Approach speeds way too high?

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eulereit
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Approach speeds way too high?

Postby eulereit » Mon Nov 14, 2016 5:45 pm

I fly the sportcruiser, I almost have my SP ticket.

The approach speed is 60 knots, according to the POH. When executing short-field landings, we're told to use 55 knots.

These speeds seem very high. Vs0 is 32 knots, making the FAA suggested approach speed for short field landings around 42 knots. Does anyone understand why the published numbers are so high?

Thanks!

foresterpoole
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Re: Approach speeds way too high?

Postby foresterpoole » Mon Nov 14, 2016 9:30 pm

Hey speaking of this question, I just did a few today! The sport cruiser and P-92 have same approach speeds 60 which is the best glide speed. My CFI has taught me to make a shallow approach just pull the throttle out and try to land on the threshold at somewhere around 40, but approach at 60. Even at 60 in the approach the Technam will be down and stopped in no time...

My understanding is it's always better to have a cushion, no one wants to stall, as eyefly eluded to wind will make big differences the slower you are...
Ed

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drseti
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Re: Approach speeds way too high?

Postby drseti » Tue Nov 15, 2016 7:45 pm

eulereit wrote: Vs0 is 32 knots, making the FAA suggested approach speed for short field landings around 42 knots. Does anyone understand why the published numbers are so high?


Even if the ASI may indicate 32 knots in the stall, be aware that airspeed indicators always read low in the stall. That's because if the wing is at a high angle of attack (the very definition of a stall), so is the pitot tube. With the air blowing across, rather directly into, the Pitot tube, the reading will of course be low. You can be pretty sure the Calibrated stall speed in any SLSA is at or very close to 45 knots (the ASTM maximum allowed). Use that instead of what the ASI reads, and using the 1.3 x Vso rule, you come up with an approach speed of almost exactly 60 knots!
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eulereit
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Re: Approach speeds way too high?

Postby eulereit » Tue Nov 15, 2016 8:04 pm

drseti wrote:
Even if the ASI may indicate 32 knots in the stall, be aware that airspeed indicators always read low in the stall. That's because if the wing is at a high angle of attack (the very definition of a stall), so is the pitot tube. With the air blowing across, rather directly into, the Pitot tube, the reading will of course be low. You can be pretty sure the Calibrated stall speed in any SLSA is at or very close to 45 knots (the ASTM maximum allowed). Use that instead of what the ASI reads, and using the 1.3 x Vso rule, you come up with an approach speed of almost exactly 60 knots!


That's a good point; the sportcruiser has a CAS of ~37 knots at the 32 knot indicated airspeed. This puts the 1.3*Vs0[CAS] at approximately 48 knots CAS, which is 45 knots IAS.

Reference: http://www.czechsportaircraft.com/wp-co ... nce_12.pdf

foresterpoole
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Re: Approach speeds way too high?

Postby foresterpoole » Tue Nov 15, 2016 8:37 pm

You know the angle of attack's effect on the pitot tube did not even come to my mind. Learning things every day!!!
Ed

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Re: Approach speeds way too high?

Postby MackAttack » Wed Nov 16, 2016 9:19 am

Great post Dr. S! I have similar approach speeds for my Tecnam Astore ... I always prefer to approach at about 60; when I try it slower or faster the outcomes are not as good. I'd rather land just a little longer (assuming it's not a short field) than to have the runway look up at my plane and say "gee, thanks for dropping in!"

Cheers

eulereit
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Re: Approach speeds way too high?

Postby eulereit » Wed Nov 16, 2016 1:55 pm

I suppose my point is, even with CAS vis IAS corrections, the short-field approach speed should be under 45 knots (according to the FAA's 1.3x rule-of-thumb). The published book numbers look to be about 10+ knots too fast.

I understand in gusty conditions, we're going to want to approach faster for obvious reasons.

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Re: Approach speeds way too high?

Postby FastEddieB » Wed Nov 16, 2016 7:39 pm

eyeflygps wrote:Use the published book numbers. That's why they are published. At the already slow speeds of LSA, 1.3 Vso leaves little margin for error, wind, bank angle, etc.



Well, being 30% over stall speed seems like a pretty good margin. It's what, in general, the FAA recommends for a normal approach, with 20% above being adequate for short fields. All this with the caveats that the POH recommendations trump all that, and we're talking steady wind states.

That said, my Sky Arrow stalls at about 40kias. I generally find myself at around 55kias on final with full flaps, which is close to what my POH calls for and is closer to 40% above Vso. I do depart a bit from the POH in that it calls for a "landing speed" of 50kias, where my normal touchdown is much closer to the 40kias full flap stall speed.
Last edited by FastEddieB on Thu Nov 17, 2016 6:27 pm, edited 1 time in total.
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HAPPYDAN
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Re: Approach speeds way too high?

Postby HAPPYDAN » Thu Nov 17, 2016 6:00 pm

drseti wrote:Even if the ASI may indicate 32 knots in the stall, be aware that airspeed indicators always read low in the stall. That's because if the wing is at a high angle of attack (the very definition of a stall), so is the pitot tube. With the air blowing across, rather directly into, the Pitot tube, the reading will of course be low. You can be pretty sure the Calibrated stall speed in any SLSA is at or very close to 45 knots (the ASTM maximum allowed). Use that instead of what the ASI reads, and using the 1.3 x Vso rule, you come up with an approach speed of almost exactly 60 knots!


So this idea may explain why the stall horn on the C162 is blaring during slow flight, full flaps 45kts. She has a very nose-high attitude, but with lots of right rudder and careful control, will maintain altitude and heading with 1900-2100 rpm. Thus, the indicated airspeed is incorrect? With any reduction in power, or increase in attitude, the bottom drops right out. So might it be closer to 37-40kts true A/S? POH says IAS stall speed is 37kts full flaps, 41kts clean. :?:

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Re: Approach speeds way too high?

Postby 3Dreaming » Thu Nov 17, 2016 6:17 pm

HAPPYDAN wrote:
drseti wrote:Even if the ASI may indicate 32 knots in the stall, be aware that airspeed indicators always read low in the stall. That's because if the wing is at a high angle of attack (the very definition of a stall), so is the pitot tube. With the air blowing across, rather directly into, the Pitot tube, the reading will of course be low. You can be pretty sure the Calibrated stall speed in any SLSA is at or very close to 45 knots (the ASTM maximum allowed). Use that instead of what the ASI reads, and using the 1.3 x Vso rule, you come up with an approach speed of almost exactly 60 knots!


So this idea may explain why the stall horn on the C162 is blaring during slow flight, full flaps 45kts. She has a very nose-high attitude, but with lots of right rudder and careful control, will maintain altitude and heading with 1900-2100 rpm. Thus, the indicated airspeed is incorrect? With any reduction in power, or increase in attitude, the bottom drops right out. So might it be closer to 37-40kts true A/S? POH says IAS stall speed is 37kts full flaps, 41kts clean. :?:


Also remember published stall speeds are power off. If you carry power it will fly slower.


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