Two useless speeds

[43Inches]

He mentions that Vx is close to stall speed - ever heard of Take Off Safety Speed? The manufacturer states this is the minimum speed until 50' to ensure controllability of the aircraft in the event of an EFATO.

Most manufacturers POH for light aircraft refer to lift-off speed and 50 foot barrier speed ( which is usually close to Vx). There is no mention of Vtoss.

The Bonanza POH states "set desired climb speed once clear of obstacles", assuming the obstacles were deemed clear on lift off there is no need to fly Vx or Vy.

Piper breaks up take-off into take-off and climb; for take-off they recommend accelerating to Vx to clear obstacles then Vy or just accelerate Vy if no obstacle, something to note is they do not say to accelerate and maintain these speeds, they are implying a minimum speed. En-route they state the Vx and Vy speed but usually have a recommended higher speed. They do not state where take-off ends and en-route climb begins, one can assume that is at a safe altitude once obstacles/terrain is no longer a threat.

From the PA-32R-300 POH (1987 copy);

"Allow the airplane to accelarate to 52 to 62 KIAS depending on weight of the aircraft and ease back on the control wheel to rotate to climb attitude"; end of normal take-off procedure.

Climb procedure next; "The best rate of climb at gross weight will be obtained at 92KIAS. The best angle of climb may be obtained at 87 KIAS. At lighter than gross weight these speeds are reduced somewhat**. For climbing en-route, a speed of 104KIAS is recommended"

So based on that if obstacles are not an issue the manufacturer is basically saying lift of and accelerate to 104 KIAS for a normal take-off, Vx or Vy only if required.

[Old Akro]

He mentions that Vx is close to stall speed - ever heard of Take Off Safety Speed? The manufacturer states this is the minimum speed until 50' to ensure controllability of the aircraft in the event of an EFATO.

Someone will correct me if I'm wrong (DJPil?), but I'm pretty sure that take-off safety speed is a uniquely Australian manifestation probably going back to the old DCA. It used to be in the old uniquely Australian performance charts, but I' can't recall ever seeing it on a manufacturers chart.

The bit about "The manufacturer states..." is wrong. The take-off safety speed is defined as 1.2 x stall speed. Co-incidently, 1.2 x stall speed is used in FAR 23 as a performance hurdle, but its not intended to be a target speed used by the pilot, its more of a certification protocol.

See here:

eCFR ? Code of Federal Regulations

I'm with John Deakins that for relatively high performance aircraft like the Bonanza that vx & vy have very little practical relevance, I'd disagree with low powered aircraft.

Take off in a C150 (yes C150, not C152) from Bang Pra airport on a hot day in Thailand with high tension wires in front of you and I can guarantee the only speed you want to know is Vy.

But, take-off in pretty much anything retractable and its Carson speeds that you want to understand.

[43Inches]

Someone will correct me if I'm wrong (DJPil?), but I'm pretty sure that take-off safety speed is a uniquely Australian manifestation probably going back to the old DCA. It used to be in the old uniquely Australian performance charts, but I' can't recall ever seeing it on a manufacturers chart.

Pretty much how I understand it.

Take Off Safety Speed? The manufacturer states this is the minimum speed until 50' to ensure controllability of the aircraft in the event of an EFATO.

The lift off and 50 foot speeds are purely performance related, ie, you must attain these speeds to achieve performance.

The only mention of a safe speed with regard engine failure is usually found in section 10-Operating tips, of a Piper manual. For instance the the Arrow III states, Section 10-10.3 (b); "The best speed for take-off is about 70 KIAS under normal conditions. Trying to pull the airplane off the ground at too low an airspeed decreases the controllability of the airplane in the event of engine failure"

Considering the lift off speed flap 25 on the chart is max weight 59 KIAS (62 KIAS at the barrier) and the stall speed is calculated at around 55 KIAS full flap at max weight, this is not 1.2 x the stall.

[CaptainInsaneO]

Jab: Perhaps in the morning you will read it again. And realise what you think you read was not actually what was written.

You're probably right there.

I didn't blow the dust of my text books last night when I read the story. A quick look at it and I could see holes in it - if you consider Vx and Vy useless speeds to all aircraft that is.

As for PSQ though, I wasn't there but I think it's a bit harsh to say that 'no way was he climbing at Vy'.... I've flown Caravans where some don't climb nearly as well as others, especially if that engine had a higher than normal ITT. It might of also have been an SOP thing to climb out at a certain ITT, he might of backed off the Tq and relied on Vy to get him up there as quickly as possible - I don't know though, I wasn't there.

I don't mean to be so critical of somebody's story, yes some theories might hold true to a particular aircraft, but I would hate for a student pilot or fresh PPL/CPL pilot to believe that Vx and Vy are useless to the aircraft they fly.

My point is: just take it with some salt. Just because 95% of Ppruners think it's true - doesn't mean it is true! This story does not prove that Vy and Vx are useless air speeds as this thread title suggests.

If people are reading the story, then reading their POH and flight manual to find the truth for their aircraft - well then I'm a fan of the story and I'm glad I stirred people up a bit.

Happy researching everybody and safe flying!

[john_tullamarine]

Most manufacturers POH for light aircraft refer to lift-off speed and 50 foot barrier speed

Probably because most manufacturers endeavour not to go too far beyond what the design standards require ... Also it is reasonable to presume that OEM paperwork is structured with a very close eye on liability ..

I'm pretty sure that take-off safety speed is a uniquely Australian manifestation

Probably not quite the case. 1.2Vs for singles has long been held as a reasonable balance between too slow and too fast in the event of a failure. The US regs reflect this. I expect djpil will endorse my view ...

Probably reasonable to opine that Oz has, for many years, used the Vtoss abbreviation as a convenient shorthand.

this is not 1.2 x the stall

Be careful of PEC. The 1.2 is not applied to IAS. As PECs can get more interesting in the near stall regime, this is a common error amongst the pilot fraternity. Indeed, we proably have all flown Mr Cessna's fine products and enjoyed the observation of "zero" IAS with the nose pulled way up above the horizon .. doesn't mean anything useful though ... and most certainly doesn't indicate that the aircraft is traveling along with no airspeed.

[43Inches]

Don't have access to the flight test data. The Piper POH I was quoting refers to IAS for stall speed calculation, max weight only. The calibration chart lists negligible change for flaps, possibly 1 kt less calibrated than indicated, and a slight decrease for flap zero at low speed changing to slightly higher at high speed. Although at low speed close to the stall this is probably a little more out than the chart indicates.

If you take the calibrated flap zero stall speed and adjust 1.2 you pretty much get the flap zero lift off speed in this aircraft. 1.2 x 59 = 71

For flap 25 1.2 x the 55 kts listed speed arrives at 66. There would have to be a big difference between CAS and IAS for this to achieve 1.2Vs, lift off being stated at 59 kts.

It makes sense why piper advises 70kias as a more appropriate speed to aim for as it is close to the Vs clean x 1.2.

One thing that's got me thinking of is how much difference is there in accuracy at various flight attitudes and configs of the Piper pitot static head vs Cessna static port and pitot tube.

[Aussie Bob]

In my humble opinion, liftoff speed is another useless speed for most piston singles. Far better to raise the nose wheel (if flying a nose dragger) slightly, keep all eyes outside and let the liftoff happen.

Tis a pleasure when you get it right which will be always with practice. In a tail dragger of course it is even more important to keep all eyes outside and again, from a tail low position liftoff is automatic.

[waren9]

Take off in a C150 (yes C150, not C152) from Bang Pra airport on a hot day in Thailand with high tension wires in front of you and I can guarantee the only speed you want to know is Vy.

eh?


45678910

[triathlon]

Vx and Vy. Useless speeds ?
Deary me.

[djpil]

From FAR 23:

(4) The takeoff safety speed, V2, in terms of calibrated airspeed, must be selected by the applicant so as to allow the gradient of climb required in §23.67 (c)(1) and (c)(2) but must not be less than 1.10 VMC or less than 1.20 VS1.

so definitely not a unique Australian term but perhaps the old Department thought that it was a more meaningful set of words to use in their standardised AFMs rather than V2 or whatever.

Apart from the climb requirements referenced in that paragraph there are one-engine inoperative performance requirements as appropriate. I'm sure that others can give examples of where they need to be able to get the climb performance provided in the AFM and that is only achievable at the right speeds.

Operating a 300 hp Pitts in the Rocky Mountains I didn't need to know Vx or Vy, I'd just point the nose over the mountain and we'd get over it. Different thing in a heavily loaded 172 taking off at 8,000 density altitude - not much rate of climb at Vy. Not going anywhere useful at any slightly higher speed.

[Walter Atkinson]

**In my humble opinion, liftoff speed is another useless speed for most piston singles. Far better to raise the nose wheel (if flying a nose dragger) slightly, keep all eyes outside and let the liftoff happen.

Tis a pleasure when you get it right which will be always with practice. In a tail dragger of course it is even more important to keep all eyes outside and again, from a tail low position liftoff is automatic.**

Aussie Bob has this right. Piston aircraft should be allowed to "fly off" by placing them in the proper attitude, NOT ROTATED off the ground like a jet. If done correctly, the aircraft will simple lift upwards away from the runway in manner almost imperceptible to passengers. "Rotating" a piston is poor form and has dangers associated with it's use.

[Trent 972]

"Rotating" a piston is poor form and has dangers associated with it's use.

Operating off a prepared sealed surface perhaps, but if operating from a 'contaminated', soft unprepared surface, is your advice poor, and has dangers associated with its use?

[Aussie Bob]

Operating off a prepared sealed surface perhaps, but if operating from a 'contaminated', soft unprepared surface,

I don't actually think so Trent, I think it is even more relevant.

Get the nose wheel up early off the poor surface, eyes outside looking for the best takeoff run, let the plane fly, lower the nose if required, finish takeoff in the air.

[Trent 972]

Thanks for that Bob, just one question though.
Are you able to explain the difference between
1. not rotating - "Get the nose wheel up early off the poor surface….. let the plane fly..." (yourself)
2. not rotating - "by placing them in the proper attitude…..the aircraft will simple lift upwards away from the runway..." (Walter)
(Full quotes are found in the posts above)
.. and the general theme of 'not rotating' a S/E piston on take-off?
Also remember to be specific to cover every eventuality or otherwise acknowledge when the advocated technique is not applicable.

[Mach E Avelli]

I hear you Aussie Bob. Bugsmashers are bugsmashers, not jets.
By Vr, a transport category aircraft is already committed to continued flight. As opposed to merely breaking the surly bonds of earth at some minimum speed determined by the airframe's performance on the day and accelerating to a safe margin above the stall while in ground effect and before attempting to climb. With no commitment to continued flight possible.
As for those who say that using airstrips which require short field techniques are pushing their luck, what about those who fly high performance singles over built up areas? If their engine quit, could they really get into that park or football field?
40 knots or less is a very desirable stall speed for any single.

[43Inches]

**In my humble opinion, liftoff speed is another useless speed for most piston singles. Far better to raise the nose wheel (if flying a nose dragger) slightly, keep all eyes outside and let the liftoff happen.

Tis a pleasure when you get it right which will be always with practice. In a tail dragger of course it is even more important to keep all eyes outside and again, from a tail low position liftoff is automatic.**

Lift off speed is not a speed to aim for lift off to "pull" the aircraft off the ground. If you read the recommended take off procedure it says to accelerate to around this speed and set take-off attitude. All things calculated correctly and the aircraft should become airborne at close to this speed.

There are three types of take-off technique usually mentioned in the POH, normal, short field with obstacle and soft field. Soft field raising the nose at the right point is important, rotate too early and the mains will be pushed further into the muck by the downward tail force. Raising the nose too early on the other forms of take-off will increase aerodynamic drag at a point where the aircraft is not able to fly yet and will consume more runway then necessary. The lift off speed gives you a rough rotate speed to achieve this.

[Aussie Bob]

1. not rotating - "Get the nose wheel up early off the poor surface….. let the plane fly..." (yourself)

In effect raising the nose wheel is rotating I guess but not in the conventional sense. I would open the throttle, apply up elevator as required to lift the nose wheel, probably to the full extension of the oleo and wait. Ease off back pressure as required during acceleration to maintain this attitude. Lift off happens in a most pleasing manner .

Also remember to be specific to cover every eventuality or otherwise acknowledge when the advocated technique is not applicable.

In the (land) singles I fly, this is the only technique I ever use anywhere ever. Not applicable for seaplanes!

A brilliant book on the subject and highly readable is "Take Offs and Landings" by Leighton Collins. I got my copy from the Kindle Store onto my Ipad.

[Aussie Bob]

Raising the nose too early on the other forms of take-off will increase aerodynamic drag at a point where the aircraft is not able to fly yet and will consume more runway then necessary.

You have to raise it pretty high to get a substantial drag penalty. In the POH for my tail dragger the recommended technique is to take off from the three point attitude. It may be marginally shorter raising the tail in the takeoff run but not much. The three point attitude is very close to the stall. IMHO raising the tail in this aircraft is a better technique than the POH recommendation.

[Walter Atkinson]

It may be worthwhile to note that from The Wright Brothers to the advent of jets, the term "rotate" was not used in flying. How do you reckon we managed to fly for over 50 years without ever "rotating" an airplane on takeoff? <g>

[Trent 972]

Here's a little bit of a hint for you.
"Rotate" is just another way of saying "pull the stick back".
How did you ever manage 50 years of aviation without "pulling the stick back".
It's not like you've discovered a new form of aviation.
As for letting it roll down the runway at full noise without ever having to pull the stick back, maybe, maybe not, but will it achieve the POH numbers with regards to take-off distance required in a limiting situation, probably not.

Challenge for you, next time you're here, visit your good mate Jaba at Caboolture and go for a spin in his mighty RV10. Give yourself min book runway distance figures for takeoff, don't touch the stick, and tell me at what point everybody's @rse starts to pucker.