k:
test gear-stall warns, hold altitude with power

gear warn: 100 KTAS
angle warn: 79 KTAS

gear warn: 102 KTAS
angle warn: 90 KTAS

gear warn: 102 KTAS
angle warn: 58 KTAS

go to 15 k:

15,000 GPS ALT:______________

165 KIAS, worst glide: (look for L/D variation with alt… NOTE GPS ALT AT THE TOP)

   alt             TAS               VVI

So the first bit is the fuel calibration:
The Evolutions (like most modern planes) let you enter the fuel on board into the avionics before engine-start.
Then, they look at how much fuel goes out to the engine to deduct that fuel from the fuel on board.
This way, you get a more accurate fuel on board reading in flight, since the gas gages are always so in-accurate due to the fuel sloshing around in the tanks and messing up the readings
Now, here is the problem: The fuel flow indicator needs to be calibrated: Otherwise it will not be able to understand how much fuel, exactly, is moving though its’ sensor!
Now, when I first got my Evo, the fuel flow was reading 41% high! These readings were so high that I just about panicked, afraid that I could never use this airplane.
BUT, before I get completely panicked, I decided to try CALIBRATING the system.
To do this, I had the plane filled with gas to the brim before one of my trips.
Then, I flew it where I had to go.
Then, I looked at how much fuel it took to fill it to the brim again!
That told me exactly how much fuel I REALLY burned.
THEN, I looked at how much fuel the fuel-totalizer THOUGHT I had burned.
OOPS! The fuel totalizer THOUGHT that I had burned 41% more fuel than I really had! Whew!
I adjusted the fuel flow ratio accordingly, and looked at the fuel flow that resulted, hoping that the fuel flow indication (and thus totalizer values) were now accurate.
The good news: The fuel flow indicated, after my adjustment, was EXACTLY the same, to within ONE GALLON PER HOUR, of the fuel flow according to the pilots operating handbook of the CERTIFIED Piper Meridian, which has the same engine! (You see, I set the same torque and RPM, at the same altitude as the Pilots Operating Handbook of the Piper Meridian, and looked at my indicated fuel flow… it was the same as the fuel flow of the Piper Meridian to within on gallon per hour in most cases! That was the confirmation that the fuel flow really was dialed in correctly… and that my re-manufactured engine was behaving like a new one! Cool!)

Now, that initial calibration was based on a few tanks of gas and a POH check of a different airplane, so ongoing calibration is still desired. TO that end, I record the fuel that ACTUALLY goes into the airplane on each fueling, and then see how much fuel the airplane THINKS is on board from the totalizer, and I can adjust the totalizer calibration (in the Garmin-1000 setup page) as needed to keep the fuel flow indication as accurate as possible. Now, I do NOT fill up every flight (WAY too much weight! 170 gallons! 1,156 pounds!) so the only way to know how much fuel I have on board is to see what the fuel gages indicate! BUT, they are NOT so accurate, so it is not wise to calibrate off of them… unless you keep track over a LONG time, over the course of MANY gallons of gas, though MANY fueling, so a few-gallon error that is bound to be in the gas gages does not account for a noticeable percentage of the total fuel the engine has gone through. So, the name of the game is to note the fuel that comes in from the truck EVERY TIME gas goes in, and be sure to add that to the totalizer EVERY TIME as well… and then see how much gas the plane ACTUALLY has on board, compared to what it THINKS it has on board from the totalizer, and then adjust the totalizer calibration accordingly.

So, those are the first few lines on the checklist there: How much fuel went in? We keep track to see how much we are REALLY burning.

Next are a dozen lines or so of cruise performance. I get the hi-altitude (up to 28,000 ft!) performance on the longer cross-countries, but on the short ones over to KCDN where I test Xavion, it is not worth going that high, so that is a good time to knock down some low-altitude cruise data. As you see, I go to a desired altitude, set a desired RPM and torque, and then note the ITT (turbine inlet temperature), Ng (gas turning % rpm… 39,000 is redline!), fuel flow, and knots true airspeed.
From this data, I can plot the speed versus fuel flow, range versus fuel flow, etc, for all different RPM settings and altitudes. The thing that is becoming clear is that you want 1800 rpm at low alt, 2000 rpm at hi alt, and you want to go as high as you can at the lowest power you can to maximize your range. (So what does that mean: “lowest power you can”? Well, the plane de-pressurizes at low power settings, since it is compressor output from the engine that pressurizes the cabin, so you HAVE to carry some pretty decent power to hold cabin pressurization! In fact, at high altitude, you have to hold way MORE power than is optimum for range just to hold cabin pressure… the next time I have my plane in the shop in Oregon, we will put smoke bombs in the plane and look at where the smoke leaks out to find out where the cabin is leaking, and solve the leaks. This will let us hold cabin pressure at lower power settings, thus enabling me to cruise at lower power settings and get much better range! Interesting!) So, in flight, I will set these various power settings and note the cruise performance that results to build my performance charts… which include curves that show the best-range speed for each altitude, so I can set to that power if ever low on fuel!

Next, I will test the stall and gear warning system. My Evo has a PreciseFlight angle of attack sensor hooked to TEENY little holes in the wing that send pressure at various places on the wing to a little computer that then estimates angle of attack and gives stall warnings if you get to slow… and gear warnings as well, by the way, if the gear is up! This system only knows when to trigger these warnings, though, if you teach it what pressure values are the ones you will get right above the stall, or right at the gear warning speed! So you have to fly to the speeds that the gear SHOULD warn, and fly to the speeds that the stall warning SHOULD warn, and push a number of buttons in flight to calibrate the system. So, that is a thing to do.

The test above will be done at high altitude for safety.
Next, while up at 12,000, I will pick up another 3,000 feet or so and then do a power-off descent (holding 165 knots indicated), noting the vertical speed every 1,000 feet. Doing this, we can measure the glide ratio of the Evolution when it is configured to glide BADLY (going fast, with the prop forwards to redline, engine at idle). We want to record BAD glide performance, just like we want to record BEST glide performance, so that Xavion can learn them BOTH and then give you an approach that is right in between the two, maximizing your margin for error! So we want to collect all the data needed to enter into Xavion, and I want to test worst-glide today, and do so at a number of different altitudes so I can see if the glide ratio varies with altitude… if it does, then this is surely something we need to teach Xavion.

So, that is the punch-card for todays’ flight.
As is typical, there are a few pieces of paper associated with each flight, since I have all of my plans, and a bit of data to record.

Now, as for actually FLYING the test… well that is of course the fun part.

First we start with the pre-flight, which seems to me to be of limited use, since the plane is so tightly faired that there is very little that you can visually check. But, you check what you can and then strap in with the various computers and paperwork all in reach, and the iPad or iPhone with Xavion running on it mounted on the dash and fired up to collect weather data by WIFI from the FBO or nearest cell tower during engine start and taxi. Once all pre-flight items done and the door is closed and latched, the batteries come on. The hydraulic pump will run a few seconds as it pressurizes the hydraulic system, which will soon raise and lower the gear. Then hit the door seals (you will hear the pump pressurizing the seals). Then on with the igniters (tick-tick-tick) and electric fuel pump (whirrrrrr). Hit the bright red START button from a Honda S-2000 and you hear the rising whine of the jet just in front of your feet. The propeller slowly starts to spin up, but it is very slow since there is no direct connection between the engine and the prop: The prop is geared to a turbine that just spins in the exhaust of the engine! Once the compressor has reached 13% of its’ 38,000 rpm redline on the starter, you push the condition lever forward to dump in the fuel. The noise of the engine rises to a crescendo at this point, and the prop starts to spin up to a roar as well. Once the engine is running for sure and the Turbine Inlet Temperature coming down from 1,000 degrees as air starts moving through the engine to cool it, you release the starter. Generator on to re-charge those batteries. Now the prop has been feathered up until now. Advance the prop control to forwards, and the propeller slowly winds up to 1,000 rpm or so. At this point, you are burning about 25 gallons per hour, and the airplane is struggling to taxi at 60 knots or so, only the brakes are holding it back. It can NOT idle any lower.

Get your taxi clearance and off you go. You have to keep pulling a trigger on the throttle to lift it up and aft over the idle-stop gate to move the prop into a flat or even reverse pitch as you taxi just to keep the taxi speed under control. This is called BETA. As you do this, the sound of the prop turns to an angry growl as the speed and airflow all change. Within 60 seconds, all of the engine temperatures are nominal: It could be negative 50 degrees outside, but the engine is still ready for take-off within about a minute or so. So, with prodigious use of beta and brakes, burning 25 gallons per hour to TAXI, you taxi to the runway and get take-off clearance. Igniters, fuel pump, and bleed air all on. Flaps set for take-off. Trims all centered. This engine can maintain over 2,200 ft-pounds of torque, but the take-off is only smooth and manageable for me if I take off around 1,000 foot-pounds or so. So we take off at LESS THAN HALF POWER. More power than that has things happen so fast that I cannot fly smoothly. Manage the rudder to try to stay on centerline and keep the ball centered on the PFD. Nose up around 80 knots or so and retract the gear when you have positive rate. You can feel the aerodynamics change as the gear comes up, and watch the gear lights change from green (down) to yellow (transit) to nothing (up) to be sure they all retracted. The hydraulic pump will run for a few moments to pressurize the system, running a light near the gear indicators: Watch to make sure it runs to pressurize the system, then goes out! If the pump does not stop in a few moments, having reached its’ “happy pressure”, then you must have a hydraulic fluid leak! Stay on the rudder: You will need to adjust it as the gear comes up, because the yaw center-point and stability are changing rapidly.

Flaps up (AGAIN with the rudder! You have to adjust the rudder pressure again when the flaps move!) and you hear the strangest groaning noise from the air as the flaps retract. You have been at less than half power up to now, but with some speed, you are now ready for full power: Add some more power in, and get on the rudder as the power, and then speed, change a lot. At this point, with enough power, you CAN climb at over 5,000 fpm… but that requires a high deck-angle, high power, and low speed of only 110 knots indicated… too little to really safely and easily absorb that engine torque. The ailerons and rudder are literally working hard to simply balance the torque and p-factor. As well, there is a pretty STRONG vibration from the propwash at high power and low speed. It is much more manageable to lower the nose and climb at half power or a little more.

Approaching cruise altitude, igniter and backup fuel pump off.
Switch fuel tanks as needed.
Fly the various tests described above.
This involves constantly-changing speeds, altitudes, noises, and control inputs as the turbine up front hums along at all sorts of different speeds, turning that big prop at sorts of different speeds and prop-pitches, changing the propwash and torque and p-factor, with resultant changes in pitch, roll, and yaw inputs to counter it, with each different test.

Now time for landing.
So this plane, power-off, comes down at 650 fpm at 110 knots with the prop feathered. And 3400 fpm at 165 knots with the prop at redline.
So you can descend any way you want to. Either way, you typically wind up blasting into the pattern pretty fast, since best RANGE speed is around 160 knots at low altitude… any slower than that and you are going BELOW your best-range speed. (Things you learn from testing the performance at all different speed and altitudes!) So you typically find that you are coming into the pattern at 160 knots or faster, and you can easily make a pattern at 3,000 feet if you want: Since you can easily come down at 3,400 fpm, you could turn base at 3,000 feet, and easily be down to runway altitude in less than a minute. This, of course, is non-standard and not smooth at all, because the rapid speed, altitude, and power changes are all very jarring. Much better to come in to the pattern at 1,000 feet, of course, carrying some power to hold that altitude. With some power already being carried, the deceleration from flaps and gear is much less jarring. Now, abeam the numbers, you can bring in approach flaps (Vfe 160 knots), and on base I bring in the gear. (No earlier because it has SOOO much drag, and lowering the gear pitches the nose UP sharply if done at high speed! Why? Maybe the nosewheel bay is pressurized by the gear extension process somehow??) So, to avoid the huge drag of the gear and the strong pitch-up that comes from lowering it when going too fast to its’ 150 knot extension speed, I lower the gear on base, with the flaps already down to approach and speed down to hopefully 120 knots or so. As soon as the gear comes down, a LOT of drag is felt and heard strongly, and you have to advance the power to 40 gallons per hour just to hold an approach speed: All the power is overcoming the huge drag of the flaps and gear. Over the fence at anything from 75 to 90 knots can be justified, and if you swipe power to idle across the threshold, the plane will decelerate at just the right speed to give you just a few quick moments to try to finesse a smooth touch-down, without wasting too much runway by floating forever. Power to idle lets the plane decel and touch-down pretty quickly, wasting a minimum of runway. If you are better than me then you can work out a smooth landing in those few moments of flare.. my landings are more “slightly bumpy”. The landing gear is VERY stiff, especially at low weights, and you really feel every imperfection in the runway as you roll out, since the gear simply does not give a smooth ride. It is much more of a stiff ride. Into beta with a prop snarl to slow down and with that and modest brakes you are down to taxi speed quickly. WHEW! OK take a breath and clear the runway, taxi back to the ramp with plenty of beta to keep the speed under control, and force the plane to stop on the ramp with heavy braking. Pull the prop back to feather and there is a snarl and shudder as it feathers and slows. The whine of the engine remains, of course. Now turn off the generator and yank the fuel lever. The engine whines down to a grumble and you feel like you can breathe again, if you have been just about holding your breath for the flight like me. Fuel pump off as the compressor comes below 10% (you want it on above 10% compressor for lubrication) and then door seals and batteries off. Ponder all the stuff you did wrong to try to fly it better next time!


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