Ground Handling | List of aircraft wheel friction values

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61

Well as far as I can tell from the SDK it’s all tied to IAS

IAS is both the the speed of the aircraft (or pitot tube) through the air plus any wind effect. So, for example you have a perfect 10knt headwind and you are taxiing into it at 10knts. Your IAS would be 20knts.

But as the wind shifts off the nose, it has less of a headwind component and more of a crosswind component. The crosswind component will not add anything to IAS.

So by tying the operation of these values to IAS it means that the aircraft will start to experience the effect of wind at different points on its roll: the more off the nose, the higher the aircraft’s own speed will be before the wind effect is introduced, with the point being determined by what speed and direction the wind has and what speed and direction the aircraft has.

In our same example, if say the minimum value had been set to 16.9fp/s (10knots), the aircraft would have to accelerate not at all before the wind effect value is applied in a pure headwind, but would need to reach 10KIAS if it were a pure crosswind.

Once that minimum is reached, crosswind effect will start to be increased proportionally until the max value is achieved. Then full wind effect will be experienced.

And as an ‘assist’ this makes sense - it’s not the headwind component that causes the issue the value is trying to address. It’s the crosswind. I can see the advantage: you.are not eliminating the headwind effects - which are ‘good’, just the ‘bad’ crosswinds

This is my understanding of how it works at least: the higher you set the minimum value, the later the cross-wind will start to be felt. If you set it sufficiently above an aircraft Vr, you would presumably experience no crosswind effect on the ground at all!

In practical terms if the aircraft was weathervaning badly in say a 10knt cross wind and had a Vr of 50, you might want to set the minimum value to 15knts and the maximum to 40? But this will in turn depend on how much rudder authority is available, the availability and effectiveness of nose wheel steering, the scale of propwash and P-factor etc. What you don’t want is the aircraft to be a pushed around in low speed crosswinds to such a degree that lateral control via the rudder cannot be obtained before the aircraft has deviated significantly from c/l.

Now, from my perspective this value is a bit of a cheat - a hidden assist baked into the FM. Using the tire friction scalars is a better route all round. But even these won’t stop weathervaning in gentle winds at low ground speeds. This is where I think using the min/max crosswind effect values have their place as you can set them sufficiently low to eliminate forces that should simply never overcome the static friction of the contact points, but not interfere with how the aircraft experiences a crosswind on take-off and landing.

All of this could do with more testing though. Like, how smooth is the transition as ‘max’ IAS value reached? A sudden increase or something more subtle? Do we even have direct evidence that they work?

I know the tire friction scalars absolutely do work (set them to zero and you aircraft becomes a contender in Tokyo Drift!). But also care is needed with the friction scalers too: too high and they may effect turning at low speeds, especially tail-draggers.

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62

This seems like a strange result. I’ve seen it in the Kodiak recently, where the plane’s steering seems to drift due to slipstream effect from the propeller just as I’m beginning to roll forward from a parking spot at low power. If the wheel friction values are set to higher values, why wouldn’t the friction prevent any vaning or drifting at the lowest speeds? If anything, friction should be maximized when the wheels are at rest, correct? I would expect friction to gradually decrease during takeoff roll as lift reduces the weight resting on the tires and the tires are rolling increasingly quickly.

63

Indeed it is a strange result and I’ve noted the same thing. Whi h is why I would advocate using the min/max values also.

I can only speculate why. It might be that the scalars are only being applied as higher ground speeds (implied in the variable name). The SDK has little to say.

64

Contact friction will indeed decrease as weight on the wheels reduces with rising transitional lift.

But offsetting that is increasing inertia and gyroscopic forces which help keep the wheel tracking and resist the lateral force of the crosswind.

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65

Thanks for your extensive explanation, way more useful than the documentation we get in the SDK.

I will use this to experiment now that I understand a bit better what the variables are supposed to do.

Best,
Raul

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66

I found your post while investigating the same issue for the Piper Arrow. I found a solution (at least for me) by using the min_available_steering_angle_pct parameter, please see my post here

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67

Thanks. Very interesting. I think this is another relatively new variable? I was aware Asobo had been using it to tune the C172 G1000 in SU12 beta, but I’d not done too much investigation of how it worked.

It sounds like another hidden assist in some ways, as for many aircraft, nose wheel steering become locked above a certain speed. But it might be a better option if the alternative was either eliminating crosswind effects or increasing rudder authority to the point where it would make the rudder too powerful/twitchy in flight.

68

Any owners of the new ATR able to report what it’s values are?

69

I took the Turbo Arrow out for a quick flight again this evening and tried neutral trim with slight forward pressure on the yoke like you suggested. The wind sock showed a slight right crosswind. The takeoff roll was rock-solid (no swerving). Quite a different experience than what I’ve been used to in MSFS.

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70

Nice. The forces that act on the aircraft during the t/o roll are quite dynamic and there is this point where the wing is beginning to create lift.

Technique really does make quite a difference. For me this becomes most clear in a tail-draggers. And it’s one of the reasons it’s so hard to isolate problems / test new values. : Landing a tail-dragger in strong crosswinds is so technique-sensitive it doesn’t take much to bork the landing - much more so than the ground physics

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71

Have not checked through all of the posts 100%, sorry if double post. but PMDG 737 and 738 has those values

ground_crosswind_effect_zero_speed=18.43905
ground_crosswind_effect_max_speed=303.806
ground_high_speed_steeringwheel_static_friction_scalar=3.82115
ground_high_speed_otherwheel_static_friction_scalar=3.42203

I got an update of pmdg today but seems to be exactly same values

Version: 3.0.69

ground_crosswind_effect_zero_speed=18.43905
ground_crosswind_effect_max_speed=303.806
ground_high_speed_steeringwheel_static_friction_scalar=3.82115
ground_high_speed_otherwheel_static_friction_scalar=3.42203

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72

Thanks! We don’t have the 737 or 738 on the list yet. I’ll add them on when I do the next batch update.

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73

Just had a look at this properly. 303!!! That means the 737s won’t experience full crosswind effect until the wind is blowing a nice 180knots. (Or to be more precise, until the aircraft reaches 180KIAS).

No wonder it doesn’t weathervane!!

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74

I guess what you’re trying to say he is the velocity of the airflow over the wing is a combination of aircraft velocity + the headwind. But isn’t ground speed the speed of the aircraft adjusted for wind speed/direction vector?

I was unsure, so stared checking and then fell into a rabbit hole of differing definitions with a bucket full of IAS, GS, TAS and pressure formulas.


Why doesn’t an increasing headwind change indicated airspeed?

Indicated airspeed is the pressure of the airplane moving through the air at the pitot tube. This pressure is called, Q force. An increasing headwind will not change the pressure Q force at the pitot tube because the entire airmass in which the airplane is flying in is increasing and and the speed across the ground, ground speed, is decreasing. However, a very rapid change in headwind or tailwind will cause a change in indicated airspeed because of inertia.

I’m assuming the Q Force he’s referring to above is:

From the velocity calculation, volume flow rate can be determined using the formula Q=AV. Flow rate Q is equal to the velocity multiplied with the cross sectional area of the duct or pipe. https://blog.dwyer-inst.com/2018/08/01/air-velocity-and-flow-measurement-with-pitot-tubes-2/

Is there a definitive reference for all these wind and airspeed type scenarios?

75

Airspeeds play out a little differently when on the ground versus airborne, when the airplane becomes part of the overall fluid of the airstream. I think I’m reading you right when you say you want a breakdown, so here goes:

Indicated airspeed (IAS) is the dynamic pressure (Q) acting on the pitot tube. As you discovered, it’s unaffected by a change in wind (unless it’s a very sudden change) because the airplane is part of the whole flow, rather than affixed to a point and simply measuring what goes past like we do on the ground. A free balloon, for example, will move with the airflow, thus traveling over the ground, but have an indicated airspeed of zero (in steady winds). IAS is calibrated for standard temperature and sea level pressure, which will come into play later.

Calibrated Airspeed (CAS) is indicated airspeed corrected for instrument error and position error. You’ll see this especially in light aircraft at low speeds and maybe full flaps. CAS is technically the correct airspeed to make all further airspeed conversions and calculations, but at higher airspeeds it is usually close enough to IAS that we just use IAS. CAS conversions are particular to the aircraft and can be found in the aircraft POH.

True Airspeed (TAS) is CAS corrected for non-standard pressure and temperature. As you climb, let’s say you’re moving the same exact speed through the fluid of air, but since the air is less dense as you climb, there are fewer total molecules entering the pitot tube. So your actual speed through the fluid is your TAS, but the as the density of the fluid drops, your CAS drops with it. The result is that for a given CAS, the TAS will generally increase by about 2% per 1000 feet of altitude above sea level. This is why an airliner at 35,000’ might have an IAS (dynamic pressure) of 250kts, but is actually going maybe 450 knots through the air (TAS). At an extreme, a vehicle in orbit may be traveling thousands of miles per hour (TAS) but has an IAS of zero because there aren’t enough (or zero) molecules of air to register on a pitot tube. Warm air also affects the relationship between CAS and TAS, as it is less dense. So even at sea level, on a hot day your TAS will be higher than your CAS, and vice-versa for cold. This is one reason we chew up so much runway when taking off or landing on a hot and high altitude day.

Groundspeed (GS) is when you add (or subract) the total movement of the fluid of air to the TAS, dependent on how the velocity vectors interact. More simply, it’s the exact amount of speed relative to the ground.

There are also Mach numbers and equivalent airspeeds that deal with compressibility, which come into play when you get fairly fast, but I’m keeping this simple, in the GA realm.

The best aviation resource for all of this is the Pilots Handbook of Aeronautical Knowledge (PHAK). There are many others, but this one is official and free (well, courtesy of the US taxpayer).

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Nav Log estimated time per leg
76

This definition, like most definitions of IAS, deals with what is going on in flight and not on the ground.

IAS will be influenced by the wind on the ground as you are not part of the airmass - you can see this most clearly when an aircraft is parked into the wind on a gusty day : a 20knt wind blowing directly onto the pitot tube aperture will be read just the same as if the same aircraft was taxiing at 20knts GS on a windless day. The less the wind is on the nose, the less it will contribute to that reading.

It was an elegant choice by Asobo to use IAS for setting the upper and lower limits of this assistance parameter as it self-adjusts on the ground and is irrelevant in the air (which you want).

I’m not sure about Devs using very high values like PMDG have: their 737 will never experience full crosswinds on the ground as the upper range is above Vr. Better to get the tire friction right, although the high values that some aircraft seem to require for the friction scalars can start to lead to odd behaviour on touchdown. Still they had their reasons I’m sure and it flies real nice.

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77

In the Warriors I fly, it’s really important to have some nose down trim or the nose tries to lift really early, as well as the right wing. It’s a little scary actually. And this is with near zero crosswind.

78

That’s interesting! I’ve never really needed nose down trim for takeoff in a PA28. Is it a balance thing? Rigging?

79

The Just Flight BAe-146 was just updated today (v 0.1.9). Wheel friction and crosswind parameters are now included:

Plane Steering Wheel Friction Main Wheels Friction CrossZero CrossMax
BAe-146 (all variants) 1.5 1.2 10 140
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80

Thanks - do please keep them coming!