A few words on brake performance and taxi testing.
Adam makes an excellent point about brake performance and monitoring brake heat. In every instance where I have seen the nylon brake lines fail, they have always been inside tightly fitted wheel fairings. I have never seen a heat failure on a plane with no wheel fairings. That brings up another point.
If one is going to be doing taxi testing, you should be doing so with no wheel fairings. In fact, your first few flights should also be without wheel fairings. Wheel fairings add no benefit to taxing the plane, or test flying the plane up until you start testing for max speeds, best cruise and max climb rates. Up to that point, you want the wheels and tires bare for easier inspection, access, and cooling during repeated ground and taxi ops. In fact, while learning the plane it is easier to fly without the wheel fairings thanks to the additional drag that makes it slow easier and land easier. Put on all the speed fairings toward the latter part of the flight testing once you know the plane.
A word on taxi testing. I test fly a fair number of planes. 5 that I can think of in the last year, all of which were new models for me to fly. My recommendations are to go very lightly on the throttle. I make a slow taxi the length of the runway (and back on the taxiway if one exists) a time or two until I am satisfied with the ground handling characteristics. Often times I find this step needs to be repeated several times while setting up the tailwheel steering as it is rarely acceptable the first time out of the hangar.
On the next test, I'll slowly bring the throttle up to where I get the tail up in the air (tail draggers), then gently ease the throttle back out to allow me to fast taxi with the tail up, then ease the throttle back to idle and allow the tail to settle to the runway. Again, I minimize braking and am testing for proper control of the aircraft, tire tracking, tail tracking, and tailwheel performance while transitioning to tail up, then settling back down. I don't move on until I am satisfied with the results.
On the third test, I slowly add throttle until the plane reaches what I expect to be the rotation speed, then allow the tail to settle back down to the pavement again. If that felt right, the next time I will bring it up to rotation speed and allow the plane to lift off. If there is still more than 50% of the runway in front of me, I'll land the plane again and allow it to roll out while minimizing braking. Otherwise, this will be the first flight.
The next test is the first flight test. At this point in time, I already know what to expect for ground handling characteristics, have a good idea as to how it lands, and know that it will lift off without doing anything unexpected or crazy. This is where I start working from my test cards to start documenting the plane.
On every test, I make sure the plane is ready to fly, and the pilot is ready to fly. Any time you get on the runway, you can assume it is possible that things can happen and you might find yourself on your first flight.
I make it a habit to always be very gentle with the throttle going both in and out. I usually take roughly 5 seconds to roll the throttle up to full throttle. There is no advantage in slamming the engine to full throttle. If you are on a short runway, hold the brakes while you gently throttle up until you get to full power, then release the brakes. Otherwise, let the plane roll while you gently throttle up, and you will save a lot of time filling and filing the rock chips in your prop.
-Jeff Scott
Arkansas Ozarks
Sent: Tuesday, July 30, 2024 at 8:38 AM
Cc: "Adam Deem" <deemaviat...@gmail.com>
Cc: "Adam Deem" <deemaviat...@gmail.com>
I think the big takeaways from this discussion are that testing is useful, but one must understand the limitations and failure modes of the system which can sometimes only be determined though testing. In the large airplane world we have numbers for brake cooling time after an aborted takeoff or landing at various weights or sensors on each brake. This ensures the following takeoff has enough thermal headroom to prevent a hot brake incident like Zach encountered. It appears his incident was primarily related to a mechanical abnormality or failure (dragging brake pad).
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