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Reveal all secret hardcoded friction and braking coefficient tables - MSFS DevSupport (flightsimulator.com)

Present MFS physics model does account for braking efficiency with contaminated runways, but I am almost 100% sure FS 2024 will bring it to a much higher level, based on the astounding ground physics available in IL2 Great Battles 😉

2 hours ago, jcomm said:

astounding ground physics available in IL2 Great Battles

How to take off in a 190 when the weather is a wee bit wet ...

 

 

3 hours ago, qqwertzde said:

WestAir got it right, an here is the physics behind it.

In my opinion, both users were right.

Both are very important, the IAS-speed for aerodynamics reasons and the groundpseed for ground-friction and -braking reasons.

deleted

Edited July 1, 20232 yr by Franz007
double post

3 minutes ago, Franz007 said:

In my opinion, both users were right.

Both are very important, the IAS-speed for aerodynamics reasons and the groundpseed for ground-friction and -braking reasons.

You are absolutely right. As I wrote in my previous post, IAS is critical for procedures in the air. However, the topic is about the ground roll on wet runways, and then IAS is much less important than GS. That's why I think WestAir's answer to the topic is the correct one.

Peter

11 hours ago, Mike T said:

We land and depart into the wind because it increases the relative wind over the wings (IAS).

Not by the conventional definition of the term, which refers to an aerofoil in motion  

The relative wind at any given IAS is exactly the same whether you’re in a 100kt headwind or 100kt tailwind. 

Takeoff and landing performance on a given runway condition and aircraft performance is exclusively determined by ground speed. 

Edited July 1, 20232 yr by 2reds2whites

12 minutes ago, Franz007 said:

and the groundpseed for ground-friction and -braking reasons

Even before actual touchdown, groundspeed along the runway in the last bit of your short final and flare determines how much runway you will use up before the wheels touch.

20 minutes ago, qqwertzde said:

You are absolutely right. As I wrote in my previous post, IAS is critical for procedures in the air. However, the topic is about the ground roll on wet runways, and then IAS is much less important than GS. That's why I think WestAir's answer to the topic is the correct one.

Peter

Got it! Thanks 🙂

12 minutes ago, Glenn Fitzpatrick said:

determines how much runway you will use up before the wheels touch

Thats what i would refer to "landing distance". Usually you assume passing the RWY-slope at a heigh of 50ft before touchdown.

14 hours ago, sd_flyer said:

Here is accurate physics but on dry runway hard breaking and consequences due to lack of directional control 

 

 

What  I see here is aircraft is drifting to left . (RIGHT RUDDER!) . Overcorrects to right and tries to takeoff . 

Here we go love these popcorn Avsim threads.

44 minutes ago, MikeH99 said:

Here we go love these popcorn Avsim threads.

This would be a 90 page thread by now if OP had asked if you use throttle for altitude or airspeed. 😂

1 hour ago, PIPER042 said:

What  I see here is aircraft is drifting to left . (RIGHT RUDDER!) . Overcorrects to right and tries to takeoff . 

What you see is loss of directional control, wrong rudder application, and incorrectly initiated go around. You can easily do same thing on a wet runway with hard application of brakes.

9 hours ago, qqwertzde said:

Friction F is independent of speed, but it is limited by the normal force N. One has

F < mu N,

where mu is the friction coefficient between the tires and the runway. At sufficiently low speed, deceleration is proportional to friction (Newton's second law, F = M A with M the mass of the airplane). Hence the maximum acceleration you can achieve is proportional to mu. We have seen above that GR is inversely proportional to A, so that the minimum GR is proportional to 1/mu.

Now to the answer to the question raised by the OP. The friction coefficient mu depends on whether the runway is wet or dry. For dry asphalt, one roughly has mu = 0.7, while on wet asphalt it decreases to 0.5 ( https://www.isasi.org/Documents/library/technical-papers/2018/Thurs/Aircraft Tyre Hydroplaning and How to Analyse it in Runway Excursion Events - Gerard van Es.pdf ). That means your ground roll increases by a factor of 0.7/0.5 = 1.4. The real problem occurs when you have aquaplaning, which reduces mu to 0.1 or less. Your ground roll is then 7 times longer than what you would normally have. This problem is much worse in a plane than in a car since the lift force decreases the normal force, und thus friction.

Hydroplaning can occur at groundspeeds above ~seven to nine times the square root of the tire press in PSI, depending on the tire.  A C172's 6.00x6 4-ply tires are normally serviced to ~30 PSI, so you can realistically begin hydroplaning above 37-48 Knots GS.  And even short of hydroplaning, the lift still being produced by the wings in a hot landing significantly reduces that normal force (weight on wheels), which is why it's important in spoiler-equipped acft to get them out during a landing/abort and dump the lift so there's maximum weight on the wheels and therefore maximum coefficient of friction.  In a C172 with no spoilers, that's normally accomplished by an on-speed full-stall landing--airspeed above that produces lift that reduces braking effectiveness...the only measure you can take to kill the lift is to get the flaps up.

We had a C-141 that landed at Iwakuni MCAS Japan back in the late 80s that landed hot and the crew didn't get the spoilers out...it ended up sliding sideways off the end of the runway, lost a wing to the post-crash fire, and was NOTAMed as an obstacle there for the next 3 years.  They eventually took the other wing off and shipped it back to the US to replace one that had also been burned off when another C-141 taxxied into a light pole.

Both groundspeed and airspeed make a difference w/r/t landing distance.

TEST Results:

IF I land the Reims Rocket C 172 at 80 knots IAS with a descent rate of less than 50 fpm (hard to do in gusty winds) - I can easily stop in less than 1000' no matter how wet the runway.  My problem was caused by the tendency of the Reims 172 to bounce if the VS at touchdown exceeds 100 fpm.  It REALLY bounces, (VS ~120 fpm at touchdown),  getting back up to 30-feet and then is impossible to settle back to the runway.  My original post was based on landing in very heavy weather with limited visibility.  I did not realize that the plane was bouncing so far off the runway.

Much testing (FStart is great for this type testing) in a standard C-172 and the Reims rocket shows a significant difference in the landing characteristics of the two planes.  The base C-172 does not bounce very much at a touchdown VS of 100 fpm where the Reims bounds back into the sky.

AND, my testing indicates that landing even the standard C172 on a very wet runway results in much longer runouts due to poor braking.