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Greatest gradient for taxi uphill?

I am wondering what determines this.

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

Administrator
Shoreham EGKA, United Kingdom

Tipping back on your haunches?

Oxford (EGTK)

You are right if flying a jet (constant thrust) and your best L/D is lower than inverse of friction coefficient (T/N) then if you can climb 30deg at Vx then you can taxi 30deg up on the ground at 1kts

In a piston, thrust is even higher at slow speeds and tarmac friction coefficient is lower than D/L at Vx, so I expect you can taxi and climb very steep climbs at 1kts, maybe 30deg on TB20 at sea level? but you should not plan to accelerate for takeoff unless Vx condition is there, something about 10deg with 1200fpm & 70kts at sea level

It only works on flat pavement, not sure about (frozen) lakes, snow & wet grass but I would not try digging small wheels with power…

Last Edited by Ibra at 18 Feb 23:27
ESSEX, United Kingdom

Ibra wrote:

In a piston, thrust is even higher at slow speeds and tarmac friction coefficient is lower than D/L at Vx, so I expect you can taxi and climb very steep climbs at 1kts, maybe 30deg on TB20 at sea level? but you should not plan to accelerate for takeoff unless Vx condition is there, something about 10deg with 1200fpm & 70kts at sea level

Is this a comparison of climbing a steep hill, to a Vx climb in flight? I sure hope that the climb in flight is at a very high power setting, and the climb on the ground is certainly not at a high power setting! Let alone possible prop damage, and overheating an engine, even approaching a steep grade creates the risk of a prop strike!

A piston power GA plane will not climb anything like a 30 degree incline. I know this because I have beached a number of amphibians out of the water up a ramp. It can take very high power to ramp up even a 15 degree incline (which seems really steep).

Be very careful with any vehicle on a steep incline, let alone a plane. If you get part way up, and realize you cannot make it, what do you do now? Apply the brakes, and the plane sits on its tail? Now it’s stuck on the hill, and possibly damaged. Or, it rolls backward out of control! If you realize you cannot make it, you’re going to turn around on the incline, and go back down? If you get to the top, you’d consider going back down? Would you think that the brakes might hold you going back down? It’ll end badly!

We have criteria for taxiing a skiplane up an incline (intended for up and over a snowdrift), it’s not 30 degrees!

It only works on flat pavement, not sure about (frozen) lakes,Quote

I have landed on many frozen lakes, I have never seen a hill on one.

Last Edited by Pilot_DAR at 19 Feb 04:23
Home runway, in central Ontario, Canada

The original question was: Vx climb => taxi uphill? it’s pure high school physics, it’s not about the risks or flying lessons for climbing at Vx or taxi uphills, for that worth checking with a familiar & qualified insutructor before trying !

This was my answer Peter’s question: you can land & taxi uphill but not go-around at Vx…

You HAVE to taxi at high power in Courcheval or StTropez and it’s 15deg-25deg uphill in +6000ft amsl (I am sure same thing is doable at 30deg at 0ft amsl is doable, again on pure power and density altitude physics but yes there are some “aircraft gemetry” limits), obviously you can land there and taxi with high power (and some inertia if density altitude is high) but you surely can’t go-around at Vx, you will learn that in the first hours of aliport checkout

I used to land & taxi motor-glider or aerotow gliders in Dunstable gliding club, it has a huge bowl which is -25deg down and +25deg up, yes you can’t climb on that grass in winter at slow speeds but we know how to operate tugs & motor-glider there, it’s not as difficult as you describe with common sense but we had people stuck and asked for help one who did break a propeller (too much power & breaks and maybe stick in wrong position)

As suggestion if you can’t climb at +25deg due to power or descend at -25deg with no breaks, try left & right zigzags at shallower angle on same surface but you should have “taxi plan” designed well ahead, especially with crosswinds and tailwheels in grass, but I am not sure if you got that much space when getting an Amphibian out of water?

Getting stuck on uneven surfaces or transition between wo is a different matter, you will not be able to taxi in deep muddy grass with 0deg angle at full power nor climb parking ramps or taxi over wheel chocks…but that’s different?

The 30deg limit om skis is it because of no available power to taxi uphill? or because of no breaks downhill?

You a right about frozen lakes, silly of me, I never landed on one but how about landing uphill on that glacier under the wing?

Last Edited by Ibra at 19 Feb 06:54
ESSEX, United Kingdom

What you learn in the first hour of the Courchevel checkout is that there are three options: one good, land on the proper touchdown spot and inertia (certainly not “high power taxiing”) takes you up the hump to the flat. And two bad: come in short and crash into the threshold embankment, or come in long, roll up the hill and come to a jarring stop into the embankment at the end of the runway. A 25-degree slope is about 45%… I’ve never seen people try taxi uphill from a stop at Courchevel which is only 18%.

Glacier landing – it depends on snow condition, but usually taxiing uphill in snow is a losing proposition considering that at full power the engine only delivers 65% of rated HP… we probably wouldn’t even land on a 25 degree upslope, let alone try to taxi uphill.

T28
Switzerland

You are right, land & roll is not same as taxi from scratch

I will let Peter answer if TB20 can climb 20% gradient on full power at the hill near Shoreham (she does +12kts climbs near 60kts)



Last Edited by Ibra at 19 Feb 07:23
ESSEX, United Kingdom

I don’t think that PA46 is related to the topic; he just came in too fast, etc.

What I was wondering is what the physical limit is on the uphill taxi gradient. You have not far off 100% engine power, so it should be close to Vx which for a typical IFR tourer is in the area of 10% i.e. 1 in 10.

Administrator
Shoreham EGKA, United Kingdom

To a simple first order calc (and ignoring rolling resistance) I think the limit should be Arcsine(T/Mg) Where T is the thrust and M is the aircraft mass.

Ibra wrote:

The original question was: Vx climb => taxi uphill? it’s pure high school physics, it’s not about the risks or flying lessons for climbing at Vx or taxi uphills, for that worth checking with a familiar & qualified insutructor before trying !

Fair enough, a thought question, with no intention of actually attempting it.

The added factor of social media, which was not a factor during my formative early years in piloting, induces some pilots to start “trying” things in airplanes. In the many decades past, pilots were trained by other experienced pilots, who may have been instructors, maybe not. In a certain piloting discipline, I am an “instructor” within the intent of the regulation, though I have never held an instructor rating.

At best, and eager adventure pilot seeks out an experienced mentor pilot (maybe an instructor, often not), to guide/warn them for a particular type of operation. At worst, they read about it on social media, and go and try it, then we read about them. Some very experienced pilots make some things look easy, and perhaps with that experience it is, how do you measure that experience?

Manufacturers start to have the shudders, when they realize what pilots may be attempting in their products. Their legal teams begin to imagine all the legal grief which could be encountered in their products by adventure pilots, and in a over abundance of caution start to use the word “prohibited” in their approved flight manuals.

Knowing the whole picture is important. So, while some high school math has enabled rough performance calculations about an airplane on a hill (’cause we always make performance calculations before we fly), the thought exercise will be more complete if we also complete the W&B calculations too… 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?

While test flying the Grand Caravan yesterday, the ground reminded me that after I parked (on very level ground), no one was to disembark until we had the signal that the tail pogo stick was in place (as we were returning from a full aft C of G test). Obviously, the ground crew knows that pilots jumping out of the cockpit could result in their very expensive tail boom being banged off the ground.

Tunnel vision is always bad for pilots, so assure that when formulating a thought question, either all the facets are either thought through, or at least presented as cautions or limitations to the other outcome – which would tend to make another outcome moot anyway…

Skiplanes require higher obstructed taxi requirements than wheel planes. This is because of the likelihood of encountering snow banks or drifts while taxiing (some rather hidden). Too high an obstruction could result in the plane hanging from the toes or heels of the skis by the check cables, which they are not designed for, and risks a very inconvenient situation. Airplanes have crashed from controlled flight because of check cable failure. So, again, mentoring from a wise experienced pilot – who may, or may not be an instructor….

Home runway, in central Ontario, Canada
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