Angle of attack

[WLIU]

For what is worth, jacking the leading edge of the horizontal stab up and down comed with its own set of issues. Both Cessna and Piper did it and then went back to trim tabs. A moveable stab turns out to be mechanically more complex, heavier, and you have the control creep issue. Both moving stabs and trim tabs have to hold the settings that the pilot selects. Cessna found out that jackscrews creep as much as a trim tab mechanism, requiring the designer create artificial friction in the linkage.

So you may want to compare the parts count, parts complexity, and weight of the two approaches.

Best of luck,

Wes
N78PS

[Ron Blum]

A. Yes, ice on the leading edge is a new configuration (most AOA computers have a setting for each flap setting with and without ice ... that's 8 configurations for most higher performance airplanes).

B. Mass balancing is only for flutter (and flight controls should be protected from accumulating ice on their leading edges (even the aerodynamic counterbalances on the tips). Remember that this is MASS balanced, they are still not aerodynamically balanced (or they would be neutrally (or un) stable). In other words, gusts will always shake the flight controls.

C. Trimmable stabs are not known for being lighter, they are know for being more aerodynamically efficient. All the bigger airplanes have them to reduce trim drag in cruise. It matters to the airlines that are operating there all the time. Even 1% cruise fuel efficiency gain is a big deal.

D. A "PS" to the icing. Certificated airplanes have to stall within 3 knots of the clean configuration or it has to be published in the AFM, too.

[steveinindy]

A. Yes, ice on the leading edge is a new configuration (most AOA computers have a setting for each flap setting with and without ice ... that's 8 configurations for most higher performance airplanes).

Fun, fun. Probably makes flight test a rockin' good time.

B. Mass balancing is only for flutter (and flight controls should be protected from accumulating ice on their leading edges (even the aerodynamic counterbalances on the tips). Remember that this is MASS balanced, they are still not aerodynamically balanced (or they would be neutrally (or un) stable). In other words, gusts will always shake the flight controls.

Right. I wonder if this might be an application for those electrical anti-ice coatings (ThermaWing) as a way of protecting the leading edge of flaps and control surfaces.

C. Trimmable stabs are not known for being lighter, they are know for being more aerodynamically efficient. All the bigger airplanes have them to reduce trim drag in cruise. It matters to the airlines that are operating there all the time. Even 1% cruise fuel efficiency gain is a big deal.

In my book, when you're burning 20+ GPH in cruise, a little gain like that can be important too regardless of whether you're in a high-end GA aircraft, a business jet or airliner. A little extra weight and mechanical complexity might be worthwhile.

For what is worth, jacking the leading edge of the horizontal stab up and down comed with its own set of issues. Both Cessna and Piper did it and then went back to trim tabs. A moveable stab turns out to be mechanically more complex, heavier, and you have the control creep issue. Both moving stabs and trim tabs have to hold the settings that the pilot selects. Cessna found out that jackscrews creep as much as a trim tab mechanism, requiring the designer create artificial friction in the linkage.

So you may want to compare the parts count, parts complexity, and weight of the two approaches.

Thanks for the information. I wasn't aware of Cessna's experience. I will have to look into that when it comes to the final detail design.

[Ron Blum]

Leading edges of rudders, elevators and ailerons are protected by the front section of their stabilizer/wing. New procedures (since the ATR accident) set a minimum holding speed in icing conditions (puts the ice accumulation where it will be anti/de-iced well. In addition, flaps are not to be used until on short final (if at all) ... unles you can prove that the leading edges are clean.

[Anymouse]

For what is worth, jacking the leading edge of the horizontal stab up and down comed with its own set of issues. Both Cessna and Piper did it and then went back to trim tabs. A moveable stab turns out to be mechanically more complex, heavier, and you have the control creep issue. Both moving stabs and trim tabs have to hold the settings that the pilot selects. Cessna found out that jackscrews creep as much as a trim tab mechanism, requiring the designer create artificial friction in the linkage.

Had the locking mechanism on a stabilator trim break on me shortly after takeoff in a C-182 (1959 model??) a bunch of years back. Everything I moved the elevator, the stabilator would reorient itself back to the path of least resistance. I ended up having to hold the trim wheel in order to keep the aircraft from porpoising. It made for an interesting landing, especially since the trim wheel was located on the floor.

[wacoc8]

Leading edges of rudders, elevators and ailerons are protected by the front section of their stabilizer/wing. New procedures (since the ATR accident) set a minimum holding speed in icing conditions (puts the ice accumulation where it will be anti/de-iced well. In addition, flaps are not to be used until on short final (if at all) ... unles you can prove that the leading edges are clean.

Most aircraft manufacturers that build FAR 23 aircraft are now limiting all approaches in known icing or if you have visible ice on the airframe to something less than full flaps. The reason is not due to ice on the wing but on the tail. The tail can stall prior to the wings with a load of ice.....and you can't see the tail from the two front seats. The worst time to change the configuration would be on short final. You will not recover. There are numerous NTSB reports to prove this. The one that comes to mind first is the Saberliner that crashed outside Detroit in the late 80's early 90's that was operated by the U.S. Marshall Service. I remember that one cuz I was flying a Saberliner 60 at the time.
As for the AOA, great explanation Ron. I know I could not have explained it that well, and I fly with one all the time.

Dave

[Ron Blum]

Although I agree with Dave, it could be residual contamination on either the wing or tail. The "Dove" that NASA uses as a test vehicle is very tail power limited in stall (in icing ... and it flies squirrelly even when it is clean). The ATR had a ridge of ice form ahead of the ailerons. Each airplane is a little different. Wing ice normally just increases the stall speed. Tail ice on the other hand could make the airplane unstable/uncontrollable. Crap. Back to the original topic.

Ice CAN make the stall AOA a little higher and stall speeds a little lower (think little vortex generators), but this is more true on older airfoils. With a modern airfoil, the stall speeds will go up and the stall AOA will go down.

[Sam Buchanan]

This is an old thread but the emphasis on incorporating angle of attack indicators in GA aircraft continues to gain momentum. Below is a news blurb from AvWeb. The complete report is here.

FAA/AOPA Committee Pushes AOA

By Glenn Pew, Contributing Editor, Video Editor

A report on approach and landing loss-of-control accidents has emerged from a work group co-chaired by the FAA and AOPA, recommending that GA "embrace to the fullest extent" angle-of-attack (AOA) systems and work to improve pilot decision making. The work group advises the General Aviation Joint Steering Committee. Its focus was derived from an FAA overview that found loss of control accounted for 40.2 percent of fatal general aviation accidents that took place from 2001 to 2010. Particular areas of concern included flying after a period of inactivity and transition training, as well as pilot decision-making.

Pilot decision-making remains a focus of safety concerns in general, as 85 percent of fatal accidents can be traced to pilot actions, according to AOPA. The report notes that controlled flight into terrain accidents have decreased and it attributes that improvement to new in-cockpit technology like terrain-aware GPS units. AOA systems, says the report, offer pilots awareness of their margin over stall and account for weight and acceleration differences, by design. The report notes that AOA systems offer substantial safety benefits but notes that cost and regulations may produce barriers for the pilots of light aircraft. The FAA "will need to identify the right level of certification," it says. It notes that the FAA should address these issues "with streamlined processes" for certifying and installing new technology that offers "a high probability of safety benefits" balanced against "low safety risk."

[WLIU]

"It notes that the FAA should address these issues "with streamlined processes" for certifying and installing new technology that offers "a high probability of safety benefits" balanced against "low safety risk."

This is the BIG problem. The process holds safety innovation back rather and facilitates it. I deal with field approvals and the process is slowing down, not speeding up. Unfortunately, "simplification" of a regulatory process often involves more paperwork.

Best of luck,

Wes
N78PS

[Ron Blum]

Actually, the FAA IS trying to streamline the process and make the process less expensive. I am on the ASTM committee, and there is a lot of talk about this topic. It needs to be done right, though, and not all AOA systems out there are measuring AOA, and in addition, the highest AOA at stall is not with flaps extended.