richbran

I wrote this in another thread but it was a bit too far off the original thread subject.


First: You MUST understand the influence of (a steady) WIND to anything that flies: that it does NOT affect the basic flying characteristics of anything flying or floating in an air(mass), once not connected to the ground any more. Like the stall speed that is a given value, irrespective of wind.
This is special amongst model pilots an often misunderstood phenomenon (and not only model pilots...). The problem lies in the fact we as model pilots see only the END result of both the movement the model makes in the air(mass) itself, PLUS the movement that this mass makes(wind), relative to the ground we are piloting from. A full scale pilot sees the speed on his airspeed indicator (IAS) which is way more advantageous as we poor ground based pilots. We need to "subtract" the wind influence in order to guess the flying speed.

To correct for wind influence, once flying, You need a "navigational" solution, NOT an aerodynamic one.
If You don't get this, find other endless and useless threads with titles as "the downwind turn syndrome"......
Unless specially mentioned, I will talk about a STEADY wind, NOT wind shear, or wind generated turbulence. Many model pilots have little or nil theoretical background in aviation, it only makes matters more difficult to comprehend if these items are brought in.
If I talk about a "heading": that will be the (compass) direction the nose is pointed to. The (ground) Track means the trajectory painted on the earth the "object" is following.

Second: while my experience with handling crosswinds spans from Piper Cubs/Cessnas, (even an amphib), small jets to a B747 (22 Years), there is little as challenging as making a perfect crosswind landing with a model airplane. I fly model panes since the age of 10.
I had a safe professional career, no damage inflicted or nasty incidents in 23000 hours, but I am NOT the best modelpilot around, a good average one I would rate myself. And I had my share of model crashes!
From a cockpit a crosswind landing can already take up quite a few bits in one's brain processor, with a model it is way more demanding. I try to avoid crosswind situations with models as much as possible.
However, if the basics are understood and a fullsize pilot got a GOOD training in his EARLY career, even landing under max crosswind a B747 can be great to do.
I would say, the lighter it gets, the more difficult to handle it will be.

In order to land You need to take off first, so let's start with that.

Basically, when taking off in a cross wind and rolling over the earth surface, till airborne and becoming part of an airmass, You will have to "transit" from the "ground mode" to a flying status.

As long as the model is connected to the ground it WILL react to wind as we know it. Leave a float plane with the engine idling and no control inputs on water and it immediately will weathervane with its nose into the wind. Land planes tend to do that too, but the wheels interact better with the ground and will resist more than water. Only once the model or full size starts to move, the nose will try to turn into the wind.
Crosswind means You will have a take off direction, which is not into the wind so there is crosswind. The moment the model starts its take off roll, You will need to correct for that weathervaning. That needs to be done with opposite rudder to keep the nose pointed into the runway direction. Trike (nose wheeled) models will have better aid from the steerable nose wheel that a tail dragger does from the small rear wheel, as it quickly looses weight-on-wheel.
Rudder deflections will vary during the take off due to several aspects I will not dive into here, In a Piper Cub on a dry and concrete runway, it is quite a rudder "tap dance"....don't think, just keep the nose lined up with fast and mostly short pulsating inputs.

The wind also has another nasty effect on that massive thing we call a wing, which we don't need at all when rolling over the ground, but in a second or so it will be great for lifting the plane or model from that ground.
If Your model is a high wing one, or a low winger with some dihedral, the wind will cause the upwind wing to lift prematurely, and try flipping the (model) airplane on its back.
So right from the beginning give a good aileron input into the wind, to keep the upwind wing down. During the roll You can release some of the roll input, but keep a bit in.
As soon as the model reaches flying speed, get it off the ground in a slightly more positive manner as You used to do, without stalling it! In order not to bounce back.
When airborne, release both rudder and aileron inputs, because otherwise You are "side slipping". Allow the model to stabilise with the nose pointed into a heading that gives a compensation for drifting, most of the time this will happen more or less automatically after T/O when releasing the rudder/aileron input.
It will now climb out with a heading that is different from the runway track, the difference in angles is we call the "wind correction angle". This WCA depends on: flying speed, and crosswind factor, which in itself depends on the angle to the ground track and strength. A 90 degree strong wind will have a greater crosswind factpor as when blowing under a 30 degree angle to the RWY.

If You need to make corrections on this heading, because the model drifts off sideways more as wanted, or if the chosen WCA proves just a bit too great, make a small heading change with coordinated control inputs, NOT by rudder (alone)!

Later more about landings.

beepee

Well done Richard. Thanks for posting. I have been flying for years and good crosswind work is always a challenge. One I have to consciously work on, all the way through roll-out. There is a great satisfaction when I get it right though!

Looking forward to landing now that you got me in the air.


Cheers,


Bedford

BMatthews

Richard, this is a good write up but as you say it's more about training the PILOT than about aerodynamics. I'm thinking that this thread might have a better home in a forum that will reach more beginner pilots that are struggling with how to take off and land in cross winds rather than the Aerodynamics forum. Maybe pick a forum that you feel is better suited and I'll move this over there.

richbran

Looks like a difficult decision.

Indeed the theme does involve hands on flying and training (a lot!), but my posting is more to explain what people might do already without knowing. Like driving a bicycle, the stability why You stay upright and how You steer, is way more complex as the activity itself is.

The item also requires as indicated some basic knowledge (and ACCEPTANCE) that a steady wind does NOT have an impact on flying characteristics of a (model) airplane.
If put in a general forum, or even beginners, I'd reckon I will end up in yet another downwind turn discussion with people who don't accept (and many times bluntly refuse) this basic given.
Or who don't get the difference between a side slip angle and a crab angle(WCA).

So my idea was to stay in an aerodynamics forum, where to a certain extent people gather who do know a bit more on basic aerodynamics, or want to learn about it, because they are interested in it.

But I'll take any good advise if there would be a more optimum place to continue!

dbacque

My Feedback: (3)

I started building model airplanes in the 60's. Moved to C/L in the 70's. Graduated to R/C in '77. So I've been flying a lot of years. But one of my favorite things to do at the field is to start the day with a first flight of nothing but touch and goes with one of my fire breathing pattern planes. I've been doing this for countless years and even after all this time, I learn something every single time I do it. But mostly about cross winds. I have actually reached the point where I land better with a crosswind than with no wind or a head wind. I know, that's just crazy talk! Seriously, that last minute touch of rudder just seems to make for awesomely smooth touch downs.

So for advice on how to handle crosswinds, I'd say it's the same as how you get to Carnegie Hall. Practice, practice, practice!

Dave

RCPAUL

My Feedback: (1)

Like most discussions of this type, it is assumed the wind is steady. However, more often than not, the wind is gusty which makes a completely different situation.

richbran

RCPaul

During many discussions about wind influence I have noticed that windshear and (wind generated) turbulence only makes the issue only more complex unless the contributors/readers fully understand the matter. I'll see how the thread evolves if I take it into consideration in a later stage, because You are right, they do have influence, and they complicate a crosswind landing even more.
But to get the basics right, I choose to leave those items out to get to the heart of the problem.
When You are learning a new language, You concentrate on the basic rules first, before getting into exemptions.

But even getting windhear right, it would probably take a lóóóng thread alone....

I will continue this thread in a few days as I am too busy right now.

bogbeagle

A crosswind landing in smooth conditions is not essentially different from a crosswind landing in bumpy conditions.

Yes, the pilot' workload is higher. Yes, he might choose to add a little extra speed to the approach. Yes, he may need to be more vigilant. But, he's doing much the same things in either scenario.

AFAICS

BMatthews

With the idea being that you're looking for discussion on the analysis of the cross wind takeoff/landing let's leave it here for now then.

Full size aircraft operate in some pretty nasty cross wind situations. But at some point enough is enough and they stay on the ground. Light planes are grounded by lower cross wind velocities than the bigger ones. Which seems to suggest that we modelers might want to figure out how the scaling works on this aspect and find out what our cross wind velocity limit is for the various model sizes. Obviously with the higher degree of maneuverability on some models compared to others this limit will be more of a "suggestion" but it might be worth including this aspect in an analysis of cross wind takeoff and landing.

Hossfly

First, richbran presents the best article that I have ever read within these RCU forums concerning crosswind for toy-airplane Take-offs and Landings. The only thing in his presentation that kind of irks me is that he has 3000 more flight hours than me!! Shucks and dad-burn it all!

It would be very good if one could instill early on into the newbie RC Pilot some of the basic aerodynamics and the importance thereof. The RC model pilot wanna-bees just have no clue, and don't want to know the importance of basic aerodynamic understanding and the simple fact that the airplane flies through the air mass, which is usually moving. In addition air moving over the surface does obtain some amount of turbulence, especially in various heating-cooling conditions, and said turbulence will change the normal wind direction along and near the surface, simply basic stuff but today's folks seem to be super experts with a computer but have little or none knowledge about the world around them. That old thing about downwind turns being difficult, etc. is simply total lack of accepting basic fact that an airplane flies in the air mass which, when moving, will create the illusion that the airplane is not going in the direction one may desire. It may be pointed in the desired direction, but the ground track will be different than the illusional direction. Many pylon racers find out what happens in tight turns around the pylon. Way back when I raced pylon, I loved strong cross-winds because I could direct the machine to use the least amount of space around the pylons. Even better when I could "downwind turn inside them" on the far-pylon and then as they were about ready to cut inside me at the base pylons, I could sucker them into mating with the pylon. I enjoyed that!
Wind is your friend if you play it right. It can also be your worst enemy. In addition, many RCers have difficulty in taking-off and/or landing because they cannot keep the machine TRACKING straight down the runway and on the runway when landing. Sad but funny when they still believe the downwind turn is bad!

richbran

Hossfly, tks!

The examples You give using wind to its max indicates You are fully into this matter! OK, You indicate being a pro pilot, so it is more or less explainable... But in defence of the model pilot that has no pro background: many full size pilots can't even handle a model airplane! When my son turned 12 he flew circles around me!

In order not to make this contribution too long, I start with some general remarks, the landing itself will be covered later.

One advantage we as model pilots have above full size flyers is that we know EXACTLY what (cross)wind is blowing over the runway!

Start with a good circuit FIRST.

Look at all the "super pilots" like jet fighter and Navy carrier pilots, ALL fly a decent and very well organised circuit before attempting a landing. This means a certain height above ground or water, and specific (timed) downwind lengths, coordinated turns, exactly flown speeds. Only then You can arrive on a final track without having to correct for earlier made mistakes, like excess altitude, overshooting the final track and wrong speeds. Planes should be trimmed to match the configuration and speed.
For airline pilots a circuit would not be practical and they follow an Instrument Landing System (ILS) which allows some 3 minutes for stabilisation, config changes, trimming, attaining the correct speeds and completing checklists. You get "in the mood" You need for a professional landing. Pumping up Your adrenalin.
Many terrible accidents are known that followed after a lousy flown (unstable) approach.
Ever seen a sniper pull his rifle and fire a target at hundreds of feet without giving himself time? Nope, they need time to stabilise, aim and even coordinate their breathing and heartbeat to the moment they fire.

So see a circuit as a part of the landing, it should be flown with the same concentration as the landing itself. Especially with complex, large, heavy and costly planes like model jets, or big radial WW2 planes. An intermediate pilot getting his Calmato safely on the deck in cross wind, needs to do the same. But also a beginner, if he respects his HobbyCity foamie should be serious in (cross wind)landing his plane the correct way. Then later in his career he will enjoy the effort he has put into honing his skills right from the very beginning.

If Your model features flaps, You should consider a few things. In many situations a pilot, whether full size or model, needs to make a compromise between two or more nasty things.

As told before, the Wind Correction Angle (WCA or crab angle)depends also on the (air)speed the model is flying. The slower You get, the more crab angle You need to track the extended centreline of Your Runway. More crab needs more effort to line up the nose for the landing itself. Which in full size is called "de-crabbing".

There is a marked difference here between jets and prop driven planes. Flaps mean higher drag and thus more power to compensate and keep the same approach angle. The higher revving prop delivers more prop"wash" on the tail feathers thus increasing controllability for rudder and elevator. However the ailerons, outside the prop wash suffer from reduced airflow when You fly a slower airspeed.
Jets don't blow anything over Your model so a lower airspeed means less controllability with ALL controls. However with (lots of) flaps they need a higher RPM on final, which gives a way better spool-up performance as compared to a jet engine running idle.

So we need to decide upon two nasty choices:

Fly slower: more crab angle to "de-crab" on landing, less controllability to fight the wind generated turbulence.
Fly faster: less crab angle to "de-crab", better control in turbulence, but at a risk of overshooting the runway at the end, during roll out.

Many GA airplane pilots will select a lower flap setting and an increased speed, airline drivers will have almost no choice in flapsettings (in a 747 You can choose between "25" and "30" as landing flap setting) but use higher speeds instead, for fighters, I dunno, never flown a fighter myself.

So if You have ample landing distance available, You might opt for a lower flap setting/higher speed.

A mistake many pilots make is that a 90 degree cross wind means You have NIL headwind! Comparable to landing in still wind conditions. So on a short runway with excess airspeed on final plus less flaps, the speed bleed off takes longer and You are eating up runway length BIG TIME!

So here we are, approaching terra firma, on speed, on "glide path", but with a cab angle, that means if You are touching down this way (especially on dry and hard surface runways) things will get quickly into a mess.... To be continued.

Richard

Bozarth

My Feedback: (15)

Hossfly,

Good points

Kurt

richbran

How about this one.......
http://www.focus.de/panorama/videos/...d_3717919.html

Villa

I used to land on a different heading when there was a strong crosswind. Then I noticed that I was crashing a lot, particularly when the plane heading was toward me. I concluded that I was loosing my ability to estimate the air speed, because the plane was coming toward me, so the plane was stalling and crashing. Now I make little allowance for the crosswind. If the crosswind is pushing me off the field, I will abort, adjust, and try again. I try to touch down directly in front of me.

otrcman

Here I am, approaching terra firma, pumping adrenaline, and you conclude Chapter 2 without getting me onto the ground. Don't wait too long for Chapter 3. I can only hold my breath for so long !

I'm anxious to see whether you'll kick it straight and land wings level or will you transition to a wing low sideslip. Maybe you will discuss the merits of both methods.

Dick

Hossfly

If one wishes to have a nice touchdown, it will behoove one to ease in a little downwind rudder, and maybe some wing down on the windward side so the tires are touching down with no side-slip, like rolling straight on the tarmac. Makes life better! Accomplished pilots do that but now and then there is NO accomplished pilot!
When I went through USAF Pilot Training, There was only one runway set-up (2 parallel RWs), 35 or 17. The wind in Laredo, TX was almost always from 330 degrees or 150 which was almost ALWAYS left wing down, and right rudder. Then after graduation, one went out into the real world and it took a bit of retraining one's self to keep those feet from working by habit while learning that there was this thing, right wing down and left rudder. Not me of course, just some kind of BUMP-BUMP and bump.

sensei

Lots of great information indeed, but there comes times when pure motor function must take over, like when taking off and especially landing an R/C model in a direct crosswind of lets say maybe 30 mph gusting to around 40 mph without tearing up your stuff. So practice, practice, practice is right and a great deal of it in weather conditions way beyond your comfort zone to become proficient enough to just react to a situation because knowing and understanding what is going on is great but it won't help you much in some scenarios, crazy things are all happening in front of your eyes way too fast for thinking about and everything is usually over if you are thinking about it and not properly reacting to it. Just one man's experience...

Bob

richbran

OK,

Now the difficult part...

You are approaching at angle to the RWY, that is on centreline but the nose not aligned with the direction of the latter. The Crab or Wind Correction Angle.

One word of warning, if aiming for the runway, aim for the upwind side of that RWY. If You let the model drift too far to the downwind side: You will be running out of options in no time, other than discontinuing the landing and making a "Go Around" (many modellers still keep referring to this wrongly as an "overshoot").

In the professional world there are a few options:

Option ONE: Airplane factory test pilots slam the test aircraft to the ground with crab angle et all, very stressing for the gear, and probably not very suitable for a model. Many Boeing/Airbus crosswind test flights can be found on YouTube.

Option TWO: The next option is to "decrab" VERY shortly before touchdown. Just by kicking in enough rudder to line upthe nose with the centreline, and keeping the wings LEVEL. Kinda "modern" way of landing larger airplanes, if timed right, a good landing will be the result. If done too early: the plane will starting drifting sideways. If done too late, the result will be like option one.

Option THREE: I was trained the old fashioned way with crossed controls: wing low and opposite rudder. Even for a B747 with underslung engines, to most GA aircraft, this is my favourite.
This works also great for models. But as stated before: it is darn difficult to find the right proportions of aileron and rudder input.

Option 2.5 would be a blend of two and three.


Here we go (option 3):
A few feet above the ground, start rolling the wing into the wind, but don't overdo that, 5-10 degrees bank is even in a strong crosswind sufficient. But try keep it there.
Not planning to dive too deep into aerodynamics: the "tilted" lift should balance out the sideways drifting caused by the wind. This needs a lot of practice to find the right amount of bank for that moment.
But at the same time You start the roll, You add opposite rudder (opposite as You would normally do in a coordinated turn), or as someone mentioned before: downwind rudder.

The rudder input here has TWO functions:

As You add bank, the nose will like to start turning into the direction of the low wing, which would be even further away from the centreline direction! So part of the rudder input is needed to stop that. BUT You still need to push the nose even further away, till it meets the direction of the centreline. So even more rudder is needed.
So a general picture would be a little bank into the wind, but lots of downwind rudder! More or less, You start some kind of sideslip, but now one that brings the nose BACK to the runway, iso away from it as a side slip in no wind conditions would show.

The perfect landing would be with the low wing wheel first, then the other and last the nose wheel. For a taildragger the tail would come down even later...

Writing this is relative easy, but doing it is another issue, it happens in seconds and control inputs must be right from the start and balanced for all conditions.
Was Your Mark One Brain Computer already busy with the roundout/flare manoeuvre, it is now even more worked with these additional tasks. Here is task saturation one of the main reasons for crosswind related incidents and accidents.

We as model pilots are in a bad position as compared to full scale, normally in this phase we would be looking at a sideways view. Drifting off the centreline will be less clear as when sitting in the cockpit. Getting the nose aligned with the runway is relative easy when inside a plane, from the outside it is just way more difficult.

One of the problems a crosswind landing brings is that both the plane's "neutral position": wings level, no slip/skid, is replaced with some bank plus a sideslip. AND with corresponding continuous control inputs.
So after correcting a jolt of turbulence, You don't release the controls to neutral, but again to the wing low/ opposite rudder situation, and continue. This makes it very hard to do correctly with a model. As if flying with the trims way out of neutral!

Another issue is that You are flying with "crossed controls", the ideal input for starting a spin/wing dip. PLUS the crossed controls increase the model's drag, so the speed will drop too, thus aggravating the danger of spin/wing dip.
So when starting the "de-crab" manoeuvre, it is wise to let the nose drop a degree or two, so that at least the speed is constant and NOT decaying.

After the landing make the roll-out as straight as possible, by adequate and mostly aggressive rudder inputs, and keep the upwind wing to the ground with increasing roll input as the decaying speed reduces aileron effectiveness.


Wrapping it up:
If You have found a way to get the model back to the ground in one piece, but without knowing exactly what You do, it is fine, If You want to know what happens and my contribution has helped a bit understanding, fine too.
But I keep saying that avoiding a heavy crosswind is always the best option, taking another take-off/landing direction, or just plainly chickening out that day. If the crosswind flips Your model over upon landing, that is a high price for a flight of a few minutes....

lobohart

good grief! if I gotta think about all this while the instructor is trying to instruct I am sure to have some repairs to do. I think the hardest part for a beginner is remembering that everything is reversed when the plane is coming at you. I froze when one of mine started drifting toward me on landing and didn't remember to turn the aircraft. it hit on the radio with the wing and knocked me down. That was my intro to crosswind landing and a hard lesson learned. I now spend more time on landing with crosswinds on the simulator. I have enjoyed reading all the posts on this subject.

bogbeagle

Richbran,

You didn't make any mention of power.

richbran

True.
I basically focussed on the aerodynamic part, technically it means landing a glider and even a jet need the same handling.
The (prop driven) planes have some advantage, in that, when rudder is used, it will be more effective during de-crab. However and then even more disturbing is that when power is cut for landing, suddenly more rudder is needed. At a time when You are already very busy.

It however can provide energy to stop the speed loss, other than by lowering the nose a bit.

lobohart

As I stated, I froze. If I had cut all power it would have landed prior to impact. This model required 15-20% power input to stay above stall speed on landing. It also required full power to just stay airborne. Believe me I will not let this happen again. The guys in the club have been showing me ways to keep from doing stupid stuff.

bogbeagle

Baby steps. You have to build a chain of skills, one at a time.

And, the crosswind landing is possibly the epitome of conventional handling skills ... except for the crosswind wheeler landing. So, try not to beat yourself up about mistakes.

A good exercise is to fly your trike the length of the runway, just a few inches above it, with the controls crossed so as to maintain the runway' heading and centreline. Do this in a light x-wind. No need to make any attempt to land.

After a few attempts, you will be able to perform the task at a suitably low power setting ... one which will permit you to fly at just the right speed for the hold-off. So, you'll be flying the length of the runway, a foot above the ground, maintaining the runway heading with crossed controls, and with the nose high, close to the landing attitude.

I can almost guarantee that you will find yourself inadvertently making really good x-wind landings.

Bozarth

My Feedback: (15)

So what happens in flight if you pull the power to idle?

Kurt

Hossfly

That depends on what you are doing, and whatever results you seek. In the real world, regardless of what the simulator instructors say the real airplane needs to be driven by a real pilot. I remember once way back when the simulator instructor was was giving me the warm-up just before an annual proficiency check and some stuff was being discussed during the break. He went on to say that any pilot that used rudder in a JET airplane did not know how to fly. (He had never flown a jet) I looked at my co-pilot, I smiled, he smiled, and we went on to the task at hand. Never told the sim. guy to explain what we did with one leg when we had a flame-out on one engine just at rotation.
When one pulls power off, then a pilot does whatever he needs with with less power. Lose an engine on one side of a multi-engine, generally the first thing to do is get that nose back straight using rudder. Throttle/s as needed to climb, descend, or remain same. Pilot's decision.
Generally pulling the throttle to idle requires descent depending on the speed when one idles the engine. You only have just so much potential energy between you and earth when you lose power. Watch many an RC pilot when he loses an engine. He tries to hold the nose up. Bad Call! Potential energy is wasted with the drag. Keep the model flying well - your call - and use that potential energy to cover distance back to the runway. Same in 1-1 scale. BTDT!