Ground Effect

Adjust your seat

2008-04-13T13:06:00Z

This essay is a confession. In the mystery that is the perfect landing I believe I have been overlooking one of the most important variables.   The height of the seat!

A recent AOPA Magazine had a short article on the important of adjusting seat vertically and showed a picture of a small cockpit fixture used as a fiducial to make sure the pilot and co-pilot had precisely positioned their seats.



It referenced a specific landing accident which was attributed in part to the poor seat position of the pilot flying.

www.mtc.gob.pe/portal/transportes/aereo/aeronauticacivil/alar_tool_kit/pdf/ap_nov93.pdf



I did read the article intently but it would not have clicked unless I hadn't then gone flying in one our club '172s.
Given that it is April 13th in Minnesota and it is STILL SNOWING I haven't done as much aviating as I normal do. Saturday was a chance to knock off the cobwebs.

The previous pilot of the 172 was probably shorter and smarter than I am -- and coupled with the slightly lower glareshield I had an expansive view of the tarmac immediately in front of me.

What a difference!

I fear that in the last years of flying I have gotten in the habit of plopping down in the seat, dutifully adjusting it fore and aft (and doing the Cessna bump and grind check) and blasting off into the clag. My only goal was to have a good grip on the rudders.

I don't think I have ever turned the crank, and God knows that flight school seat cusion is long gone!

Perhaps some early experiences in my formative years flying from the aft seat coupled with the instrument fixation -- conditioned me that you didn't need to really see the runway?

The aircraft designer, and the pilot who set up the 172 before me knew this:

The design eye point is located to allow the pilot to see a length of approach or touch-down zone lights which would be covered in three seconds at final approach speed. This represents a distance of approximately 600 – 750 feet along the flight path.



Here is an excellent article (also from AOPA)

http://www.aopa.org/asf/publications/inst_reports2.cfm?article=4702

Here is an excellent discussion of the specifics:

http://www.avia141.com/Ch5_overview.htm

While I should write the substance of these on the chalkboard 100 times, rather than plagarize them here I post the links and offer this entry as confession.

I may not be as dilligent as this F-18 pilot, but as I am sharing my flying steeds with pilots much taller than I am I will now be very deliberate to check the seat height.   In those cases where its not adjustable that old skanky boat cushion might be more important than the 40lb swag bag of gadgets ...



Todd out

Waunakee

2008-03-22T00:30:00Z

Many years ago I had a cessna 170 based at the Waunakee Wisconsin airpark.   I was
coming in to land and a little fellow cuts towards the runway on his bike.   As I overfly he drops it right
on the centerline and bolts. Now there was enough runway left in the denominator and with careful
attention to any more animal crossings I land and scoop up the offending bicycle.

Pretty soon his lawyer shows up with some sort of explaination I cannot remember in an effort to get the bike back. It gives me a good chance to give the Airport Safety Lecture.   I suggest that if he would like to sit in
the plane and put the headphones on that is fine.   Now the rest of The Little Rascals are hiding behind rocks and bushes nearby watching to see what happens. I suggest that if someone can produce a responsible parent
there might be enough daylight for a ride or two. Eventually some parents are located and it is another chance to reinforce the airport safety lecture (and make sure bail was posted and a sentencing hearing was scheduled).

One mom had been in the Navy which made her most qualified to interview hobo pilot and she asked various questions about my freshmen flying background (she even looked at my logbook).   One or two of the kids got Young Eagles certificates. One of the others certainly learned his lesson as I can be sure Navy-Mom narced on him.

I did give him his bike back.
Todd

lost comm, hold at the IAF until your ETA

2008-03-16T02:36:00Z

In the case of lost communications the Instrument Flight Rules provide explicit guidance.   It is expected that if you arrive at your destination prior to your filed Estimated Time of Arrival that you will hold at the IAF until that time.

Sec. 91.185 - IFR operations: Two-way radio communications failure.

(a) General. Unless otherwise authorized by ATC, each pilot who has two-way radio communications failure when
operating under IFR shall comply with the rules of this section.

. . .

(3) Leave clearance limit. (i) When the clearance limit is a fix from which an approach begins, commence descent or descent and approach as close as possible to the expect-further-clearance time if one has been received, or if one has not been received, as close as possible to the estimated time of arrival as calculated from the filed or amended (with ATC) estimated time en route.

(ii) If the clearance limit is not a fix from which an approach begins, leave the clearance limit at the expect-further-clearance time if one has been received, or if none has been received, upon arrival over the clearance limit, and proceed to a fix from which an approach begins and commence descent or descent and approach as close as possible to the estimated time of arrival as calculated from the filed or amended (with ATC) estimated time en route

This is a question.   Would you, or should you hold at the Initial Approach Fix in a legitimate lost communication situation?

If you are soldering thru the scud in your single radio FLIB and the radio does fail, what are the chances that it is portending a more serious total electrical failure. If you had a functioning navigation radio to get you to the IAF
is there a chance that the the same malaise that infected your marconi might also silence your only tool to accomplish the approach?

If you have had a more aggregious electrical failure and are improvising with a handheld, should you consider this a full emergency and accomplish the approach and land as soon as practical?

I cannot imagine that if ATC is tracking your primary only target, or has lost track of you entirely, that they won't feel oblidged to sterilize your most likely approaches up until you land safely -- regardless of how crisply you are holding an an initial approach fix.   By continuing to hold all you do is; consume fuel, give the weather an opportunity to deteriorate, and suspend all the operations at the airport.

They would have no way of knowing what precise time your Big Pilot Watch was displaying, or that you might not make a dash for the ILS at any time having imagined you now smell smoke, or are seeing the ammeter twitch.

No, I would seriously consider flying the approach as soon as I reached my filed destination.   Assuming I couldn't raise ATC on my hand-held backup -- I would chant 91.3 and exercise my emergency authority to get the airplane on the ground.

Sec. 91.3 - Responsibility and authority of the pilot in command.

(a) The pilot in command of an aircraft is directly responsible for, and is the final authority as to, the operation of that aircraft.

(b) In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency.

Opinions?

Spins from Coordinated Flight (part 2)

2007-12-28T03:38:00Z

I wrung out the debate on uunet (on rec.aviation.piloting, and rec.aviation.aerobatics).


I managed to provoke several dozen responses and was ultimately called an idiot in an email
exchange from a well known retired test pilot.   Standard fare for a newsgroup interaction I suppose.

However, one profound suggestion was to reach out to Rich Stowell.   He is the evangelist
for the P-A-R-E recovery acronymn.

Power (to idle)

Ailerons (to neutral)

Rudder (against the spin)

Elevator (briskly forward to break the stall).

I must say he drafted a very thoughtful response to me in a direct email.   In so doing he affirmed
what most of the denizens of rec.aviation polled (including the test pilot). There must be two ingredients
to spin:   A stall, and yaw.

The only conclusion I can offer to this essay is to point to his web site, and his book(s):



http://www.richstowell.com/

He articulately explained that the inclinometer is not a precise indication of coordinated flight and that
some form of yaw is a necessary ingredient to the spin.

It really is a deep subject and I would steer anyone to his texts (he sent even sent me PDFs of the relevant
sections) or to the Aerobatics text by Williiam Kersher:





http://www.amazon.com/Basic-Aerobatic-Manual-William-Kershner/dp/0813800633

Spin from a Climbing Turn

2007-12-25T20:11:00Z

During a recent uunet debate I asserted that an aircraft would not depart, or spin, from a coordinated turn.

It was my understanding there had to be yaw, in order to risk a spin.   Note that I have demonstrated spins or received specific spin instructions in five different airplanes, on six occasions; and I am by no means an expert.



I was chided that a coordinated climbing turn could produce a dramatic wing drop (and potentially a spin) due to the different angle of attack experienced by the outside wing.

The responder offered the following references:

Full power stalls in a balanced climbing turn tend to result in the outer wing stalling first, because of the higher aoa of the outer wing, with a fairly fast wing and nose drop (particularly so if the propeller torque effect is such that it reinforces the roll away from the original direction of turn and the aircraft is a high wing configuration) and likely to result in a stall/spin situation that any pilot lacking spin recovery experience may find difficult to deal with. If the climbing turn is being made with excessive bottom rudder then the lower wing might stall first with the consequent roll into the turn flicking the aircraft over. Recovery from a stall in a climbing turn is much the same as any other stall – ease the control column forward to about the neutral position, stop any yaw, level the wings and keep the power on.

http://www.auf.asn.au/groundschool/umodule11.html#climb_turns

When the aircraft stalls in a climbing turn, the high wing is at a greater angle of attack than the low wing and therefore stalls first, which results in a rolling motion toward the high wing, creating asymmetric lift and drag. The down-going wing will stall further as a result of less lift and more drag than the up-going wing. A deeper stall, generated by aft C of G, will aggravate these asymmetries, increasing aircraft rolling and yawing moments into the down-going wing. In addition, the aft C of G reduces the distance from the C of G to the centre of pressure of the vertical fin, thus reducing directional control authority, making recovery more difficult

http://www.tsb.gc.ca/en/reports/air/1994/a94o0316/a94o0316.asp?print_view=1

In a climbing turn, the outside or upgoing wing is meeting the relative wind at a slightly higher angle of attack than the lower wing.   If we pull on the column to the stalling bite, then the upgoing wing will reach it first...The upgoing wing suddenly drops and the wing falls away from the original direction of turn.

http://www.casa.gov.au/fsa/2000/sep/FSA34-35.pdf

The Transport Canada Guidelines on Stall Training and Spin Awareness specifically requires demonstrations in coordinated climbing turns:

http://www.tc.gc.ca/civilaviation/general/Flttrain/TP13747/stalltrain.htm

I am not certain you could actually accomplish a spin in a certficated airplane, stalling it in this manner
from coordinated flight. My original thesis in the debate insisted that if the ball was in the center (and controls
were properly applied thru the stall) there would be no spin.

If a climbing turn on the verge of Vs risked departure would preparations for a chandelle turn carry specific
warnings? I could find none in instructional texts? Granted, at the point of minimum airspeed the bank angle is
being relaxed to zero; however the hamfisted manuevers I have demonstrated should have risked
catastrophe.

I did a cursory check of the NTSB database and could find no correlation between chandelle and spin, however,
there were incidents of loss of control following a climbing go-around.

At this time, I do not know the answer to the question; departing from coordinated flight. However, the
discussion has challenged my thinking:

The convergence of insufficient right rudder and a slipping turn, the left turning tendencies and the
assymetrical stall could gang up on our hapless pilot resulting in a quick snap and
spin during a climbing right turn away from obstacles in the departure path.

(High Wing) Slips with Flaps

2007-12-01T23:10:00Z

The subject of slipping, specifically with high wing Cessna's; entertains a lot of discussion.

Many years ago a fellow named Gene Seibel rallied a T-shirt design on rec.aviation.student.
It was supposed to mirror the theme of "Runs with Scissors."

I still have the shirt in my pajama drawer.  I guess it was appropriate for last years polar bear plunge,
at MapleLag, in Detroit Lakes Minnesota.   A careful observer will note that I didn't plunge very deep that year.
It is a yearly tradition, but the level of committment varies.   That is another story ...

The debate has always fascinated me. Perhaps becuase I have flown older taildraggers, or perhaps because
I am invariably too high and too fast.    My first exposure to the "controversy" was with an FAA Safety Inspector.

------

I have mild CP and walk with a limp. It affects the strength and dexterity with which I can apply my left foot.

As such I required a Statement of Demonstrated Ability in order to receive an unrestricted third class medical
and this required a flight test with the Milwaukee FSDO just prior to completing my private pilot training.
My second class waiver was awarded some years later based on operational experience alone; perhaps
I will detail the process in a later entry.

The flight test was fairly straightforward and the inspector simply asked for some of the practical test maneuvers; all the while pointing out that if I had been properly organized this could have accomplished the flight test. Perhaps the only unusual element was an "oscillatory" stall performed by pinning the control yoke full back and applying rudder corrections while the airplane stalled and recovered in succession.

As you can imagine, retiring my anxiety about the medical barrier was a large hurdle in my early training.
So when the examiner seemed satisfied, I was more than eager to point the Cessna 150 at MKE and tie it
down before some kind of fumble.

At approximately 3000ft AGL over the field, and headed the wrong direction out to sea; the tower controller volunteers runway 7 with the sporty "can you make it work?"    I picked up the microphone and began to squeek "Student pilot would like a long slow series of gentle vectors around" when the nice lady from the FAA snatches the microphone, responds "yes this will work."   She barks to me, "your flying is fine, and I am late for dinner." Her arrplane.

As we curve around out of the sky she sets up a full control deflection side slip and asks "has your instructor ever talked to you about slipping with flaps?"   I recited the mantra and she admonished that the placard
reads "extended slips are to be _avoided_"  It is not a formal limitation and they are not forbidden. She indicated that it was a valuable tool to adjust the approach angle in a power off landing (or in this case, increase descent rate to make up time when late for dinner).

I watched her honk us down out of the sky and accomplish a very nice landing.   It may not have been very dramatic but I was quite early in my flying career and caught up in the event.

-----

The following paragraph is copied from the book "Cessna, Wings for the World" written by
William D. Thompson, an Engineering Test Pilot and later Manager of Flight Test and Aerodynamics at the Cessna Aircraft Co.

With the advent of the large slotted flaps in the C-170, C-180, and C-172 we encountered a nose down pitch in forward slips with the wing flaps deflected. In some cases it was severe enough to lift the pilot against
his seat belt if he was slow in checking the motion. For this reason a caution note was placed in most of the owner's manuals under "Landings" reading "Slips should be avoided with flap settings greater than 30° due
to a downward pitch encountered under certain combinations of airspeed, side-slip angle, and center of gravity loadings". Since wing-low drift correction in cross-wind landings is normally performed with a minimum
flap setting (for better rudder control) this limitation did not apply to that maneuver. The cause of the pitching motion is the transition of a strong wing downwash over the tail in straight flight to a lessened
downwash angle over part of the horizontal tail caused by the influence of a relative "upwash increment" from
the upturned aileron in slipping flight. Although not stated in the owner's manuals, we privately
encouraged flight instructors to explore these effects at high altitude, and to pass on the information to their students.

This phenomenon was elusive and sometimes hard to duplicate, but it was thought that a pilot should be aware of its existence and know how to counter-act it if it occurs close to the ground.

When the larger dorsal fin was adopted in the 1972 C-172L, this side-slip pitch phenomenon was eliminated, but the cautionary placard was retained. In the higher-powered C-172P and C-R172 the placard was applicable to a
mild pitch "pumping" motion resulting from flap outboard-end vortex impingement on the horizontal tail at some combinations of side-slip angle, power, and airspeed.

----

The following link points to an online video from Ron Wanttaja, www.BowersFlyBaby.com.   It displays the slip seen from a digital movie camera mounted to the landing strut.



                                                                   Fly Baby Slip to Landing

Finally this is a video from the ground of a slip in a glider:




                                                           YouTube, slip to a landing in a glider

Ground Effect Part Deux

2007-11-19T03:26:00Z

Notice that this Cessna 182 firewall is slightly concave near the bottom and that the top nosegear mounting is twisted slightly.

.

Here is an undamaged airframe for comparison.

Why do we have to be suspicious of buckled firewalls and nose gear damage on high wing Cessna aircraft?

Recall that the tail plane generates a down force, this is responsible for the natural pitch stability of the aircraft:

In fact, in general, the rearward Center of Gravity limit represents the aft most CG for which the tail plane is still generating a down force.

The forward Center of Gravity limit represents the point at which the tail is unlikely to reach critical angle of attack before the wing during the landing flare; at full flaps and with a range of power settings.

Let us picture this. The flaps are extended which changes the mean aerodynamic chord of the airfoil, deflecting the downwash further.

The tail is an inverted airfoil, so this downwash angle increases the angle of attack of the tail surface.

This is why you feel the need for forward trim after flap extension in most high wing aircraft.

Now lets look at a sample weight and balance problem.

Two 200lb adults and full fuel in a Cessna 182Q:

Add up the weights and moments:

And note that while we are in the Center of Gravity envelope, we are very close to the forward limit:

The common explaination for wheelbarrow landings in the Cessna 182 is poor technique and pilot induced oscillation.

Could it be that under reasonable circumstances, the airplanes are often flirting with their forward CG limits? The tailplane stalls as the pilot applies up elevator in ground effect; and the nose drops to the runway suddenly?

It is speculation but I believe you see more nosewheel damage and flexed firewalls on the high wing Cessna's than the low wing Piper Indians ...

Back Pressure

2007-11-16T01:28:00Z

This essay is to discourage the description of control "pressures" rather than control inputs and control surface deflections.

The terminology is common parlance:

"In a turn the necessary additional lift is acquired by applying back pressure to the elevator control"

- The Pilots Handbook of Aeronautical Knowledge FAA-H-8083-25

It is even used in an advisory circular on stall recovery no less than four times!

"Every aircraft may require a different amount of forward pressure or relaxation of back pressure to regain lift"

- AC-61-67C Stall and Spin Awareness.

Interestingly an article from Boeing chooses to phrase it like this:

"To recover from the stall, angle of attack must be reduced below the stalling angle. Nose-down pitch control must be applied and maintained until the wings are unstalled"

- AERO Magazine Issue 03 -July 1998

Remember, control pressures are a function of airspeed and trim condition, not aircraft attitude.

Using the terminology of "pressures" masks the real goal of deflecting control surfaces as appropriate to control pitch, back and yaw. It may convey a dangerous misconception for aircraft recovery.

Ground Effect

2007-10-13T02:39:00Z

Ground Effect

It is possible to fly an airplane just clear of the ground (or water) at a slightly lower airspeed than that required to sustain level flight at higher altitudes. This is the result of a phenomenon, which is better known than understood even by some experienced pilots.

Pilots Handbook of Aeronautical Knowledge FAA-H-8083-25

Let us develop a clear explaination of this effect.

In the simplest description the wing planiform moving thru the air results in a lower pressure above the wing and a higher pressure below it.

In the simplest formula lift is calculated by multiplying a fixed coefficient by the angle of attack by the surface area of the wing element.   This generates a force normal to the relative flow of air across the airfoil.   If
we increase the angle of attack this normal force increases in direct proportion until it reaches a point of stall.

In most free body diagrams the pure vertical component of this normal force is labeled lift and the horizontal component of this normal force is labeled induced drag.   Induced drag results from the production of lift.

This convention may have originated from the Wright Brothers and Dayton Ohio; this diagram is from a correspondence  with Octave Chanute:




However for this discussion we will use a very specific definition; lift is the force perpendicular to the chord line of the wing and induced drag is the force parallel to the chord line. This is the engineers definition; as lift and drag are purely a function of the wing regardless of its relationship to the horizontal.


If the wing had infinite span, the relative flow of air across the wing would follow the chord line thru the airfoil and the normal force would be perpendicular to the wing chord.

However, the aircraft wing does not have infinite span, and the high pressure air below steals over the wingtips to the top surface at each end.   This continuous flow forms a vortex.




                                                         Picture courtesy of DANTEC Measurement Technology.

These vortices have two immediate effects. The airflow reduces the difference between low pressure air above the wing and high pressure air below it (reducing the total normal force that translates into lift).   The swirling vortices also deflect the trailing airstream downward.   The downwash deflection alters the effective path of the relative wind downward and tips the normal force back (moving the lift vector backwards).



The resultant lift vector has a smaller overall vertical component and a slightly increased horizontal (backwards) component.   Thus the wing is producing less lift and more induced drag directly as a result of these effects at the wingtips.

However, when the wing is operating within half a span of the ground or water; the downwash is obstructed and these wing vortices are distrupted.

In this way, the normal reduction in normal force (and lift) and the larger induced drag due to downwash is diminished.

The wing in ground effect produces more lift for a given angle of attack, and less induced drag.

The venerable Pilots Handbook of Aeronautical Knowledge goes on to comment:

"While the aerodynamic characteristics of the tail surfaces ... are altered by ground effects, the principal effects ... are the changes in aerodynamic characteristics of the wing."

This simple statement glosses over the horizontal stabilizer as an airfoil and overlooks a potential grave misunderstanding of the consequences of center of gravity limits and weight and balance ....

TWD 12 September 2007