Crosswind Limits - Some thoughts

denningte

Member
Over on the other site Tom D and Jim W were answering up on a XW and VG issue and noted that XWs at different field elevations (lower) are harder to handle than the same XW at high altitudes where they both fly and so do I.

Here is some thoughts on why that is real and not perceived:

Gentlemen,

The point you bring up is a classic merge point of IAS/CAS/TAS/GS understanding for the landing zone.

If you consider the difference in measurement between a pressure differential system (pitot-static IAS) and how wind speed is measured on the ground (free-wheeling cup or rotor vane system), it becomes clear than at sea level you are dealing with a different wind triangle for landing than you would at higher density altitudes like CO or a hot NV day. The wind speed measurement given at a field from AWOS or a local controller is closer to TAS than CAS or IAS but not truly TRUE because of minor drag inefficiencies of the rotors or cups in the air stream. ( I estimate negligible.) The reason TAS is preferred for the landing area is because it is next to GS in most calculations and the runway and tires only care about GS during and after contact. I would suggest that a way to figure out what is “like home-drome” limiting conditions elsewhere is to convert your regular approach/landing IAS to TAS and use the same RATIO of forward to cross as a guideline for operating in other elevations.

I also note that the placard and POH in my aircraft AND the extra POH I have from Aviat for an A-1B all say 15 MPH which is 13 KTS. 15 KTS is 17.2 MPH. When you’re on the edge of performance a difference of 15% can be eye-watering (or other dampening farther south:eek:).

Before the next section is read, it is important to understand that CAS (IAS corrected for installation error) is what your wings and control surfaces care about and describes the amount of controllability in compressible flow. (We will need a few more mods from Tom to get the Husky into the transonic region:D)

So, if the factory says 15 MPH at AFO on “normal” as opposed to “standard” day where the temperature is say 20C, the recommended XW approach speed is 68 MPH IAS (77 MPH TAS) but the felt max demonstrated crosswind of 15 MPH true would be equivalent to 13.25 MPH CAS. Now let’s say that in this speed range a good installation makes the difference between CAS and IAS negligible. I am using the recommended approach speed only as a reference point for comparison. The hardest controllability problem will be around 40-20 MPH in the roll-out but the CAS is not available there. If you tried to read it there I would suggest that your tail-dragging priorities need a reshuffling.:rolleyes: This is where in my experience the VGs can come out to bite you after you think you’ve lived through a high XW touchdown.:eek:

77/15 = 68/13.25 = 5.1/1 forward to cross ratio in the same speed type (IAS/CAS/TAS).

At sea level std day – 68 IAS/15 IAS is the same as 68 TAS/15 TAS = 4.5 forward to cross. The smaller number means you are dealing with more “felt XW” throughout the rollout than in a high density altitude environment.

TD:D
 

denningte

Member
That is the benefit of having a Husky. The world is an airport with infinitely variable runway directions.
 

barbwire

New Member
("The world is an airport with infinitely variable runway directions")

Crosswind Confessions,

Last Sunday 12knt gust to 20 @ 240-250. Returning from a trip with the better half. Could not land at home on grass as it was a direct crosswind with a narrow short strip. Headed for the local concrete 7000' by 100' and 31. Made one approach went missed, could not hold the centerline. Tried again and on short final I saw the sock sag a little so I forced it on the ground. (big mistake):eek: Of course it bounced, I caught it and got it back on the ground but in doing so lost directional control (wind sock straight out again) and now I am about 20* to the centerline heading for an edge light, so full power, pulled back on the stick wallowed in ground effect, now out over the grass ditch for a few l-ooo-ng seconds untill the dog caught its breath, then climbed out to about 50 feet turned left directly into the wind across the runway and landed in the grass. A little rough out there but with the 29" bushwheels uneventfull. Needless to say I was PO'd### at myself :mad:
I had nearly lost my baby, oh yeah the wife too :D and I had options, 100 acres of airport property.
--My Penance was 10 Hail Mary's and thanked God I did not injure the dog,
I will repent for some time as stupidity like that hangs with you.


The moral of the story is proclaimed above: thanks TD
"The world is a airport with infinitely variable runway directions"

signed: AN0NYMOUS :cool:
 

NevadaGwen

New Member
I landed a C-141 once at Diego Garcia in a 40 knot direct cross in driving rain. No more choices, no more fuel, no more options. This was in the days when the weather forecast (at Clark AB) was based on 24 hour old satellite photos.

I don't have to do that crap anymore. Especially in my prize pedigree dog.

If it gets that windy (i.e. over a crosswind I'm comfy with), I'm going somewhere else, or landing on a ramp, taxiway, or other quasi-flat area.

On the other hand, when we do get a 10-12 knot wind here, I'll try to find an appropriate runway to practice crosswind landings.
 

denningte

Member
Brought over from the mail list in response to a thought about keeping the CG between the mains on landing:

The CG is and always will be between the mains unless you take the mains off and lean them up against the hangar wall.

It does not take the motion vector being outside the triangle made between the CG and the two mains. You are well beyond the end swapping point in that condition. The Husky CG is not very far behind the mains. The mains are at station 61.56 and the aft CG limit of the A-1B (for example) is 80 inches. That puts the CG at the aft limit just behind the front seat – about where the aft stick position is. To get a vector outside the mains is in excess of 70 degrees right or left. You are well into the departure from controlled taxi by then.

My cues from the back seat when instructing are based on side load felt toward the outside of the turn. In the front seat the cues are the same but because you have more window and less structure moving against the outside scenery and a flatter seat back it is harder to detect because your visual senses tend to override tactile senses. The eyes tell the butt – “naw, you ain’t in trouble yet!” From the instructor vantage point the visual cues are stronger and the seat back is curved a bit so you get tactile reinforcement of side load much sooner than the front seat.

On the ground, you will never SEE the vector of the aircraft deviate to one side or the other before it is too late. In the air, before correcting for a demonstrated limit crosswind, even at sea level, the mains will always be outside the flight path even viewing from the back seat.

SO, the real cueing is all about side load felt when the mains are on the ground. The ability of the rudder to correct the condition decreases with loss of speed but a tail wheel locked in the detent adds some resistance to departure. Landing in high XW can result in a free tail wheel, but by the time you are below 40 mph you should have the TW in the detent and the stick firmly in the aft and locked position (to the upwind side of course).

Power and go around options are not always possible. Whether on a one-way strip or confronted by an obstacle due to the swerve, you should NOT have that option developed to instinct. What you should do is recognize that two conditions lead to ground loops – inability to control the aircraft in a XW (most prevalent) and the locked brake condition that opened this conversation.

The two cures are easy.

For the locked brake, add “5. Parking Brake – RELEASED” and “6. Main Gear –CHECK VISUALLY” and run your before landing checklist every time – GUMPSS for you who eschew a written checklist. Safe flying conditions are not all related to wind and precipitation. The muck you drop your tires into are demonstrated as a point to consider through experiences of the contributors to this thread. If you suspect you will have a frozen brake or locked wheel, land with that wheel downwind and be prepared to hold it off until you can’t anymore or land on a slick surface like wet grass, snowpack, or ice. You may get a GL anyway, but at least you can pick the slowest speed where it will only damage your ego.

For the XW condition practice making full stops regularly in XWs up to your comfort limit or the demonstrated limit. If you don’t pack the gear to cope with XWs find an instructor that can work you up to the aircraft capability and firmly seat your known limitations in your aeronautical decision making.

I fly with a back seater or in the back seat all of the time and regardless of the weight and CG the cues are the same – side load, side load, side load. Learn it, love it, avoid it like the plague.
 

bumper

Well-Known Member
As TD states, advancing throttle for a go-around isn't always an option. When rudder and/or tailwheel aren't enough to regain directional control (or there's a broken or disconnected tailwheel spring), applying brake on the outside wheel will often do the trick . . . but not if the departure has progressed much beyond the initial swerve.

bumper
 

bumper

Well-Known Member
"full power and take off over the grass or obstacle can save the day many times"

But, of course, not all the time.

Jack


BUT . . . in those instances where it is probably not the best option, yet the pilot decides to go ahead and advance the throttle anyway, the result is often spectacular and sometimes has substantial entertainment value too.

In those instances, the best we can hope for is, "they managed to walk away".

bumper
 

mvivion

New Member
Todd,

I'm confused. Not a huge dissappointment at my age, but....looking for clarification:

Here's the quote from your initial post: "So, if the factory says 15 MPH at AFO on “normal” as opposed to “standard” day where the temperature is say 20C, the recommended XW approach speed is 68 MPH IAS (77 MPH TAS)"

Where does the factory or anyone else advocate that high an approach speed? Where on approach would you actually USE that high a speed?

In my experience, I don't add ANY speed to approach speeds, UNLESS the wind is GUSTY. And, even there, the recommendation from the FAA is half the gust factor.

Please clarify where you're coming up with that high a recommended approach speed. I musta missed something.

Mike
 

bumper

Well-Known Member
When I first got my Husky, I carefully read and adhered to the POH recommended approach speed. I often bounced down the runway like a bunny on steroids and meth. Slowing it down a bunch really helps smooth things out.
 

denningte

Member
Mike V,

Those numbers are taken directly from the Aviat Aircraft A-1B POH dated Jan 28, 1998 page 14 of 29 in the Normal Operating Section under "Cross Wind Landing." My A-1A manual is the same.

Specifically it states 68 mph IAS for 30 degrees flaps and 75 mph IAS for zero flaps under bullet one of the XW landing checklist. Since I can't read Mark Heiner's mind or get anything out of the factory like I am used to investigating USAF issues I have to use what is printed for a starting point. My assumption in the discussion is that if they placed those numbers in the XW section and there is a corresponding 15 mph demonstrated limit that they are there for a reason. It seems to run counter to the fact that in a big XW you would be cross-controlled to a fair amount and indicating SLOWER because the pitot tube is flying at an angle in the RW and losing some pressure off the nose of the tube in the process.

I agree with you that these are uncomfortably high speeds for an experienced Husky driver in normal conditions. I teach new fliers to use 60 mph and depending on the installed prop an RPM of 1200-1300 for a smooth round-out and flare. In most conditions I encounter the real issue with XW is the dynamic variability of it in the flare. It is not unusual at KVGT to have four wind socks pointing four directions and the American Flag next to the tower at yet a fifth. :D

For that reason I teach that the runway center line never lies for current conditions the aircraft is flying through. Ailerons (bank) to hold your CG over the centerline and rudder to align the nose for touchdown. As you know, the upwind wheel should always be the first point of contact and smooth input of full XW aileron should accompany touchdown of the downwind wheel. Rudder is as required to keep it tracking the CL. :) Big smiley when done right.

One last point I would caution the readers about the POH since you make an excellent point that these numbers are suspect. This involves the Vx/Vy stated in the newer A-1B manuals. If you graph those out you find the Husky should have a service ceiling just short of 10k MSL. The older A-1 and A-1A manuals graph out the absolute ceiling to be around 20k MSL. that makes a whole lot more sense. The aero engineer will tell you that there is a merge point of Vx, Vy, and absolute ceiling. Here is a reference:

"Vx increases with altitude and Vy decreases with altitude. Vx = Vy at the airplane's absolute ceiling, the altitude above which it cannot climb using just its own lift." [ame]http://en.wikipedia.org/wiki/Rate_of_climb[/ame]

I have asked this question of Vx/Vy difference and the apparent contempt for physics in the newer manuals before. The silence is deafening. Perhaps someone poured out their bong water into the Afton water supply back in '97 when they were drafting the manual. :eek:

Currently flying one of your machines - N9618F - with the new owner!
 

mvivion

New Member
Todd,

Thanks for clarifying. I assumed that's where the numbers came from, just wondered why anyone would advocate using them...:)

18F was a great little airplane. It served well till replaced with the first production B model. Unfortunately, 18F has sat outside much of its days, and last time I saw it it looked a little rough. I hope the new owner treats it well.

Mike
 

denningte

Member
Mike - instructing in a litigious environment requires a nod to the POH as a starting point in any discussion. In the case of the discussion at hand, slower speeds make the original point more acute than less. (That was the issue of surface winds always being reported in TAS).

My review of 9618F's POH states XW landing should be 3-pt with a 60 mph approach. I suspect every lawyer in the ambulance chasing business would find fault with teaching A-1 techniques in an A-1B so I am not advocating anything here. I just used the A-1B POH numbers (most prevalent list model) as an example to show the effect of 13kts XW reported at a high altitude field versus a sea level field :)

BTW - 9618F IS a great flyer!
 
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denningte

Member
Re: Crosswind Limits - Some thoughts (carry over from list)

Original list post:

I have just finished my insurance required 10 hours of dual. I flew with a very good, old time tail wheel guy. He had never flown a Husky until he flew with me. He taught me the wheel landings and most of the tail wheel guys around our area in Windy Wyoming say wheel landings 90% of the time and especially on windy days. I just ready Husky 101 and it calls for three point landings for cross winds and anything except for calm days. How do you Husky guys land? I was also taught to lift the tail at 40 and lift off about 65 but the book talks about not lifting the tail on takeoff… Any thoughts?

Thanks
Garry Chadwick
Cheyenne, WY


Garry,

Husky 101 follows the Aviat factory guidance which calls for three point landings in cross winds. I have talked with the author on a few occasions about his preferences and he is not only consistent but has good rationale behind his position. Wyoming is no different than Las Vegas for wind speed and volatility of direction and gust and I have tested all the techniques and can fly them as well as instruct them. As the Husky has matured, the manuals have diverged on some numbers to include crosswind guidance. You may have seen my discussion with Mike Vivion [below] regarding that subject. Legally, an instructor and owner should start with the manual; however, the mistakes, vague language, and contradictions in some areas of the POHs make the POH a minefield for litigation if an owner ever pressed the point against an instructor. As an instructor I have a copy of each version. As an owner, you probably don’t and these go unnoticed.

The two points of view come from experience, proficiency and conditions. I find trainees will gravitate to one technique or the other fairly quick and getting the inertia to reverse toward the other technique is a challenge in primary training.

Let’s take the wheel landing as the first case. The positives for a wheel landing in cross wind are that the controls retain effectiveness longer through the landing. As the aircraft touches down, your need to move the controls into the crosswind are slower and more easy to do consciously. Wheel landings are excellent demonstrations of crosswind control because with a little power an instructor can freeze the condition to show what is going on with the stick and rudder. The wheel landing allows an extended training exposure to the control physics and hence you are likely to learn them more quickly if this is your first exposure to conventional gear aircraft. The extended “flying” time in the landing is also what the three pointers will tell you is bad about the wheel landing technique.

The three point technique positives are that the aircraft stops flying shortly after contact with the runway. This reduces but does not eliminate the likelihood of the upwind wing lifting in a gust. In a true XW landing you will actually make a two-point touchdown with the TW and upwind main gear followed immediately by the downwind main gear. The aileron input in this short time between the mains contacting is a more abrupt application of cross control and can generate more required rudder input to counteract the abrupt aileron application. What I see in training is the three point is a little more wobbly in yaw in the learning phase until the trainee learns the anticipatory pressures required on the rudder. I can talk all day about it but the learning takes place only when the trainee makes the connection between main touchdowns, aileron input, and rudder taps required at each point. Before that they seem to be behind the aircraft and on the verge of losing control. Once they get it the 3-pt touchdown is solid as a rock.

Once these connections are made, three point landings have some undeniable strengths: 1) shorter time exposed to wing-lifting condition, 2) less wear on the outer tire tread, and 3) shorter landing roll.

If you intend to fly in the back country with your plane I would recommend you continue your learning to master both techniques. A fully proficient pilot will adapt whichever technique makes sense for the conditions. For example, I have a very short one-way strip I use for practice that invariably has a direct crosswind most days. There is no room to wheel land on it. It is smooth enough that I don’t have to worry about TW abuse. Protection of the tail wheel in very rough strips is a good rationale for keeping the tail up on landing. In a strong quartering headwind, you may be able to do a wheel landing on a short bush strip, but those conditions are rare in mountain valley strips.

Lifting and dropping the tail is a similar experience/proficiency/conditions issue – let me make some observations: The POH says to take off from the three point attitude. The factory owner has stated in published articles that he lifts the tail to see over the nose. Other vertically challenged pilots often say the same. From the back seat I have no choice but to revert to classic TW methods for takeoff so I angle the aircraft to inspect the takeoff zone then line up on it using peripheral cues. The best way to avoid an obstacle in the takeoff run is to get flying over it as soon as possible. If there is any doubt about the takeoff zone, there is absolutely no harm in walking the zone before you crank up (towered airports excluded of course). The three point attitude with 20/30 flaps will get you airborne faster than the tail lift first technique. AGAIN, if TW abuse is a factor, that is a reason to get the TW up as soon as possible but you are going to cost yourself some TO roll.

Welcome to the club and never stop learning about your airplane! :)
 
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