Homebuilder's Workshop: Fuel
By Ed Wischmeyer
I have very little going on in the homebuilder’s workshop, as I’m recovering (on schedule) from yet another spinal surgery, but my guys have been busy on the fuel system of the RV-9A. And I hope to restart flying within the next month, accompanied of course by a babysitter CFI on the first flight.
The most recent project was the fuel gauges, as the Garmin G3X glass cockpit lets you calibrate those puppies. Although calibration had been done when I bought the airplane, they didn’t seem to be reading right. And besides, I had removed the analog fuel gauges to make room for the second G3X touchscreen, possibly confusing the electrons. Anyway, the fuel gauges are now calibrated, as much as possible, that is.
The tanks in the RV-9A are in the leading edge of the wing, the wings have dihedral (are bent upwards towards the tips a little for in-flight stability), and the fuel sensors are float-type sensors mounted in the root (inboard and lower) end of the tank. This means that the floats float up to the top of the inboard end of the tank and read full before the tanks are completely full at the outboard end of the tank. Each tank holds nominally 18 gallons, but the left and right fuel gauges show full at 14 and 15 gallons. What’s more important, though, is that they be accurate when the fuel quantity is low.
But do the tanks actually hold 18 gallons? Yes and no. The guys were able to get a little more than 18 gallons in each tank. But if you’re in a hurry, the ribs inside the fuel tank can hold air bubbles, and you might not get the tanks completely filled. When I was an active CFI, I always told my students not to calculate fuel reserves based on completely full tanks for just that reason.
This plane has a fuel-flow sensor that connects to the engine interface unit part of the G3X system, but the GTN650 nav/GPS/comm also has a function to track fuel flow and hence, fuel burn. There’s no provision in the G3X to export fuel flow in the serial data format (RS232) that the GTN650 wants to see fuel flow in, and the GTN doesn’t have the ability to count the pulses that come from a conventional fuel-flow sensor like the G3X can – you need a fuel-flow computer to convert pulses to RS232. Ain’t a gonna happen – don’t want to run more wires, break into the fuel system for an extra sensor, and of course, don’t want to spend any extra money.
BeLite, maker of the ultralight, has fuel gauge senders that are based on pressure of the fuel at the bottom of the tank due to gravity. No surprise, the senders are calibrated through a microprocessor. This seems like a really cool widget, but similarly, not a project at the top of the list.
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As an AirVenture Chairman (one of perhaps several hundred, each in charge of a group of volunteers with a given mission), I get a heads up on all of the preparations for AirVenture. In this month’s briefing, we learned of all of the airplanes coming to AirVenture (impressive!) and all the arranging that is required (overwhelming).
Part of what makes AirVenture special is that all kinds of airplanes and groups of airplanes show up. On the other hand, it seems that an “anniversary” is any multiple of five years that some group wants to use as an excuse for a group event. Let ‘em come, I say. The economic and political clout of EAA comes in large part from AirVenture, and that clout in turn comes from the broad participation across the aviation spectrum. If AirVenture was just homebuilts, politicians and the more mainstream press would ignore it, and the homebuilt movement would wither to a few very hardcore individuals.
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The RV-9A has an angle-of-attack system installed so that I can determine for myself what AOA features are actually benefits and which ones are part of the hype. This (Garmin G3X) system does not have a prominent visual display on the primary flight display, but it also generates a series of beeps as the angle of attack increases. At lower angles of attack, the beeps are infrequent, but as the angle of attack gets close to stall, the beeps become more frequent till at stall there is a continuous tone.
The system was extremely easy to calibrate in flight. It took longer to read and understand the directions than it did to do the calibration.
Frankly, I think that an aural tone for AOA is the way to go––think of the Cessna vibrating reed stall warning systems that got ever more annoying as the angle of attack increased. After all, if a pilot is too preoccupied to watch the airspeed indicator, why would he or she be able to watch an AOA display?
We shall see.
