Flying the Reims Rocket Cessna 172 into KORS (Orcas Island - US San Juan Islands at Canadian border).  The thunderstorm I waited for moved from over the airport but left it soaked.  I landed on Rwy 35 with touchdown @ 80 knots IAS with a 25 knot headwind and mostly clear skies.  That gives me a groundspeed of about 55 knots at touchdown?

In 1900-feet of runway I could only get the IAS down to 47 knots and slid off the runway into the beach and saltwater with a groundspeed of maybe 15 mph.  I was braking heavily but had no indication of hydroplanning - the 172 just would not slow down. It stayed straight  on the runway but brakes seemed worthless.

Is this something that would happen in real life?  

I was afraid to land at a lower speed because I had encountered a lot of windshear on the prior attempt and on final during this failed landing(sudden loss of 20-knots IAS).

There's no accurate runway physics modelled in MSFS, I think even a soaking wet runway changes nothing in regards to friction in MSFS. Unless that changed in one of the recent updates (?). So no idea what was happening to you, maybe a wind shift?

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

 

 

People have complained about the brakes in the rocket, but they are not normally that bad.  It is a great plane, and the developer says the braking is realistic, so only a real pilot could tell us.  My gut instinct would say your experience doesn't sound right though.

I would expect braking even on a wet runway to be a lot less than a modern car, maybe about 25% based on tyre surface contact area, but it should be a couple of knots per second at least even in the wet.

Just speculating here, but with your much too high touchdown speed of 80 kts (should be between 40 and 50 kts) you probably bounced and floated along the runway for a while with no braking action at all.

 

80kts not a speed for landing in a C172. Also ground speed means nothing, it's indicated airspeed you need to worry about. Come across the threshold at ~55kts IAS, touchdown when the stall horn sounds (40 - 50 kts).

If you're landing at an airport with a 20 kt wind sheer then you shouldn't land at that airport whether you're in a C172 or a B747.

25 minutes ago, Mike T said:

Also ground speed means nothing.

I'm gonna harp on this one statement and say that ground speed when landing and departing is one of your most important speeds to consider.

We land and depart into the wind to decrease ground speed and subsequent roll distance. Take a look at your Cessna POH; Switching from a 10 knot tailwind and landing at 50 knots to a 10 knot headwind and landing at 50 knots nearly cuts your runway usage in half. Groundspeed is insanely important.

A good rule of thumb is that double the speed is 4x the energy. An airspeed at touchdown of 60 knots with a 20 knot headwind (40 knot groundspeed) is 4x less energy than an airspeed at touchdown of 60 knots with a 20 knot tailwind (80 knot groundspeed). Also important to remember that when you're crashing into something and want to survive.

2 hours ago, Fiorentoni said:

There's no accurate runway physics modelled in MSFS, I think even a soaking wet runway changes nothing in regards to friction in MSFS. Unless that changed in one of the recent updates (?). So no idea what was happening to you, maybe a wind shift?

wet surfaces have behaved differently since launch.

20 minutes ago, WestAir said:

I'm gonna harp on this one statement and say that ground speed when landing and departing is one of your most important speeds to consider.

We land and depart into the wind to decrease ground speed and subsequent roll distance. Take a look at your Cessna POH; Switching from a 10 knot tailwind and landing at 50 knots to a 10 knot headwind and landing at 50 knots nearly cuts your runway usage in half. Groundspeed is insanely important.

A good rule of thumb is that double the speed is 4x the energy. An airspeed at touchdown of 60 knots with a 20 knot headwind (40 knot groundspeed) is 4x less energy than an airspeed at touchdown of 60 knots with a 20 knot tailwind (80 knot groundspeed). Also important to remember that when you're crashing into something and want to survive.

We land and depart into the wind because it increases the relative wind over the wings (IAS) - you don't base rotate or landing speeds based on ground speed. In a C172 with a rotate speed of 55 kts and an approach speed of 65 kts with full flaps I've got no reason to calculate ground speed unless I'm coming into a short field where I'm worried about rollout distance. If you touch down where you are supposed to then rollout distance is not a factor at 45 kts and a 1300 ft roll out, given a typical airport runway of 3000 feet or more it is not a consideration.  For flying legs and navigation it's a different story.

2 hours ago, Mike T said:

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

Straight from https://monroeaerospace.com/blog/why-planes-take-off-into-the-wind/

"pilots take off into the wind because it reduces the required ground speed."

Another article: https://pilotteacher.com/why-do-airplanes-take-off-and-land-into-the-wind/

"Taking off into the wind allows the pilots to lift off the ground at a slower ground speed and using less distance."

From the FAA Pilot's Handbook: https://www.sheppardair.com/download/faa-h-8083-25.pdf

"The effect of wind on landing distance is large and deserves proper consideration when predicting landing distance.Since the airplane will land at a particular air-speed independent of the wind, the principal effect of wind on landing distance is due to the change in the ground speed at which the airplane touches down."

2 hours ago, Mike T said:

you don't base rotate or landing speeds based on ground speed.

This is true, but I never stated that you would base rotate or landing speeds based on ground speed. My argument is that ground speed determines roll distance, and airspeed does not.

2 hours ago, Mike T said:

given a typical airport runway of 3000 feet or more it is not a consideration.

I disagree? There's a reason 3,000 feet isn't beyond the scope of the take off and landing distance chart. This is on a paved, level, dry surface. I can get the roll to be in excess of 5,000 feet on a wet strip that's down-sloping.

The idea that ground speed is not an important consideration in take-off and landing sounds wrong to me, especially when discussing contaminated runways during 20 knot gusts. Your other comment of approaching at 65 knots with a 20 knot gust is also something I would like to discuss, because the Vg diagram stall graph (I could only find the Cessna 180 Vg. Move the Vs1 to the left 10 knots) suggests a 20 knot gust will throw you into an accelerated stall at level flight at 65 knots in a clean configuration, which is a configuration I was always taught to fly with 20 knot gusts. (The argument made to me was that with flaps down at those gusts, the wind will be strong enough to catch your wing and flip you over, and you'll also have too much lift on landing for effective braking action).

I understand that you're talking about everyday situations, but the OP is discussing extreme situations at the edge of the aircraft limitations: Wet runway, high gusts, high approach speed. I don't know if the sim models hydroplaning, but your average Cessna tire will hydroplane at 56 knots ground speed on a contaminated runway.

Edited July 1, 20232 yr by WestAir

Wow. I see you're familiar with Google. 😀

So a couple of things. Please show us the chart with the v(Ground Speed) calculation required for take off and landing. Don't waste your time, there isn't one. All the speeds to which you are referencing are IAS speeds not Ground Speeds. Of course if you take off with a tail wind your take off / landing distance is increased and you risk stalling which is why you don't do it, much less with windshear.

In the OPs scenario, in real life you would not have landed at that airport if the WX required you to attempt to land a Cessna 172 at 80 kts IAS. Here's why: Let's say the winds give you a 30 kt headwind and you are calculating that you're only moving at 50 kts over the ground (GS) and use your logic. Your GS is 50 kts but since you're not driving a car, your WINGS have a relative wind of 80 kts! When you attempt to put the aircraft on the ground at 50 kts GS (80 Kts IAS) it is NO where close to finished flying (which is why a proper landing in a C172 includes hearing the stall horn at touchdown) so you will sail down the runway and off the end just like the OP did.

I don't know how to make it more clear to you. You worry about what the wings are doing at a given speed, not what your speed over the ground is doing at a given speed (unless you are calculating legs and navigating). Which is also the reason that Cessna never saw fit to put a speedometer in there - just saying.

But you fly as you see fit sir!

 

 

Edited July 1, 20232 yr by Mike T

I know when I was in Okinawa when it first rained and you had coral dust on the concrete it could get slicker than normal.

 

23 minutes ago, Mike T said:

Wow. I see you're familiar with Google.

Wow. That was rude and unnecessary... I used Bing. 😂

 23 minutes ago, Mike T said:

Please show us the chart with the v(Ground Speed) calculation required for take off and landing. Don't waste your time, there isn't one.

You misread my statement, and if you think debating with you is a waste of my time, I assure you I waste more time on far less fun activities. Back to calculations:

• What I am not saying: That the pilot has a speedometer in the cockpit showing wheel rotation and is taking off when the wheels show 55 knots groundspeed.
• What I am saying: The distances that your POH will give you to determine your landing ground roll depend entirely on the ground speed at touchdown. Period. Full stop. They determine ground-speed by asking for your true airspeed corrected for wind. Do you know what true airspeed corrected for wind is? It's ground speed.

All of the speeds used to determine ground roll, which is what you care about when landing any airplane, depend on the energy you have when you land. 2x the speed, 4x the energy.

 23 minutes ago, Mike T said:

Of course if you take off with a tail wind your take off / landing distance is increased and you risk stalling which is why you don't do it

Why would you risk stalling if you take off with a tail wind? This one I'm genuinely asking because, how it was explained to me is that, if you take off at Vr and climb out at Vx or Vy you can have a 100 knot tail wind notice no difference in aircraft performance. I was taught relative wind is all the wings care about, but it sounds like you're saying a tail wind affects Vs1?

 23 minutes ago, Mike T said:

In the OPs scenario, in real life you would not have landed at that airport if the WX required you to attempt to land a Cessna 172 at 80 kts IAS. Here's why: Let's say the winds give you a 30 kt headwind and you are calculating that you're only moving at 50 kts over the ground (GS) and use your logic. Your GS is 50 kts but since you're not driving a car, your WINGS have a relative wind of 80 kts!

Shirley you can't be serious? I never wrote anything contrary to this. There was certainly no need for you to be condescending ("You're not driving a car!") when I never once argued that the aircraft had its full weight on the wheels during an 80 KIAS rollout. What I did say was that at 56 knots groundspeed the tires will hydroplane, and that during a 65 KIAS flaps up approach with winds gusting 20 he risks an aggravated stall on final.

 23 minutes ago, Mike T said:

I don't know how to make it more clear to you. You worry about what the wings are doing at a given speed, not what your speed over the ground is doing at a given speed (...) Which is also the reason that Cessna never saw fit to put a speedometer in there - just saying.

I don't know how to make it any more clear that: A) Your ability to stop on a given surface depends on your ground speed, and B) we can have a debate without being rude or condescending. I'm having fun here and I don't think it's fair if I'm the only one.

I think we all know the basic physics regarding ground speed and IAS. Looks like you are just talking past each other here. Maybe both of you can agree that a slower groundspeed because of a headwind does reduce landing distance when landing with a proper IAS but doesn't really help if your IAS is way too high on touchdown. The aircraft will be prone to keep flying, bouncing and floating and be hard to control on the ground because of the high IAS even with a lower ground speed.

I think that is the point which is relevant to the OP.

Edited July 1, 20232 yr by RALF9636

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

Let's start with the different speeds. In an airplane, you have an indicated airspeed (IAS) that is measured by a pressure difference between the pressure at a forward pointing location of the airplane (the front of the wing in a Cessna) and a second point at the side of the fuselage. IAS differs from true airspeed (TAS) because air pressure also depends on the air density, which decreases with increasing altitude. TAS is your actual speed relative to the air, while IAS is some "fantasy" speed (i.e., it is not a velocity that an object actually has, as measured from a fixed reference point). Nevertheless, IAS is critically important during flight since aerodynamic forces (lift and drag) also depend on air density. Hence, when determining whether you are close to stalling or other operational limits, you want to know IAS, not TAS. Plus, when you are at low altitudes (above sea level), both are pretty much the same. For what happens during landing at KORS, we can therefore set TAS = IAS.

To determine ground speed (GS), you need to add the velocity of the wind (WS) to TAS. GS matters to determine the time you need to travel for a leg, which in turn is relevant for how much fuel you consume. During flight, GS only matters for timing, but after touchdown and while decelerating on the runway, GS is the most important speed (see below). IAS is not that important anymore since you don't need a lift force, but it still relevant to determine the maximum deceleration you can achieve. That matters mostly right after touchdown, but gets irrelevant once your speed has decreased significantly.

The main reason we land into the wind is to reduce GS. If you land directly into the wind, you have GS = TAS -WS, but with a tail wind, you have GS = TAS + WS. Assuming a constant deceleration A (it is not, but this approximation is good enough for the general argument), the ground roll GR to full stop is given by GR = (GS^2)/(2A). Hence, your ground roll grows with the square of GS during touchdown. If you land with a GS of 80 Kts instead of 50 Kts, that's a factor of 80/50 =1.6. The square of that is 1.6^2 = 2.56, so your GR will be about 2.5 times longer. 

Now to the forces, which determine deceleration A. There is lift, drag, and weight force as during flight, but also ground friction force and a normal force (the ground pushing back on the plane, i.e., the force that keeps us from falling to the center of the Earth). The forces that contribute to deceleration are drag and friction. Drag is proportional to speed squared. Initially that can be strong, but it becomes irrelevant once your speed is significantly reduced. To determine ground roll on a wet runway, it is relatively unimportant, friction matters much more.

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.

I don't know how this is modelled in MSFS, but it can be modelled without too much problems. In FSX and P3D, FSCaptain models the effect of wet runways, for instance. 

Peter

Edited July 1, 20232 yr by qqwertzde