bubi352

It seems that a lot of pilots have a misunderstanding of slips and skids. One needs to understand the forces involved in a slip and skids to truly appreciate its applications and limitations. Even the FAA teaches improperly uncontrolled stalls to initial CFIs. It has become a viscious circle and leads to pilots saying "it's not a good idea to be cross controlled on final for crosswind landings... if you stall you will spin". Ask any experienced aerobatic pilots.

Without going in too much details, if you slip you create two forces in opposite direction: horizontal (created by bank) and fuselage lift (created by Beta angle). If the two forces are in balance, stalling the aircraft with cross controlled inputs will not be any different than stalling it coordinated. This is exactly what the FAA teaches and it is completely wrong. You can do this maneuver forever and all you see is you descending rapidly like a falling leaf. What needs to be taught is the danger of skids not slips. It is so hard to spin a slipping aircraft because much of the load is carried by the fuselage. The only way would be by loading quickly the fuselage (pulling Gs).

In a skid, you create also an horizontal and fuselage lift but both go in the same direction. It is impossible to maintain heading in a skid. Impossible. If you stall in a skid, all forces go in the same direction and if you were to spin you would enter in a spin.

What I am getting at is the best crosswind technique is the slip. You can achieve far better result. This is the prefered technique used by test pilots. This being said, it cannot be done on all aircrafts because it is often limited by geometry (engine pylons, wing swept,...).

ryan1234

I think I understand your point... however directional stability is most critical at very high beta and alpha values (and I'm sorry if anyone believes this to be too technical) - frankly from my understanding it depends on the actual design.


Also I think you're talking about postive beta values (to the right?) and not negative, since +Cn plus - beta = instability. +Cn plus + beta will be stable until a point where it becomes neutral and then unstable. This is all at low alpha values.... when high alpha values are thrown in, it decreases static directional stability (including some lateral stability) -reducing the stable slope of Cn versus beta - also at high alpha, the fuselage boundry layer increases around the vertical stabilizer causing reduced effectiveness- so an airplane can enter a spin cross-controlled - especially at negative beta values (and obviously very high alpha). (especially pronounced like you said in swept wings) - so overall, this could make sideslips produce more stable stalls versus skids... to a point.


Of course there are other factors like rudder float/lock etc... but that's a whole other can of worms.

I'm not trying to argue with you either...in fact, I can see your point of slips/stalls not being that bad in moderation.... just trying to shed light on teaching the limits of each technique and the critical dynamics of each.

For example the FAA teaches about how bad it is to stall in a turn or accelerated stalls..... they're really not that bad (coordinated), especially depending on the structural strength/limits.

KC10 FATboy

I think they teach them because so many people end up getting into them while in the traffic pattern which makes them unrecoverable.

The first lesson I had in an airplane was a turn to stall on final approach ... at altitude. Will never forget what happened after that wing stalled.

ryan1234

Most of the ones I've done in various aircraft just buffet a little (in a coordinated turn), reduce the AoA/bank and it recovers before anything bad happens. Cross controlled/side slip it's a little different.

Also, Fattie... I've been doing some research into vertical gear loads, etc. The DC-10/MD-11 was an interesting aircraft with the vertical load design.. and damage to the wing spar,etc. I would concede to you all about over all loads during normal crosswind landings... however if a slip is allowed to produce an excessive sink rate, the one wheel landing (especially with less than normal flaps) would be a serious thing.

bubi352

This is exactly my point. There is this misundersanding that because your rudder is pressed one way and your ailerons are the other way that you are uncoordinated. Yes, you are uncoordinated in terms of control inputs but not necessarily in terms of aerodynamic forces.

I can demonstrated this over and over in an aerobatic aircraft and all they see (with a slipping stall) is a high pitch attitude falling leaf. Some even have a hard time believing they are stalling because there is almost no dramatic pitch change. That's when I point to the VSI and altimeter. But when I put my inputs in the same direction (skid) then it is a complete different story.

cospilot

If I needed to lose altitude quickly in a single-engine airplane, I would forward-slip the airplane; opposite aileron and rudder and no power but remember to maintain the proper airspeed or you will get a really nasty aggrevated stall.

In a multi-engine piston or turbo prop, simply making the props flat, gear down, flaps down, flight spoilers (if you have them) and engines idle is enough to get a good descent rate, but again, watch your airspeed.

Finally, we don't typically do forward-slips in the jet. Instead we crap all the way down to the flare, then side-slip the airplane, aligning the nose with the centerline and a bit of aileron into the wind. Ideally, one main down (upwind main), next main down, then nose wheel down, full aileron into the wind.

In the CRJ200, the wingtip is only 5 feet off the ground, so you have to be very careful when banking into the wind...and watch out for wind gusts, that is usually when you get a wingtip strike, ironically on the other wing from over-correcting! Go figure!

If we have to lose altitude quickly in the CRJ, usually idle thrust, gear down, flight spoilers (till 30 degrees of flap) and full flaps down will get you down...fast! If you can't make the runway in that configuration, it is time to do a go-around anyway.

Good question!

Jeffrey

tomgoodman

Hmmm... I'd vote for the forward-slip.

Cubdriver

Side slip is often used interchangeably with the expression wing-low method when applied to landings, true. But I have never heard the term forward slip used synonymously with crabbing. They are not synonymous in terms of attitude, purpose for being, or aerodynamics. I think you may have confused the two because the longitudinal orientation of the aircraft can be the similar. It is a coincidence only. In fact, forward slips are most effective when the airplane is misaligned with the prevailing wind, not aligned with it like a crab. This is to make the fuselage blunt to the wind and it slows the airplane down by creating additional drag.

Forward slip is a high drag, a nose-down maneuver for the purpose of bleeding off excess altitude. But the first thing that happens when it is applied badly is airspeed runs away, even beyond Vfe if you do not manage it. Applied with a little skill they are great at this. Crabs by contrast, orient the airplane like a weathervane. In fact, the term weathervane is a pretty good synonym and a lot of times I will instruct "fly the airplane like a weathervane here".

Distinguish between ground track and relative wind for the answer to this question. In a crab the ground track does indeed form an angle with the airplane longitudinal axis, but the relative wind does not. In contrast, in a sideslip the relative wind does form an angle with the centerline of the airplane, but the ground track does not because the sideslip is ideally applied in counter to wind drift on the ground or runway. Since this is it's main reason for being, it is important that we learn to apply sideslips proportion to wind drift. Not all pilots are able to do it well, and every year there are a bunch of runway excursions off the side of the runway due to winds blowing the airplane away.

One other point I have to hammer here as an instructor. When you slow down after landing and you are still on the roll, you need to keep adding aileron into the wind until it hits the stop. I think the tendency is to go whew, I am safe now and stop flying the airplane, but as the airplane continues rolling at say 45 knots, there is a lot of aerodynamic potential still there and you need to address that. Wings can go back into flying well after the landing roll is past theoretical stall speed and positive aircraft control can still be lost.