Coping with significant crosswind

[motzartmerv]

Here's a question for yas. Take the acft in the video, zero forward speed into wind but indicating 50kts ( say) what happens when he turns downwind? Continues at 50 kts? Or Stalls instantly ?

 

 

[kaz3g]

Here is a backward flying Piper Super Cub:It seems to have flown backwards right off my page, 80 knots...

 

<edit> Ahhh. It just arrived and I see the pilot wisely kept a lot of height while he tried that trick

 

kaz

[kaz3g]

Here's a question for yas. Take the acft in the video, zero forward speed into wind but indicating 50kts ( say) what happens when he turns downwind? Continues at 50 kts? Or Stalls instantly ?

Hi Merv

 

As best as I remember it goes like this...

 

Simple theory says the pilot (obviously a bloke <grin>) is flying in a block of air and his airspeed is relative to his motion in the air, not the ground, so he just keeps on flying. Inertia bears on the outcome, however, because the aircraft has to accelerate to counter the loss of the headwind component and this doesn't happen instantaneously, especially if he turns quickly. In a Cub he probably needs to lower the nose, open the throttle and pick up another 30-40 knots of airspeed before starting the turn.

 

The problem is even worse if you have a significant headwind on takeoff. Close to the ground, the headwind is slowed by friction (wind shear) but, as you climb it becomes stronger. Beware the climbing turn downwind on takeoff performed without plenty of airspeed, or worse, decaying airspeed.

 

My question is would a Cub be more or less affected in this situation than say a Piper Warrior?

 

kaz

 

 

[facthunter]

Sneaky Andy...

 

If you want to know what happens just imagine the earth is not there. Gone!. The plane only knows the air mass and a balanced turn it will describe a circle in the air mass and the airspeed will not change. If you were in a balloon the plane could fly around you at a fixed radius and at a constant speed. You have a different observation point to watching it from the ground.

 

ALL pilots would know from a practical viewpoint that a turn at lift-off downwind is a different matter you have to gain the extra airspeed or fall back on the ground, and conversely if you turn towards the wind you gain airspeed, but this only happens if you do it as you lift off. Confusing eh!. Nev

 

 

[turboplanner]

Not at all professor, great explanation!, and about 10 pages shorter than our friend from Stick and Rudder.

 

 

[eightyknots]

Not at all professor, great explanation!, and about 10 pages shorter than our friend from Stick and Rudder.

More like ten chapters and 15 diagrams. However, his book is a great read -and very readable for a book of that era- and still sold frequently even though it was written well over half a century ago. I noticed that the author only ever recognises the pilot as "he". Female pilots were few and far between when this was written. Someone once told me that Stick and Rudder holds the record in the number of books sold about aviation; it only had a minor update/revision about forty years ago so it is relatively unchanged.

 

 

[turboplanner]

He was so good with his theory that it's almost timeless.

 

 

[kaz3g]

Jim Davis had a an article in Australian Flying a while back which makes entertaining reading at

 

www.australianflying.com.au/news/dragons-of-the-downwind-turn

 

The problem advanced by Merv attracts strong comments from supporters of each of the theories but it seems to me if you are more or less stationary in a 50 knot headwind relative to the ground you are relying totally on the headwind for your airspeed. If you then turn crosswind, especially if you turn quickly, you have lost all airspeed except for whatever has been gained by accelerating under throttle which means overcoming inertia. That doesn't happen straight away.

 

This is different to the closed system argument that is often also considered... The bee flying inside the moving railway carriage.. and is a really interesting discussion. I'd love to hear some other opinions.

 

Kaz

 

 

[rgmwa]

... but it seems to me if you are more or less stationary in a 50 knot headwind relative to the ground you are relying totally on the headwind for your airspeed. ......

I'd love to hear some other opinions.

 

Kaz

Kaz, your engine and prop are producing the 50 kts that the ASI is showing. The ASI would read the same in nil wind conditions. It's just that with a 50 kt headwind you can see you're getting nowhere fast, but the aeroplane doesn't know that, and is still flying happily at 50kts, as confirmed by the ASI. On the other hand, if you were on the ground and pointed into the wind, your ASI would also read 50kts, but you wouldn't be flying. It it did happen to lift off the ground, you'd be blown backwards. If you managed to crank up the engine quick enough to avoid being blown backwards, you'd be back to square one - flying at 50kts but with a ground speed of zero.

 

As you turned 90 deg onto crosswind, your air speed would remain at 50 kts, but the wind would start blowing you sideways. When you were exactly 90 deg crosswind, your airspeed would be 50kts in the crosswind direction and you would similtaneously be moving sideways over the ground at 50kts. As you came around 180 deg, your groundspeed would increase to 100kts, but your airspeed would still be 50kts.

 

This is different to a wind shear situation, where you are transitioning between layers of air that are moving at different speeds (and directions), and where the inertia effects you mentioned come into play. Just my opinion, of course.

 

rgmwa

 

 

[facthunter]

Windshear is another ( and significant ) matter.

 

This subject that the crafty Motz has posed is a hard one to get your mind around but the plane is not on a pole, it's flying in a moving airmass which is similar to a ship turning around in a flowing river. To a watcher on the bank it will appear to be doing anything but a constant turn as it disappears downstream, going slower and faster and going sideways too, but on the boat it's a constant turn with the same speed through the water etc. Nev

 

 

[David Isaac]

He is crafty alright ... and is sitting back right now p1ss1ng himself with laughter ... LOL

 

 

[turboplanner]

.....Or he's caught the train and is running around the carriage making aircraft noises......................

 

 

[facthunter]

That's why they put windscreen wipers on the inside of aer lingis cockpit windows. Nev

 

 

[kaz3g]

Kaz, your engine and prop are producing the 50 kts that the ASI is showing. The ASI would read the same in nil wind conditions. It's just that with a 50 kt headwind you can see you're getting nowhere fast, but the aeroplane doesn't know that, and is still flying happily at 50kts, as confirmed by the ASI. On the other hand, if you were on the ground and pointed into the wind, your ASI would also read 50kts, but you wouldn't be flying. It it did happen to lift off the ground, you'd be blown backwards. If you managed to crank up the engine quick enough to avoid being blown backwards, you'd be back to square one - flying at 50kts but with a ground speed of zero.As you turned 90 deg onto crosswind, your air speed would remain at 50 kts, but the wind would start blowing you sideways. When you were exactly 90 deg crosswind, your airspeed would be 50kts in the crosswind direction and you would similtaneously be moving sideways over the ground at 50kts. As you came around 180 deg, your groundspeed would increase to 100kts, but your airspeed would still be 50kts.

 

This is different to a wind shear situation, where you are transitioning between layers of air that are moving at different speeds (and directions), and where the inertia effects you mentioned come into play. Just my opinion, of course.

 

rgmwa

That's a good argument, rgmwa, and that's what Jim Davis says. But, ponder this... Consider your aircraft is stationary pointed into say a 40 knot wind on the ground and the engine is giving revs sufficient to keep it sitting there without moving ie you are below takeoff speed but you have 40 knots indicated. Now, someone grabs the tail and swings it 90 degrees ie crosswind. Do you still have 40 knots indicated or do you have to accelerate first?

 

Kaz <big grin>

 

 

[kaz3g]

He is crafty alright ... and is sitting back right now p1ss1ng himself with laughter ... LOL

Ladies just "wet" themselves... Kaz

 

 

[motzartmerv]

Teehee.. I have seen the question cause many a chin to get scratched, head to be rubbed and mates want to punch mates...lol..sorry for being sneaky.

 

Ive been studying for my last cpl exam...Performance..eerrgghh.... I knew there was a reason I left it till last...

 

Who the hell invented the echo??... I would like their number, I intend to write them a strongly worded letter..

 

cheers

 

 

[turboplanner]

That's a good argument, rgmwa, and that's what Jim Davis says. But, ponder this... Consider your aircraft is stationary pointed into say a 40 knot wind on the ground and the engine is giving revs sufficient to keep it sitting there without moving ie you are below takeoff speed but you have 40 knots indicated. Now, someone grabs the tail and swings it 90 degrees ie crosswind. Do you still have 40 knots indicated or do you have to accelerate first?Kaz <big grin>

Going back to Nev's post, you've put the earth back, or you've put it on the floor of the train carriage.

 

If you imagine you're flying alongside a balloon drifting with the wind, you could do a complete 360 around him at your normal cornering rpm and you'd both drift the same distance from start to finish.

 

 

[kaz3g]

Ok... I give up. Hardly a new bite anywhere. Only the usual suspects have bought in.

 

Jim Davis and TP, rgmwa, David, Nev and a few others are all correct. The aeroplane doesn't know it is stationary and neither did the bee know it was rattling along in the train. Everything is relative to the block of air in which the Cub and the bee found themselves.

 

I'm sure MM knew that too and has just been teasing us! Good luck with the CPL Merv :-)

 

Please do read Jim's article because it is a good one, I have his book "PPL and practical skills to fly safely" which is a really handy reference for anyone studying for their licence and beyond.

 

Kaz

 

 

[kaz3g]

And just to clear up any confusion I have caused with climbing turns after takeoff. The aircraft is climbing through "layers" for want of a word, of air each travelling a little faster than those below. This is because the wind closest the ground is affected by friction represented by the wind shear effect (the direction of the wind also veers... Anyone know why?)

 

So as the aircraft climbs and turns down wind it is effectiVely losing airspeed (it takes time for the aircraft to accelerate to the new downwind velocity due to inertia) and can easily end up stalling if the change is great enough.

 

Kaz

 

 

[motzartmerv]

Thanx kaz. Im really not enjoying the subject at all... It doin my head in..

 

All the other subjexts have kept me amused. Meteorology is my best so far with 98%. so i wont spoil your fun by answering your question. Hehe.

 

Anyway... Back to this rotten echo..

 

 

[rgmwa]

That's a good argument, rgmwa, and that's what Jim Davis says. But, ponder this... Consider your aircraft is stationary pointed into say a 40 knot wind on the ground and the engine is giving revs sufficient to keep it sitting there without moving ie you are below takeoff speed but you have 40 knots indicated. Now, someone grabs the tail and swings it 90 degrees ie crosswind. Do you still have 40 knots indicated or do you have to accelerate first?Kaz <big grin>

If you've got enough power on to hold stationary on the ground (you could save fuel and just use the brakes instead) while pointed into the wind with the ASI showing 40kts (ie. wind speed), and someone suddenly swings the tail so you're pointed crosswind, the ASI will drop to zero. If the wind then doesn't catch your upwind wind and send you cartwheeling down the runway, you will commence a crosswind takeoff, with the ASI rising as normal until you get airborne. Once you're airborne at say 50 kts, you could turn into the wind again and the ASI would keep reading 50kts all the way around (assuming you're flying a balanced turn).

 

rgmwa

 

 

[facthunter]

This is the plane impaled on a pole which doesn't happen in the real world. Of course you have NO forward speed if you turn instantly at 90 degrees to the prevailing wind., if your forward air speed was equal to the wind speed. This is NOT the way to analyse this situation.Nev

 

 

[pudestcon]

I was doing a shark patrol around Caves beach near Newcastle in the mid 60's and I sat it into wind with a bit of flap out and didn't move forward at all for a while. There was a surf carnival on at the time. They rang up the radio station I was broadcasting on and told us to P8ss off as no-one was watching the surf carnival. They were watching this plane flying backwards.. Nev.

Bet it wasn't the 1st or last time you were told to POQ Nev.

Pud

 

 

[facthunter]

Absent yourself, sounds nicer. Think I'm still going backwards. Nev

 

 

[turboplanner]

Just have a shave now and again and the boundary layer turbulence will drop, the balance will change and you'll start to go forwards.