172capt

Just wandering on what everyones take on the Max Demonstrated Crosswing componet.... is it a limitation or just a guideline. The way I see it is that its just a guideline of what the test pilots took it up to. So yes if you exceed it you would be going into uncharted territory and if something bad happened the FAA could nail you but if you were to exceed it and the FAA saw would that be a violation? Anyways just wandering so any ideas would be great.

KiloAlpha

Guideline.

But you have to ask yourself: Who is a better pilot, you or the test pilot?

HSLD

In my aircraft it's a limitation and the FH doesn't allow us to exceed that limitation. Of course everyone understands how that limitation was derived - strongest winds demonstrated during certification (38 knots). Also, there are additional (lower) crosswind limitations for Cat II/III, and irregular conditions.

rickair7777

If it says "demonstrated", it is not a limitation (unless your company says it is).

The actual limit would probably be the amount of x-wind which causes

A) You to run out of rudder to keep the nose straight OR
B) The up-wind wingtip or engine pylon to hit the ground


If you exceed demonstrated, you don't know when A) or B) will happen.

multipilot

The way an FAA examiner put it, is that the demonstrated crosswind component is just that, its demonstrated. The reason why is because when they flight test aircraft they have no control over the actual wind velocity. The maximum demonstrated crosswind component may have been the strongest crosswind component the test pilot may have encountered during flight testing. So, when you land with a stronger crosswind than what is labeled as demonstrated you essentially become a "test pilot".

Cubdriver

If you're flying a 172 with a high wing clearance you could get away with more sideslip than in a low wing configuration and I think that's why bush pilots prefer them. Experience on this is pretty useful, should you ever have a real emergency. If you have a reliable ASOS plus a day with steady winds, you can experiment with crosswind capacity by low-passing on the non-aligned runway at your field. If the airplane will remain aligned, then that crosswind factor is tolerable in that airplane. Crosswind landings are a matter of skill and practice- your max crosswind may be different than mine.

FlyerJosh

Remember however the aerodynamics of a low pass that doesn't actually touchdown are different from those where the plane actually touches down and decelerates.

For example, in a tailwheel aircraft, it is often possible to fly a low pass with a higher crosswind than it is to land. This is due to the loss of rudder effectiveness as airspeed decreases. While not as critical for tricycle gear aircraft, this needs to be taken into consideration. The taxi/ground handling limits of the aircraft in a crosswind can have just as much of an impact on max crosswind as the flight characteristics.

TankerBob

I think they can find some max crosswinds with formulas that determine your Vmcg... I have no idea how... but I am thinking that is a way to determine it also. but don't quote me on this.

Cubdriver

Vmc calculation for multiengine aircraft is taught in aerospace school and yes it can be calculated by taking the 3 simplified equations of motion for the three axes (longitudinal, pitch, and lateral) and plotting them using an iterative computer code that plots velocity on the abscissa against rudder, aileron, and thrust on the ordinate. You have as many equations as unknowns, hence the need for iteration. Of course you would have one engine instead of two and crosswind velocity instead of still air since it's a Cub. And of course, it is a Cub since that's more fun. Minimum velocity of rudder effectiveness (Vmre) could be picked off the plot as a function of chosen variables, like how much roll you would be willing to allow and stall speed, or other factors like gust variation and allowable yaw.

I haven't done this actual problem since it's not very important to flight dynamics, but it would be an hour or two of fairly easy code writing. You have to know some stuff about the actual airplane to do it, namely the coefficients for the many terms appearing in the equations of motion. As it turns out the data can be very hard to come by and for many aircraft the data has not been published even if it exists. So in the end it would be something of a guess.

Probably easiest thing is to just make low passes until you get a feel for the decay of rudder effectivness, and subtract a margin for loss of dynamic pressure as the airplane settles.

ryane946

If you really wanted to get hard core, you could put a model of your airplane into the wind tunnel and measure most of these coefficients, mainly CL.
But that would just be crazy. Leave that to the aerospace engineers.

I would have to look up some equations for rudder effectiveness, but as for wing scraping the ground (if that happens before rudder effectiveness diminishes)...

The closest estimate I could give you for wing scraping the ground would be to use the life equation (lift = cl*dynamic pressure*wing area) where dynamic pressure = 1/2 * density * velocity^2, and measure lift needed for your landing speed (say 40kts). Then solve for the component of lift needed to offset the crosswind. Use trig to solve for that angle needed to offset the wind, and see if that is larger than the angle between the bottom of the wheel and the tip of the wing. Give yourself some room for gusts!!