Power Off - 1-G vs. "Controllable" stall

Discussion in 'Rules and Regulations / Flight Safety / Better Pil' started by undean, Jul 1, 2019.

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  1. Jul 2, 2019 #21

    BJC

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    IIRC, Part 23 suggests (requires?) a deceleration rate of no greater than 1 knot per second when determining stall speed.


    BJC
     
  2. Jul 2, 2019 #22

    pictsidhe

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    Lets say that 'Harry halfwit' decides to build his own 103
    A bit of rummaging and he finds that the LE has a CLmax of 3.00 Woohoo, no need for an silly oversized wing on a 103, then!
    He then sees that the FAA doesn't care if a 103 is a few kts over. Heh, he'll be fine at 30kts.
    Harrys also 300lb, since the FAA isn't too worried about weight either, 50lb of extra material to carry his bulk seems like a good idea.

    His 'almost legal' 103 is actually going to stall at over 50kts. I have a suspicion that if some FAA suit who's having bad day sees a '103' flaring at twice the speed that it should be, Harry may soon be having a bad day, too. If there are too many Harrys, the rest of us will be very upset too.

    Guys, please don't extract the urine, I like having 103.

    I'd suggest keeping actual stall speed under 30kts. 10% extra because you aren't 170lb, 10% because you want to cut wing size...
     
  3. Jul 2, 2019 #23

    BBerson

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    On page 12 of AC103-7 it says the pilots weight will be considered 170 pounds for the purpose of stall speed calculations.
     
  4. Jul 2, 2019 #24

    REVAN

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    A power off stall speed will be descending. It isn't the lift alone acting on the airframe. The drag is also relevant, and ultralights usually have a lot of it. A round parachute has no lift, only drag, yet it will still bring you down at speeds below 25 knots. High drag and a steep glide slope makes this increasingly important to account for its contribution to the power off stall speed.
     
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  5. Jul 2, 2019 #25

    Dana

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    Depends on whether the plane would be reusable after hitting the ground in that condition...

    But seriously, that's post stall behavior. The defined stall point (silly humans, always want to draw lines) occurred at Clmax.
     
  6. Jul 2, 2019 #26

    BBerson

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    No requirement to fly it to the ground in a stall. Disregarding my deep stall comment, gliding an unpowered UL at CLmax might be a 45° glide angle (as Revan mentioned because of high drag). So a GPS would measure horizontal ground speed and get a significantly lower reading than actual airspeed. And even if actual airspeed was measured in the 45° glide it might be lower than the lift formula because of excess drag component. As Revan said, a round parachute has no Cl but still has a low speed unpowered.
     
  7. Jul 2, 2019 #27

    12notes

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    Would putting the pilot on a swiveling angle of attack type vane give an accurate airspeed at steep angles? Something like this:
     

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  8. Jul 2, 2019 #28

    BBerson

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    Putting the pitot (not pilot :eek: ) on a swivel would help get closer to calibrated airspeed. Also needs to be out two feet in front of the wing. I think the FAA offered AC103-7 to avoid all this complication with a flight test. It also gives heavy pilots a free pass.
     
    Last edited: Jul 2, 2019
  9. Jul 2, 2019 #29

    Aesquire

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    Impact type, pitot tube, airspeed sensors are typically wildly inaccurate at high AOA.

    We found with hang gliders that a trailing bomb type airspeed indicator on a 20 foot line was needed to get really accurate information. The flow field extends further than you'd expect.

    You really have to creep up on a stall to get an accurate, not accelerated number. Change, stabilize, change, stabilize, repeat.

    It's also possible to fly with the center section of a wing stalled, but the outer sections still lifting. ( twist,cuffs, etc. )

    But my opinion is some folk just reverse engineer stall numbers. You do the math & make up numbers until the equation looks not too fake and call it a day.
     
  10. Jul 2, 2019 #30

    pictsidhe

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    Didn't consider that most people would use a commercial GPS rather than build their own datalogger that will GPS absolute velocity...
    Hmmm, maybe this would be good product. Hardware parts would be perhaps $100.
     
  11. Jul 2, 2019 #31

    BJC

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    Not velocity, but my AFS EFIS shows where the airplane is going with an indication on the ground, horizon, or sky.


    BJC
     

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