Backcountry Pilot

[Zzz]

A few years back while I was studying to get my Private, I read a piece online by a weathered old flight instructor about indicated airspeed(IAS) error during forward slips. Basically, when you are slipping, the relative wind can apply a ram pressure to the static port, and not as much ram pressure on the pitot tube, so your airspeed is in error and indicates lower than if you were in coordinated flight. MY flight instructor at the time said to keep this error in mind, and that you can look at it one of two ways; That the IAS is erroneous and you actually have a higher airspeed, OR that since the wing is not flying directly into the relative wind, that the IAS should be regarded as correct, since you have less direct airflow over the wing to produce the required lift. Read this excerpt I found:

The airspeed indicator does subtraction in the process of indicating speed. Airspeed is the difference between the static port pressure and the pitot tube pressure.

A static source on the left side of an aircraft will indicate correctly only when the relative wind provides no ram or vacuum effect on the static port. In a slip, high or low airspeed variations will occur depending on the direction of a slip. In a slip the airspeed error cannot be predicted.

Slipping into the static port causes ram air effect on the port. Thus the difference between the static air pressure and the pitot air pressure will be less. Indicated airspeed will be less. Any time the relative wind is not directly into the pitot tube the pressure will be lower. The result is a lower indicated airspeed unless countered by the much greater effect of static port pressure. Relative wind is any wind created by motion will act opposite to the direction of motion.

Now a stall occurs when the wing exceeds the critical angle of attack and the airflow delaminates from the wing surface. Is this a danger during a slip? Of course. But how should we regard the relative wind since it is striking the leading edge at an angle, effectively making a longer wing chord? Should the IAS be trusted as our actual airspeed, or disregarded as erroneous?

The forward slip is an important technique for getting into some backcountry strips, so you guys must have given this some thought.

Zane

[Redbaron180 offline]

Practice with your plane intill you know what it does, different planes do different things. Stall it in the slip untill you are comfortable with the recovery. The amount of time it takes to recover from a slip depends on how close to the edge you are. For a short runway you need to be on the edge or you will gain nothing. depending on the plane of course. Is that vague enough? Ron

[hotrod180 offline]

Sometimes nothing but a slip will do, even for airplanes with flaps. But I have found another technique that works better than slips for some situations, like too high on final. Generally speaking,my target approach speed is about 60-65 mph, which is 1.3X vso. I find that when I'm high, instead of slipping, if I pull the nose up to reduce airspeed to about 55 it dramatically increases the sink rate. I've heard a guy I know talk about "stalling it down", but I think this might be what he's actually doing.
If you're not careful you can arrive down at the runway without enough energy to flare. Oh, you can get the nose pulled up, but there's not enough energy left to arrest the sink rate- you just keep on descending to slam down in a carrier-style landing. To avoid this, as you come over the fence, push the nose down to increase airspeed to regain enough energy to arrest the sink rate when it's time.
It's kinda hard to do at first, it's very counter-intuitive: too high, pull up the nose; and getting close to the ground: push down the nose. I'm sure lots of you guys know all about this, but for those who don't, give it a try.

Eric

[Marc Olson offline]

This is an area where practice at altitude will tell you how your plane responds at different airspeeds and slip configurations. I've been experimenting with my PA-12 after getting some instruction at landing over obstacles last Fall. I'd been using a slip at relatively slow airspeeds as the high AOA wants to make the plane 'float' more, even though it's descending. I've found that if I do a more forward slip I lose altitude much more quickly but then end up having to bleed off the extra airspeed.

Another pilot with much more experience than I suggested an experiment to get at which technique is more effective at landing short. I've found that a slower slip in my plane results in more horizontal distance covered but an arrival at the ground at a slower speed. If I forward slip (higher airspeed==more saftey margin over the stall) then I need to aim for a point ahead of my desired touchdown spot, but that I can do so more predicably than the 'floating' or 'near stall' down technique. In my plane this approach also means better visibility as the high AOA needed to keep the plane slow obscures the view of the landing area.

Practice, practice, practice...

A post from the sc.org site on the aerodynamics of a slip can be found here:

http://www.supercub.org/phpBB2/viewtopic.php?t=2176

[punkin170b offline]

Good topic. When doing a slip I have noticed a large increase in indicated airspeed when I transition from uncoordinated back to coordinated flight on short final. Part of the acceleration is real: It is due to the sudden reduction in drag and the energy of the high descent rate being re-vectored in a forward direction. But I am also convinced that the airspeed indicator in our 170b reads quite low when slipping. The wind noise and the feel of the yoke provide great cues for airspeed control. It's way too noisy and not mushy enough at 65-70 IAS in a full slip. It's gotta be going faster. At high altitude I have practiced the feel of the imminent stall and recovery from the full stall (insipient spin at the very least) out of a slip and am pretty comfortable indicating ~50 in the descent. The wind noise sounds like ~65 and the feel is still solid enough for comfort. Nice to have johnson-bar flaps and be able to dump them all at once when you kick the slip out too! Lands nice and short.
Matt

[Zzz]

great reply matt, thanks.

[Supercubber offline]

Slipping with full flaps on a calm day usually will not present much of a problem. You can get into precarious situations when slipping with full flaps combined with varying wind conditions where blanketing will occur.

Many people are of the opinion that you have to deadstick a forward slip for landing. Carrying a little power in a forward slip is not unheard of.

Slipping with full flaps or doing anything else for that matter outside the perimeters of the POH makes you a test pilot with all the associated perils.

Airspeed indicators are famous for being inaccurate to begin with. There may be a time when you find it indicating zero after some bug decides to crawl inside the pitot system. Much better to know what your aircraft is doing by feel.