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Greatest gradient for taxi uphill?

Pilot_DAR wrote:

So what will be the C of G position of a tricycle and taildragger airplane when on the surface at a 30 degree angle to horizontal? The C of G limit for taxiing, should be presumed to remain between the three or four wheels, will it?

Agree, while high school physics may say it’s possible “power-wise” (the law for taxi uphill with T/N friction froces is the same as L/D aeordynamics on Vx climbs but ofc there are load of caveats), first, there are not that many “bumps & holes” in the air compared to the surface, second, things are not very obvious as far as WnB & GC is concerned, they just don’t work the same way when speed = 0 as “you can’t hold it with the trim/elevator” and surely you can’t climb at heavy weight about square-root(VX/VS)*MTOW but you maybe able to taxi at 300% MTOW if your wheels are strong

Here is a funny example of WnB & GC that surely worked on paper at ASI = Vx but not at ASI = 0 when Ben Mallah & Co go inside, this was 0deg in the parking



Last Edited by Ibra at 19 Feb 13:27
Paris/Essex, France/UK, United Kingdom

One interesting point here is that a VP prop should deliver close to 100% power, whereas a fixed pitch prop will deliver far less.

Administrator
Shoreham EGKA, United Kingdom

Back in 2019, I was at an Italian grass strip with an average gradient percentage of 16%. It was like 12% (at the threshold) up to 20% or even more at the very end (total runway length was 300 m). Since the parking lots where alongside the middle of the runway, I had to taxi out (uphill) to the start, but the Dimona with a 914-Rotax was unable to reach the top, with a DA around 7.000 ft and loaded for +/- 750 kg. I gave full power, but we didn’t manage to reach the last 20% of the runway. Nonetheless, there was still enough space to take-off safely due the downhill gradient. We might have reached the top if the runway was concrete or asphalt.

Last Edited by Frans at 19 Feb 15:37
Switzerland

Peter wrote:

One interesting point here is that a VP prop should deliver close to 100% power, whereas a fixed pitch prop will deliver far

Good point for fixed pitch, I heard about “slow climb props”, “fast cruise props” but not heard yet of any “slow taxi props”
Static RPM in C172 is 2300 and max rated is 2700, so probably 20% less thrust and maybe 20% less on gradient

Last Edited by Ibra at 19 Feb 16:11
Paris/Essex, France/UK, United Kingdom

2300 v 2700 is much less than 100% power.

Administrator
Shoreham EGKA, United Kingdom

I’m pretty sure that from the Courchevel rating, I remember that I was told a 172 or DR400 180hp wouldn’t climb the hill if it came to a stop and would need to be rescued. That’s 18deg?

After touchdown and when on all 3 we add a large amount of power to maintain the speed to ensure making it to the top.
Unless the brakes can be guaranteed (who can say that) if you do stop, try immediately to turn across the slope. Running back at Courchevel could be life threatening.

United Kingdom

GA_Pete wrote:

I’m pretty sure that from the Courchevel rating, I remember that I was told a 172 or DR400 180hp wouldn’t climb the hill if it came to a stop

Is that slop steepness? or lack of power at 6000ft versus sea level?
I am sure 180hp Robin can cope with 20% tarmac slope at sea level

Dunstable GC has pretty steep 25% bowl in the middle and 180hp Robins have no issues tugging two-seat gliders along that, but it’s more in down first, then up fashion

Taxi back when grass is wet need some planning & zigzags…

Last Edited by Ibra at 19 Feb 17:21
Paris/Essex, France/UK, United Kingdom

People seem to be talking about power, when they probably mean static thrust (which is so easy for us to measure that everyone should know it for his aeroplane).

So if a certain Jodel D140 with a garbage Hoffman “cruise” propeller develops, say, 250 kg of static thrust at or near zero density altitude and is loaded to 1 tonne all-up mass, it should just about hold position on a 1 in 3.9 slope (i.e. about 14 degrees) at zero DA.

Fixed and constant speed propellers generally develop a bit more thrust at low speed than when stationary, so it helps to keep moving.

The steepest part of the runway at Courchevel is not quite 1 in 5 (11 degrees), but I’ve never been there on a zero DA day, so I wouldn’t stop my Jodel on that slope, even when fitted with a less catastrophically useless propeller.

With a lightly-laden proper bushplane like a Maule or Husky and almost 400 kg of thrust from a modern 82” prop (and more than two feet of prop to ground clearance), I’d be much happier to stop to pick up a hitch-hiker half way up the LFLJ runway.

Last Edited by Jacko at 19 Feb 23:06
Glenswinton, SW Scotland, United Kingdom

but not heard yet of any “slow taxi props”Quote

They exist, rare though… I had Sensenich make one for me years ago. It was designed to red line the engine RPM of the O-470 at 60 MPH, so it had super thrust at low speeds, and was useless above 60 MPH. It worked wonderfully. I had it made for the Fire Department airboat I’d installed the O-470 in. There was lots of need to climb out of the water, up onto the ice, and no need to ever go more than 60 MPH on the ice.

I did the flight testing of a glider tow Bellanca Scout with a four blade Hoffmann ground adjustable prop. The purpose of the flight testing was for me to determine the desired pitch, and then approve the fixed pitch prop which Hoffmann would make for the plane. My first takeoff attempt at the randomly selected fine pitch saw 2700 RPM at 30 MPH, so takeoff was not possible. After the third setting, I was able to takeoff, though could not fly more than 70 MPH, so landed back straight ahead. Eventually we found just the right pitch to produce the desired performance, but not overspeed when I dived the plane. I approved that pitch prop as a fixed pitch by STC on the Scouts operated by that glider group. They made lots of neighbours happy with a lot less noise than the two blade!

Home runway, in central Ontario, Canada, Canada

Peter wrote:

I wonder if the steepest taxi gradient is – to a first order, at least – equal to the climb gradient at Vx.

for my plane the answer is simple. Static thrust is 300 dN (old ~300 kp). MTOM ~800 kg. Taxi grad. is arcsin 3/8 = 22°. Best rate of climb is 1600ft/m at 70kt, i.e. arcsin 8/36 = 13° (if I’m right…). Nice question, Peter.

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