Brainteaser Question: Crosswind Landings

A

asicer

Final Approach
Joined
Jan 1, 2015
Messages
9,601
Display Name
Display name:
asicer
Apologies if this has been asked before, but upon cursory examination I haven't seen it here. I have my own ideas on this topic but was curious how others would chime in:

A C172/PA28-181 is doing pattern work on runway 27 with the winds steadily blowing 270 at 5kts. The pilot has been turning final in the same place, at the same speed and applying the same power/configuration settings each time resulting in a touch down on the same spot each time. After about an hour of this, the winds suddenly shift to 180@5kts just as the plane is rolling out on final.

1. If the pilot continues making the same power/configuration changes and using the same indicated airspeed but uses a crab&kick to compensate for the crosswind (i.e. the pilot makes no other compensation for the crosswind other than a crab angle in order to stay on the centerline), will the plane:
a) touch down on the same spot as before
b) touch down closer to the approach end of the runway
c) touch down farther down the runway

2. If instead of a crab&kick the pilot uses a forward slip where will the plane touch down relative to when the winds were 270 at 5kts?

3. Where will the forward slipping plane touch down relative to using the crab&kick?
 
I would think that the extra speed and extra drag would be very close to canceling themselves out in the slip to land, and the crab and kick pilot will touch down a bit further down the runway.
 
Not really a brain teaser.

Not enough info for the slip. Drag is dependent on many things, and you may get a different answer for different airframes, different W&B, and so on.

The ground speed change is easily calculable, and the "crab & kick" airplane will land further down the runway.

It's much less cut'n'dried if the pilot is doing the instrument approach thing and approaching at constant ground speed.
 
From a theoretical point of view, if a plane is having to crab into the wind coming at 90 degrees or less, to maintain its desired course over the ground, it is going to slow the plane down. So it will have less ground speed than True Airspeed due to the crab. There are some further complicating scenarios, but that is enough for now..
 
The final answer is, "It depends", yes there will be an impact, it will vary plane by plane, regardless however it's not going to be significant.
 
1 - c.

2 - forward slip or side slip? Side, gets you c. Forward gets you a or b as desired.

3 - crab / side slip - probably pretty close. Forward slip gets you down shorter.
 
A slip is a slip is a slip. The only difference between a forward slip and a side slip is what result you are looking for; to alter direction of travel to the relative wind without altering the direction of alignment, or altered energy state through the introduction of increased drag. The mechanism itself is the same.
 
Last edited:
The crosswind with crab will require a little more throttle to reach the same spot as the headwind landing.

A forward slip to land will lose altitude faster so you will need more throttle to reach the same spot if you are not already high enough.
 
I said "C172/PA28-181" in the second paragraph. Do I need to get more specific with regards to the airframe?
 
I tend to wonder about why this is trying to be thought through?
What happens should be an automatic application and correction response
of the input from your eyes, not something you think your way through (especially for 5 knots)
 
Not answering your question per se, but....

Not a fan of crab & kick in light airplanes. It should either be slip, or "crab, kick, and dip". IMO a light airplane can be influenced by wind in the final couple of seconds resulting in a side load. Heavier airplanes not so much.
 
Henning said:
A slip is a slip is a slip. The only difference between a forward slip and a side slip is what result you are looking for; to alter direction of travel to the relative wind without altering the direction of alignment, or altered energy state through the introduction of increased drag. The mechanism itself is the same.
Click to expand...
Exactly.

Just enough slip to keep lined up (so called side slip) or punch the rudder to the floor and enjoy the elevator ride (so called forward slip).
 
Kritchlow said:
Not answering your question per se, but....

Not a fan of crab & kick in light airplanes. It should either be slip, or "crab, kick, and dip". IMO a light airplane can be influenced by wind in the final couple of seconds resulting in a side load. Heavier airplanes not so much.
Click to expand...
I always liked the rudder to the floor slip then kick in a Cessna 120.
 
In all cases - closer to the approach end.

I turn to Mssr. Newton for my answer. Some amount of energy is directed toward countering the crosswind component and that energy is not available for forward movement. To say that it was equal or greater would require answering where the extra energy is coming from.

This assumes the power is set on final at a constant location and altitude and you're not looking for some crosswind effect happening on base since you didn't specify a left or right pattern.
 
Last edited:
bflynn said:
In all cases - closer to the approach end.

Some amount of energy is directed toward countering the crosswind component and that energy is not available for forward movement. To say that it was equal or greater would require answering where the extra energy is coming from.

This assumes the power is set on final at a constant location and altitude and you're not looking for some crosswind effect happening on base since you didn't specify a left or right pattern.
Click to expand...

Sink rate is about the same whichever way the nose is pointed (assuming no significant slip). Time from the turn to the runway will be longer with a headwind compared a small crosswind (headwind component will be much less) thus, the headwind results in a shorter final path. Crosswind you land further down the runway.

Pull out your E6-B and do the wind triangles.
 
Last edited:
Both (b), but can't tell for sure if slip or crab/kick will be more "efficient". Intuitively, the slip seems to be more efficient from a drag point of view hence closer to the original landing spot.
 
Capt. Geoffrey Thorpe said:
Sink rate is about the same whichever way the nose is pointed (assuming no significant slip). Time from the turn to the runway will be longer with a headwind compared a small crosswind (headwind component will be much less) thus, the headwind results in a shorter final path. Crosswind you land further down the runway.

Pull out your E6-B and do the wind triangles.
Click to expand...

Yeah, that's twice I mentally missed the original headwind. I went from no headwind to crosswind.

Let's just say I'm mental...
 
bflynn said:
In all cases - closer to the approach end.

I turn to Mssr. Newton for my answer. Some amount of energy is directed toward countering the crosswind component and that energy is not available for forward movement. To say that it was equal or greater would require answering where the extra energy is coming from.

This assumes the power is set on final at a constant location and altitude and you're not looking for some crosswind effect happening on base since you didn't specify a left or right pattern.
Click to expand...

The energy coefficient changes with the change in direction.
 
asicer said:
The pilot has been turning final in the same place, at the same speed and applying the same power/configuration settings each time resulting in a touch down on the same spot each time.
Click to expand...

I challenge your assumption. This has never happened.
 
Capt. Geoffrey Thorpe said:
Pick up the wing as you kick the rudder to align with the runway just before touchdown.
Click to expand...

I think the two of you are confusing slips.:lol: He's talking about crab and last moment kick into a slip for a cross wind, and you're talking about kicking out of a slip to increase descent then landing; if I'm reading both of you correctly.
 
I never crab and kick.

If you can't hold centerline on final, you sure won't on touchdown no matter what method.

Flying a C-180 ground loop queen that is ...
 
txflyer said:
I never crab and kick.

If you can't hold centerline on final, you sure won't on touchdown no matter what method.

Flying a C-180 ground loop queen that is ...
Click to expand...

Many airplanes you MUST crab and kick.
I flew the Citation X for years, which have a huge wing sweep. If you flare five degrees, you have five degrees of dip before scrape a wing. You crap and kick, with NO dip. A little aileron is needed because of the wing sweep when you kick. In a plane like that you can bank just as easily using only rudder.
 
Kritchlow said:
Many airplanes you MUST crab and kick.
I flew the Citation X for years, which have a huge wing sweep. If you flare five degrees, you have five degrees of dip before scrape a wing. You crap and kick, with NO dip. A little aileron is needed because of the wing sweep when you kick. In a plane like that you can bank just as easily using only rudder.
Click to expand...


How about a C-180 like I said. Flown one of those for years?

It's pilot's choice according to what he's flying, his skills, and unicorns.

I've had it happen more than once that a crab and kick would have bent metal I'll bet 100%. I could not hold centerline with full rudder and dipping a wing, so touchdown was scrubbed at that field.
 
txflyer said:
How about a C-180 like I said. Flown one of those for years?

It's pilot's choice according to what he's flying, his skills, and unicorns.

I've had it happen more than once that a crab and kick would have bent metal I'll bet 100%. I could not hold centerline with full rudder and dipping a wing, so touchdown was scrubbed at that field.
Click to expand...
Never flew a 180 specifically.

But I'm not sure I get your point here..
 
oldShar said:
I tend to wonder about why this is trying to be thought through?
What happens should be an automatic application and correction response
of the input from your eyes, not something you think your way through (especially for 5 knots)
Click to expand...

Thank you! Yes it should be automatic and five knots is not a cross wind its a cross breeze, hardly worth noting.
 
txflyer said:
How about a C-180 like I said. Flown one of those for years?

It's pilot's choice according to what he's flying, his skills, and unicorns.

I've had it happen more than once that a crab and kick would have bent metal I'll bet 100%. I could not hold centerline with full rudder and dipping a wing, so touchdown was scrubbed at that field.
Click to expand...

That's why "conventional gear" was replaced for the most part, what once caused a ground loop and bent metal now is self correcting. That's also why so many triangular airports and flat aerodromes from the WWII and prior now have one or two runways, the equipment is not nearly as wind direction restricted. Luckily many airports still have diagonal taxiways to use in a pinch.;)
 
Henning said:
That's why "conventional gear" was replaced for the most part, what once caused a ground loop and bent metal now is self correcting. That's also why so many triangular airports and flat aerodromes from the WWII and prior now have one or two runways, the equipment is not nearly as wind direction restricted. Luckily many airports still have diagonal taxiways to use in a pinch.;)
Click to expand...


You oughta hear the comments when I land in the grass nowhere near the runway or a taxiway!

It literally ****es some people off even though I perform a great landing.

It's my insurance, my plane, my ass, so why the angst? :dunno:
 
txflyer said:
You oughta hear the comments when I land in the grass nowhere near the runway or a taxiway!

It literally ****es some people off even though I perform a great landing.

It's my insurance, my plane, my ass, so why the angst? :dunno:
Click to expand...

Five knots in a taildragger is a non event even in a direct cross wind. 10 knots is an attention getter ( direct) and fifteen , in a Stearman will cause me to be nervous, especially on hard surface. Before WW2 most small airports had one runway, usually into the prevailing wind if possible as money was key to operating any airport, ( the depression!) after the war, things boomed , money was available due to huge amounts of govt. Spending during the war, jobs etc.
 
Jimmy cooper said:
Five knots in a taildragger is a non event even in a direct cross wind. 10 knots is an attention getter ( direct) and fifteen , in a Stearman will cause me to be nervous, especially on hard surface. Before WW2 most small airports had one runway, usually into the prevailing wind if possible as money was key to operating any airport, ( the depression!) after the war, things boomed , money was available due to huge amounts of govt. Spending during the war, jobs etc.
Click to expand...

A lot of airports before the war were also built as aerodromes, smooth fields capable of taking traffic in any direction and you always landed into the wind.
 
Isn't this just a simple wind triangle problem? So with identical power settings and a course of 270 what is your ground speed going to be with a direct headwind of 5 knots as opposed to a wind of 5 knots from 180 degrees? Forget about where the nose is pointing just pull out yer old metal landing detector.
 
Jimmy cooper said:
Five knots in a taildragger is a non event even in a direct cross wind. 10 knots is an attention getter ( direct) and fifteen , in a Stearman will cause me to be nervous, especially on hard surface. Before WW2 most small airports had one runway, usually into the prevailing wind if possible as money was key to operating any airport, ( the depression!) after the war, things boomed , money was available due to huge amounts of govt. Spending during the war, jobs etc.
Click to expand...


Ask any taildragger pilot what direction the wind is coming from at any point in the flight and chances are they can tell you.

I catch myself on x-country looking for clues. And if that doesn't work, I'll look on the weather page of the 696 and see what the nearest station says.

I'm a simple day VFR flyer, but if the mill ever quits at altitude, I don't think you guys will be talking about me... ;)
 
Silvaire said:
Isn't this just a simple wind triangle problem? So with identical power settings and a course of 270 what is your ground speed going to be with a direct headwind of 5 knots as opposed to a wind of 5 knots from 180 degrees? Forget about where the nose is pointing just pull out yer old metal landing detector.
Click to expand...

No, it's not that simple because it adds variable drag to the equation.
 
You must log in or register to reply here.
Back
Top