V-G Diagram Discussion with APS Training

Comments

1. A. M. CArter, Jr. says:

   May 13, 2010 at 1:22 pm

   First reaction: An excellent presentation on a much needed subject by a super qualified pilot.
   I immediately forwarded it to mu grandson who is starting primary flt training at an AF base in OK, with the suggestion that he not only review it, but also digest it!
2. Paul Logue says:

   May 17, 2010 at 4:35 am

   Good presentation....

   If you are level on the x-axis and begin a roll and it goes bad ....dishes off toward the y-axis while descending rapidly .....and you pull to recovery while you are pulling..

   Is the g meter correct at the point? I am thinking that the g is actually greater ...?
3. pransbury says:

   May 18, 2010 at 8:21 pm

   Hi Paul - That is an astute observation. The g-meter is correct in relation to the aircraft, however, geometrically the curved fight path towards the ground in an inverted flight attitude increases by an additional radial G due to the gravity vector.
4. Rahim says:

   October 11, 2010 at 11:15 pm

   Thanks a lot. An excellent presentation but if it would be possible to take a copy or record for more analysis it would be perfect. Thank you for your efforts!
   Rahim
5. Jose says:

   November 11, 2010 at 1:30 pm

   Thank a lot for producing a presentation like this,because for me it was the first time i had a really deep explanation about this diagrams.
   well,i will also like to know if someone can give me an explanation about something the person doing this presentation said in video number 3,it was actually about the cero G,and he says that you can have 100 kts of speed with no lift,but i read in the books of principles of flight that i have where they say that there are 2 ways of producing lift,the first is by increasing the angle of atack,and the second is due to the speed of the aircraft.
6. cmcneace says:

   November 17, 2010 at 7:56 am

   Hello Jose,

   Thank you for your excellent question: how can you produce no lift if you still have an airspeed of 100 kts? (If this is not what you were trying to ask, let me know, please.)

   The lift equation for a wing (also spelled out) is:

   Lift = CL x ½p x V2 x S

   Lift = Coefficient of lift multiplied by one-half roe
   (relative air density or p) multiplied by velocity squared multiplied by surface area of the wing.

   While flying, air density (p) is not a major concern to us in the production of lift. It is a concern in terms of stability and controllability above 25,000 feet. But air density is typically a pre-flight concern because we have to determine density altitude before we takeoff to ensure we have enough runway to get the true airspeed we need to get airborne.

   Surface area of the wing (S) is also not a major concern since it is basically fixed. Certain flap types can increase surface area of the wing but once the flaps are extended, we as pilots do not concern ourselves with them until it is time to bring the flaps up.

   Airspeed (V) is certainly important. We have to have airflow over the wing to create lift. No airflow means no lift.

   Coefficient of lift (CL) is simply an aerodynamic term to describe a wing's lift efficiency with airflow. CL is simply a mathematical ratio that gives us a number. It is a number based on that wing's design at various angles of attack. We as pilots do not care what that number is but when you plot all of the various CL numbers for that wing at various AOAs, you get a CL curve for that wing. We as pilots absolutely care about where we are on that curve. Where we are on the curve is based on AOA, not airspeed or attitude.

   Practically speaking, when it comes to creating lift in our aircraft, we pilots are continuously managing AOA and airspeed. So it should make sense to you that if airspeed goes to zero, you have no lift. And by the same token, if you have airspeed but the AOA is reduced to zero, you will have zero lift. (Note: I am assuming the wing is symmetrical. If the wing is non-symmetrical like most aircraft, the AOA will need to be reduced to a slightly negative AOA to reduce lift to zero.)

   Well, that was a long-winded answer but I hope that helped. If not, please feel free to email me with more questions.

   Best Regards,

   Clarke "Otter" McNeace
   Director of Flight Training and Standards
   APS Emergency Maneuver Training
7. Tom Turton says:

   November 16, 2011 at 8:46 am

   Very good and thorough presentation. Might consider adding a 6th segment showing how the V-n diagram changes with: Weight, Flap Settings, Altitude.

   With regards to the altitude effect, most V-n diagrams I've seen are plotted vs Ve(equivalent airspeed) so the altitude effect isn't seen.

   Finally one point which may or may not be relevant...I'm concerned with saying at maneuver speed you can just put in "full elevator" by yanking back. At a very high rate of increase of angle of attack, you could experience a "dynamic stall" where your instantaneous Coeff_of_Lift exceeds the steady state Coeff_of_Lift. So you might actually (in the short term) experience a higher G-load than the structural limit, even though you are at the maneuver speed.
8. APS Director of Flight Operations and Standards says:

   December 29, 2011 at 7:59 am

   Thank you, Tom, for your comments.

   Yes, showing the effects of weight & flap settings on the V-g diagram would be beneficial as well.

   I checked with a researcher at the NASA Langley Research Center and existing literature confirms your concern of a “dynamic stall.” Understanding that the V-g diagram curve below Va is COMPUTED from the lift curve is important. The lift curve is typically a function of wing design and control position. Extremely high pitch rates could create “dynamic lift” that overshoots the peak steady lift momentarily before stabilizing. However, it is unknown whether any transport aircraft have the control authority to create these high “dynamic lift” concerns or if the additional load is significant.

   Needless to say, irrespective of whether the aircraft has the ability to exceed published load factors while at Va; unmeasured full AND abrupt control inputs are discouraged during an upset event. Why would a pilot indiscriminately use “full elevator” by abruptly yanking back? That kind of control input is likely made by a pilot in a panicked state of mind during a loss of control event. Any pilot who indiscriminately yanks back on the elevator is likely not trained in proper upset prevention and recovery techniques and could make other aggravating control inputs (such as cyclic movements) during an upset event.

   Based on FAR Part 25 Section 331a, an aircraft’s design maneuvering speed (Va) will be determined, and at or below that airspeed, you should be able to have confidence that the aircraft will not be overstressed due to a maximum effort pull on the elevator by the pilot. But that is not the end of the story. This type of aggressive control input can be made safely (with due consideration of the dynamic lift discussion above) but not in a cyclical fashion. In other words, a pilot cannot pull full up elevator and then reverse with full forward elevator and then reverse again with full up elevator. This type of abrupt cyclical control input could result in structural failure even if the airspeed is below Va. The crash of American Airlines Flight 587 is a likely example of cyclic rudder inputs while flying below Va that resulted in structural failure of the vertical stabilizer leading to the subsequent crash.

   The FAA has issued last year a Final Rule (14 CFR Part 25, Docket No. FAA-2009-0810; Amendment No. 25-130) on Va definition clarification for transport category pilots. This final rule amends the airworthiness standards to clarify that flying at or below the design maneuvering speed does not allow a pilot to make multiple large control inputs in one airplane axis or single full control inputs in more than one airplane axis at a time without endangering the airplane’s structure. The FAA issued this final rule to prevent pilots from misunderstanding the meaning of an airplane’s maneuvering speed, which could cause or contribute to a future accident.

   In conclusion, Va and it’s relationship within the Vg diagram is an excellent training tool for visually giving pilots a practical understanding of their aircraft’s flight envelope. This increased understanding of the flight envelope can help the pilot to mitigate (with proper training) potential LOC-I threats including the unacceptable reaction of just “yanking back” on the elevator.

   Thank you so much for your excellent concerns and comments.

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