Vx & Vy Speeds?

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Vx & Vy Speeds?

Postby reecewallace » Wed Feb 03, 2021 2:18 am

I'm seeing discrepancy in speeds in my 1955 170B's POH for climb out.

In Section III, Operating Details, for short field take offs with 50' obstacle, it reads:
"Climb at 62mph indicated airspeed until obstacle is cleared"
*20 degrees flaps

However, in Section V, Operational Data, it reads:
"Climb at 67mph indicated airspeed to clear 50' obstacle'
*20 degrees flaps.

In my old Cessna 150, it had very clear Vy & Vx speeds.

I believe the Vy is 88mph, but what is the Vx?
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Re: Vx & Vy Speeds?

Postby rnealon1 » Wed Feb 03, 2021 5:10 am

I think you will find a few different threads on this topic, and I just brought up the 62/67 discrepancy a few days ago, with no explanation. The following information is my opinion but no doubt others will be able to add insight.

The performance section of the AFM and the Owner's Manual for Flaps Up gives a climb speed of 76 to clear a 50 foot obstacle, and this is generally accepted as Vx:

Capture.PNG
Capture.PNG (51.53 KiB) Viewed 413 times


The 62 number exists only in the Operating Details of the Owner's Manual and not in the AFM or Type Certificate, so I use 67 for Flaps 20.

For Vy 88 is the generally accepted number based on this on page 23 Operating Checklist of the Owner's Manual but does not appear in the AFM or Performance Data:

Best Rate of Climb.PNG


However, Operating Details on page 27 has this statement:

Best Climb.PNG


And both the AFM and Owner's Manual have this table in the Normal Rate of Climb section:

AFM Best Climb Speed.PNG


Climb speeds are variously referred to as Best Climb Speed, Normal Rate of Climb, and Best Rate of Climb, and none of these agree with each other. So Vy is somewhere between 78 at 7000' and 89 at Sea Level.

My best understanding is that certification under the (CAA) rules at the time did not require speeds to be defined as they are now (and for the last 50 years or so).

Bob
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Re: Vx & Vy Speeds?

Postby wabuchanan » Wed Feb 03, 2021 10:28 am

reecewallace wrote:I'm seeing discrepancy in speeds in my 1955 170B's POH for climb out.

In Section III, Operating Details, for short field take offs with 50' obstacle, it reads:
"Climb at 62mph indicated airspeed until obstacle is cleared"
*20 degrees flaps

However, in Section V, Operational Data, it reads:
"Climb at 67mph indicated airspeed to clear 50' obstacle'
*20 degrees flaps.

In my old Cessna 150, it had very clear Vy & Vx speeds.

I believe the Vy is 88mph, but what is the Vx?


ReeceWallace,
I think to be fair, you need to understand that our airplanes don't have POH's ( Pilots Operating Handbooks ). We do have an owners manual, which is what you are referencing. Our Legal document is our AFM (Airplane Flight Manual), which does not list V speeds for climb as noted by Bob in the post just under yours.

Climb speeds are based on altitude and temperature, if you look clear across the referenced section in the AFM. Thus, you get to calculate your proper climb speed for the conditions you are in. And nothing said as to Vx or Vy. In addition, it only references the McCauley prop, and does not specify whether it is the 51/53/or 56 pitch.
Unless you are running a Sensenich prop, which is one more chart below the McCauley.

Swap out to the 80/42 prop and you get to really figure out your own Vx and Vy speeds.

The charts also reference an approach speed of 71mph for their landing distances. I personally approach at 60mph, and sometimes 50 for short field, so those numbers don't help me much.
I generally use 70 for Vx in my plane ( but can go as low as 60 ), and 80-85 for my normal climb speeds. But that is after much testing and experience in my own aircraft, and with a climb prop.

Welcome to classic aircraft ownership, built in the days before POH's and comprehensive performance charts :wink:
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Re: Vx & Vy Speeds?

Postby GAHorn » Wed Feb 03, 2021 4:58 pm

Some of this confusion may result from “reader error”... in missing the difference between IAS and TIAS. Some (most) of the charts and data rely upon TIAS... while the specific text of the recommended technique for avoiding obstacles uses IAS.

(Reece, are you certain your Section V is accurately quoted in your previous post...?? Or did you inadvertently copy the earlier text and use the latter number of 67..?? My Section V does not make that exact quote you posted.... and it switches to TIAS.)

While there may be slight differences between various year-models (where we’ve measured with the micrometer) .... the practical recommendations (where we marked it with the grease-pencil) .... and the specific data becomes muddy (because we cut it with the hatchet.)

In an effort to make some sense of it, from yet another Owner’s Manual (172C) which is essentially the Same Airplane (same engine and prop, same pitot system, same essential performance numbers) comes an “Airspeed Correction Table” which offers the “correction” to be applied so we can convert the data in the performance charts which use TIAS.... into the actual INDICATED airspeed IAS we actually use while operating the airplane.
And, BTW, the Production-Standard of the airplane was the McCauley DM7653 prop and is to which all the data pertains.)

(What’s the difference between IAS and TIAS?
IAS is what we see on the instrument. TIAS accounts for installation error,** and Calibration error,.)

Here’s the chart (and notice that 62 IAS matches 67 TIAS.... the IAS and the TIAS do not merge until entering into the 90-100 mph ranges, largely because the pitot-tube is more aligned with relative-wind ):

9CD85E3C-4E98-4052-8214-90BCC202693D.jpeg


**Later airplanes like the 172 switched instrument mfr’s, and pitot tube mfr’s/design and do not apply to our 170s. This chart is from the 172C which is the identical wing, engine, propeller, instrument, installation, pitot-tube, and performance charts as the 170B.)
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Re: Vx & Vy Speeds?

Postby rnealon1 » Wed Feb 03, 2021 10:00 pm

Here is the reference on page 27 under TAKE-OFF Minimum Run Take-off:

62 Takeoff.PNG


As you note, this reference is IAS, whereas the Performance Data is TIAS.

I had also obtained the same 172 Airspeed Correction Table; this probably partially explains why I can fly at 40 MPH IAS without stalling; other factors such as a 172 style pitot tube and a different length/pitch propeller no doubt contribute to the discrepancy.

I cannot locate a definition of "True Indicated Airspeed".

Bob
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Re: Vx & Vy Speeds?

Postby IA DPE » Thu Feb 04, 2021 2:59 am

GAHorn wrote:(What’s the difference between IAS and TIAS?
IAS is what we see on the instrument. TIAS accounts for installation error,** and Calibration error,.)


By George's definition, TIAS (of yesterday) is what we call CAS (Calibrated Airspeed) of today.
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Re: Vx & Vy Speeds?

Postby TFA170 » Thu Feb 04, 2021 4:11 am

IA DPE wrote:
GAHorn wrote:(What’s the difference between IAS and TIAS?
IAS is what we see on the instrument. TIAS accounts for installation error,** and Calibration error,.)


By George's definition, TIAS (of yesterday) is what we call CAS (Calibrated Airspeed) of today.

I was wondering if that was what he was getting at...
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Re: Vx & Vy Speeds?

Postby IA DPE » Thu Feb 04, 2021 11:51 am

TFA170 wrote:
IA DPE wrote:
GAHorn wrote:(What’s the difference between IAS and TIAS?
IAS is what we see on the instrument. TIAS accounts for installation error,** and Calibration error,.)


By George's definition, TIAS (of yesterday) is what we call CAS (Calibrated Airspeed) of today.

I was wondering if that was what he was getting at...


Looking at a TCDS for a glider I'm interested in (1958), I see the FAA used that term at that time as well.
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