Two useless speeds
Thread Starter
Two useless speeds
How long are instructors, text books and know all's going to keep raving on about two useless speeds (Vx & Vy) often expected to be memorised?
Great article here!
Tree leaves in the undercarriage does not mean a lack of understanding of these speeds!
Great article here!
Tree leaves in the undercarriage does not mean a lack of understanding of these speeds!
They could be irrelevant - even dangerous - speeds to be flying close to the ground in a single. But Vxse and Vyse in a twin need to be known, and flown when appropriate.
The old provocative article title trick ... the question shouldn't be whether Vx and Vy are 'useless' or not, but when might it be a good idea to use them and when otherwise.
Mmmm ... tricky ... if I'm at one altitude and I need to climb to another quickly, what speed should I use? Not that bloody Vy, it's useless!
Mmmm ... tricky ... if I'm at one altitude and I need to climb to another quickly, what speed should I use? Not that bloody Vy, it's useless!
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Pretty big call to state Vx and Vy are irrelevant unless you are happy to drag your gear through the trees one day (or worse still power lines), Im sure Cessna, Piper et al are just wasting their time working out these speeds.
Why not have these speeds filed away in a handy document in the cockpit that can be revised on the day or quickly referred to if needed- they might just save your life. The author may have 5000+ hours of bush flying but if he thinks these speeds are crap he has never really had to do any serious bush flying.
As an aside re the cylinder head temps being increased (refer article in link) Vx is usually used to to deal with obstacles so will only be flown for a very short period of time (at least in my experience in "the bush") so CHT will not be a factor.
The problem with these sort of articles in these forums is that student and inexperienced pilots take this sort rubbish as gospel without thinking.
Why not have these speeds filed away in a handy document in the cockpit that can be revised on the day or quickly referred to if needed- they might just save your life. The author may have 5000+ hours of bush flying but if he thinks these speeds are crap he has never really had to do any serious bush flying.
As an aside re the cylinder head temps being increased (refer article in link) Vx is usually used to to deal with obstacles so will only be flown for a very short period of time (at least in my experience in "the bush") so CHT will not be a factor.
The problem with these sort of articles in these forums is that student and inexperienced pilots take this sort rubbish as gospel without thinking.
Last edited by Nunc; 2nd Apr 2014 at 21:42. Reason: spelling
Saying "could be" is not the same as saying "are". Think about why I would have used the conditional.
In general I do agree with him, but at the little 400 metre strip where I have based the bugsmasher there are power lines almost at the thresholds. Of course I have to fly best angle..... but only until the wheels have cleared the wires.
If the engine quit before then it would be seriously bad, but I would be trying to crash under the wires rather than through them.
The author's point - if I read it right - is that flying Vx or Vy very close to the ground when it is not necessary is taking an unjustified risk and of little relevance for climbing en route.
In general I do agree with him, but at the little 400 metre strip where I have based the bugsmasher there are power lines almost at the thresholds. Of course I have to fly best angle..... but only until the wheels have cleared the wires.
If the engine quit before then it would be seriously bad, but I would be trying to crash under the wires rather than through them.
The author's point - if I read it right - is that flying Vx or Vy very close to the ground when it is not necessary is taking an unjustified risk and of little relevance for climbing en route.
Its as simple as what triathlon said. Instructors are only following suite.
Imagine going into an instructor rating test/renewal with CASA and putting your own theories on the board... There would be only one result.
I reckon if an engine failed on climb out in a single (for the average pilot), close to the ground, it wouldn't matter what speed you were doing, the result would have a lot to do with luck as it would skill. There are so many external factors.
Imagine going into an instructor rating test/renewal with CASA and putting your own theories on the board... There would be only one result.
I reckon if an engine failed on climb out in a single (for the average pilot), close to the ground, it wouldn't matter what speed you were doing, the result would have a lot to do with luck as it would skill. There are so many external factors.
Its good to see John Deakins back writing again. I've missed his thought provoking articles.
My criticism of many of John''s articles is that they are about 90% correct and he fudges a bit which can be misleading. I also wish he would move on from red boxes.
For high performance aircraft, I agree completely that that Vx, Vy should never be used. However, if the engine stops and you need to glide. an understanding of which are the efficient speeds for the airframe is pretty important.
For high performance aircraft the work that Bernard Carson (ex US Naval aerodynamics professor) did on fuel efficiency and the "Carson Speed" is a much more valuable concept.
Peter Garrison (Flying Magazine) did a summary of this in 2007. See here:
http://www.flyingmag.com/very-best-speed-fly
The cafe foundation has one of his original 1980 papers (not a red box anywhere) on their website:
http://cafefoundation.org/v2/pdf_tec...B.H.Carson.pdf
A very nice summary is in the Aerostar magazine:
http://www.openclip.net/Benchmark/AO...rSweetSpot.pdf
Carson wrote a number of papers and you can find them all in the proceedings of the AIAA conferences. Some of them are hard work, but for high powered aircraft and extended climbs to altitude, Carson is THE authority.
In a light twin, I want to get above blue line, cleaned up with enough altitude to give a safety factor as fast as I can, although this varies a bit with runway length and terrain. For 3 or 4 minutes the thermal mass of the engine and cowl flaps will protect the cylinder heads. After that, its all about Carson speed & leaning according to the Deakins bible.
In a low performance aircraft (especially an underpowered antique type) flying to Vx or Vy accurately might mean the difference between climbing or not climbing. Those of us who did multi training in aircraft like Travelairs know that a couple of knots off Vy will mean the difference between climbing and not climbing. So, Vx & Vy are important concepts that must be understood.
My criticism of many of John''s articles is that they are about 90% correct and he fudges a bit which can be misleading. I also wish he would move on from red boxes.
For high performance aircraft, I agree completely that that Vx, Vy should never be used. However, if the engine stops and you need to glide. an understanding of which are the efficient speeds for the airframe is pretty important.
For high performance aircraft the work that Bernard Carson (ex US Naval aerodynamics professor) did on fuel efficiency and the "Carson Speed" is a much more valuable concept.
Peter Garrison (Flying Magazine) did a summary of this in 2007. See here:
http://www.flyingmag.com/very-best-speed-fly
The cafe foundation has one of his original 1980 papers (not a red box anywhere) on their website:
http://cafefoundation.org/v2/pdf_tec...B.H.Carson.pdf
A very nice summary is in the Aerostar magazine:
http://www.openclip.net/Benchmark/AO...rSweetSpot.pdf
Carson wrote a number of papers and you can find them all in the proceedings of the AIAA conferences. Some of them are hard work, but for high powered aircraft and extended climbs to altitude, Carson is THE authority.
In a light twin, I want to get above blue line, cleaned up with enough altitude to give a safety factor as fast as I can, although this varies a bit with runway length and terrain. For 3 or 4 minutes the thermal mass of the engine and cowl flaps will protect the cylinder heads. After that, its all about Carson speed & leaning according to the Deakins bible.
In a low performance aircraft (especially an underpowered antique type) flying to Vx or Vy accurately might mean the difference between climbing or not climbing. Those of us who did multi training in aircraft like Travelairs know that a couple of knots off Vy will mean the difference between climbing and not climbing. So, Vx & Vy are important concepts that must be understood.
I wouldn't say Vx and Vy are useless, they give information to the pilot regarding the maximum performance of the aircraft. The pilot must know the points at which the aircraft will achieve best rate/angle of climb so you can manage your climb appropriately. This does not mean you have to use them though.
Should Vy and Vx be adopted on a take-off regularly?
I think what is being pushed is that if you are having to use Vx on take-off maybe you are cutting it too fine.
Cylinder heads aside, in many high performance aircraft Vx especially will be at a very high nose attitude, and usually very little stall margin. As the article states if an engine should fail at low altitude in this configuration an aerobatic manuevre would be required to successfully land, and that is still most likely with substantial damage. Read the thread on the Arrow accident at Orange. Considering you were doing this to avoid an obstacle, you probably just went through it as well. Flying at a higher speed you will have more time to adjust to glide, more visibility to avoid obstacles and find a suitable landing zone.
Where is Vy useful?
As already stated most light twins need to be flown accurately at this speed following an engine failure to get any climb performance. Climbing below blue line in a light twin means you will have to trade altitude for speed in most pistons at high weight following a failure.
If you are trying to get to absolute ceiling, possibly for maximum range.
If you just want to get to altitude quickly.
Where is Vx useful?
Avoiding an obstacle is about it. Why are you pointing at it? This is a speed to be flown if you have stuffed up, ie put yourself in a situation that you are trying to get out of. You use it after a terrain warning, or you suddenly realise you are in the wrong place and need to climb back to a safe altitude without hitting something. If a strip is so short with an obstacle that requires a Vx climb to negotiate then why are you there?
This discussion is very different when applied to transport category aircraft though.
Should Vy and Vx be adopted on a take-off regularly?
I think what is being pushed is that if you are having to use Vx on take-off maybe you are cutting it too fine.
Cylinder heads aside, in many high performance aircraft Vx especially will be at a very high nose attitude, and usually very little stall margin. As the article states if an engine should fail at low altitude in this configuration an aerobatic manuevre would be required to successfully land, and that is still most likely with substantial damage. Read the thread on the Arrow accident at Orange. Considering you were doing this to avoid an obstacle, you probably just went through it as well. Flying at a higher speed you will have more time to adjust to glide, more visibility to avoid obstacles and find a suitable landing zone.
Where is Vy useful?
As already stated most light twins need to be flown accurately at this speed following an engine failure to get any climb performance. Climbing below blue line in a light twin means you will have to trade altitude for speed in most pistons at high weight following a failure.
If you are trying to get to absolute ceiling, possibly for maximum range.
If you just want to get to altitude quickly.
Where is Vx useful?
Avoiding an obstacle is about it. Why are you pointing at it? This is a speed to be flown if you have stuffed up, ie put yourself in a situation that you are trying to get out of. You use it after a terrain warning, or you suddenly realise you are in the wrong place and need to climb back to a safe altitude without hitting something. If a strip is so short with an obstacle that requires a Vx climb to negotiate then why are you there?
This discussion is very different when applied to transport category aircraft though.
There are Carson speeds for most efficient cruise and most efficient climb. I have all his original papers on file, but in an effort to find hyper-links quoted only the cruise related papers.
In an effort to find the climb articles, I've just found this recent article specifically on efficient climb. The author has done a really nice job of this and its worth reading.
http://www.openclip.net/Benchmark/IntroducingVz.pdf
The author is proposing a new speed (Vz) for the optimal climb speed.
In an effort to find the climb articles, I've just found this recent article specifically on efficient climb. The author has done a really nice job of this and its worth reading.
http://www.openclip.net/Benchmark/IntroducingVz.pdf
The author is proposing a new speed (Vz) for the optimal climb speed.
Thread Starter
Imagine going into an instructor rating test/renewal with CASA and putting your own theories on the board... There would be only one result.
But, not that bad, the FOI's I have dealt with with would be approachable on differing methods so long as you knew your stuff 100%. Its only a blackboard brief for the purpose of demonstration.Now picture this, you are on an airstrip in a single with trees that require exactly the precise angle and all that. You paced it out. My question is what are you doing there in the first place? You are a crash waiting to happen.
Worse still, you wait until Vx appears on the ASI before rotating. This is sadly what I was taught.
All good stimulating inputs here but I'd like to think that any pilot whom might be considering using either speeds for a T/off might look at the whole picture not just some number/s written in the POH. There's lots of variables here to also think about b4 launching off a very short strip, old machine, prop less than efficient, same goes for the donk, instrument inaccuracies, bloody nasty turb where the speedo is all over the place anyway, pilot skills & apart from the surface & wx conditions what about an escape path if it all goes tits up? Don't just assume that holding such a book figure will save you!
Not much point having the machine pointed towards the moon in order to clear the tall pine trees at the end when the donk fails & you fall from grace into the trees anyway.
Big picture stuff, not many of us are test pilots in nice new machines
Wmk2
Not much point having the machine pointed towards the moon in order to clear the tall pine trees at the end when the donk fails & you fall from grace into the trees anyway.
Big picture stuff, not many of us are test pilots in nice new machines
Wmk2
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Some of you may be familiar with an old distinguished FOI of the Newcastle area who I had the pleasure of doing several instructor rating tests and renewals with. He was very much of the school of flying what 'felt about right' and was not big on flying published 'certification' figures. This would inevitably launch into a long conversation about the difference between certification flying and operational flying. While this approach and the associated apparent uselessness of Vx and Vy might be applicable to a lot of pilots and their Australian experience, there is something about this discussion that gets under my skin. Just because you can't see use for it doesn't mean it doesn't have an application somewhere, somehow to some aircraft. Breeding an attitude in lesser experienced pilots of 'I know better' could also result in inefficiencies at best, or worse, putting themselves in danger.
Perhaps the best argument would be for me to explain by way of example times when these speeds ARE useful both 'low to the ground' (not necessarily the same as 'low level') and at higher altitudes. Flying a fully loaded piston single in PNG there were many times I did fly Vx climbing with within confines of terrain and positioning to cross ridges. Granted, I sometimes had the luxury of JPI instruments and the timeframe and temperatures were monitored. It may not have been flown for the entire climb but in cases where performance is critical, this speed IS important. For example, Cessna even publish two Vx speeds for the TU206 - one at SL and one for 10,000ft - why? They are six knots apart and it makes a difference. They don't make up these numbers just to fill manuals with 'useless' information!
If you want to talk high performance aircraft it becomes perhaps more important, not less. Take for example a single engine turboprop. ATC asks you for 'max rate' climb through 10,000ft. What speed are you going to fly? In one SE TP that I know there are at least 3 Vy speeds published depending on altitude. The speeds can easily vary by 10 knots between SL and 20'000ft. Does it make a difference? You bet! Now what about when you are sitting in cruise and start encountering moderate/severe icing and you need to climb straight away to get above a layer of cloud (if you don't act fast you will lose the ability to climb at all). What speed are you going to fly? The one I would hope you pick would be the one the manufacturer gives you as being the appropriate speed for that altitude that also gives you stall protection in icing conditions!
I say again, manufacturers do not write this 'useless' stuff just to fill manuals - ignore at your peril.
Perhaps the best argument would be for me to explain by way of example times when these speeds ARE useful both 'low to the ground' (not necessarily the same as 'low level') and at higher altitudes. Flying a fully loaded piston single in PNG there were many times I did fly Vx climbing with within confines of terrain and positioning to cross ridges. Granted, I sometimes had the luxury of JPI instruments and the timeframe and temperatures were monitored. It may not have been flown for the entire climb but in cases where performance is critical, this speed IS important. For example, Cessna even publish two Vx speeds for the TU206 - one at SL and one for 10,000ft - why? They are six knots apart and it makes a difference. They don't make up these numbers just to fill manuals with 'useless' information!
If you want to talk high performance aircraft it becomes perhaps more important, not less. Take for example a single engine turboprop. ATC asks you for 'max rate' climb through 10,000ft. What speed are you going to fly? In one SE TP that I know there are at least 3 Vy speeds published depending on altitude. The speeds can easily vary by 10 knots between SL and 20'000ft. Does it make a difference? You bet! Now what about when you are sitting in cruise and start encountering moderate/severe icing and you need to climb straight away to get above a layer of cloud (if you don't act fast you will lose the ability to climb at all). What speed are you going to fly? The one I would hope you pick would be the one the manufacturer gives you as being the appropriate speed for that altitude that also gives you stall protection in icing conditions!
I say again, manufacturers do not write this 'useless' stuff just to fill manuals - ignore at your peril.
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Some interesting points raised, but a little common sense could be put into play here.
1) If you are operating an aircraft into and out of a strip that a few knots is so critical that you may or may not collect the terrain, trees or obstacles at the end without being familiar with that aircrafts performance... then you are a much braver man than I.
2) Practical applications of Vx and Vy are used often as some have already alluded to. Climbing towards terrain (And where I am right now, there is some serious terrain that needs to be cleared which leads to sustained Vx climbs), climbing through weather (My current Vy gives around 35-40kts protection between it and stall), or when being asked by ATC to give best rate through to a certain altitude.
Perhaps either of the two could be used for climbing to MSA before entering IMC as well.
3) I'm sure most of the pilots on this forum have the ability to adjust the speeds a little to give that particular aircrafts Vx/Vy due to its age/wear and tear/general straightness of the airframe.
1) If you are operating an aircraft into and out of a strip that a few knots is so critical that you may or may not collect the terrain, trees or obstacles at the end without being familiar with that aircrafts performance... then you are a much braver man than I.
2) Practical applications of Vx and Vy are used often as some have already alluded to. Climbing towards terrain (And where I am right now, there is some serious terrain that needs to be cleared which leads to sustained Vx climbs), climbing through weather (My current Vy gives around 35-40kts protection between it and stall), or when being asked by ATC to give best rate through to a certain altitude.
Perhaps either of the two could be used for climbing to MSA before entering IMC as well.
3) I'm sure most of the pilots on this forum have the ability to adjust the speeds a little to give that particular aircrafts Vx/Vy due to its age/wear and tear/general straightness of the airframe.
Hey Jabba took ya a while to front up here buddy, careful though don't add any material that could be construed as free advertising
It would be unusual for ATC to say "max ROC" to a particular Alt more like Best ROC & that 'rate' will be at the discretion of the commander not what some book figure says due all sorts of reasons.
Wmk2
It would be unusual for ATC to say "max ROC" to a particular Alt more like Best ROC & that 'rate' will be at the discretion of the commander not what some book figure says due all sorts of reasons.
Wmk2
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What a load of crap!
I hope no one takes this guy seriously - it's pub talk.
"I went and did 3 flights with a doctor (to help me record the time and draw 3 graphs) and I have proved all the engineers and test pilots in Cessna and Beechcraft wrong" - seriously??
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.
For aircraft where the Vx is lower than Vtoss, the manufacturer states you should fly at Vtoss, then adopt Vx for obstacle clearance. Maybe this bloke should just read his POH.
Vy is there for a reason in singles, maximum height in minimum time. If you have an engine failure after 5 minutes, you got as high as you possibly could to give you maximum gliding range and maximum gliding time.
Has anyone got a link to the report about the Caravan out of Townsville that had that happen to him? VH-PSQ I think it was. I'm sure he was glad he had time!
Also if you're engine is getting hot at Vy, get it sorted out by the engineers. Leave your mixture at full rich and if it's still too hot, get them to tweak it so it gives you more fuel when the lever is fully in, lean back once you adopt a cruise climb.
I could go on all night and pick this rubbish to pieces, but I've got better things to do.
There are very good reasons for Vx, Vy and especially Vtoss.
Instructors just don't make it up or teach it because they were taught it. They read the POH and research credible information and don't listen to pub talk...well the good ones do anyway.
Cheers.
"I went and did 3 flights with a doctor (to help me record the time and draw 3 graphs) and I have proved all the engineers and test pilots in Cessna and Beechcraft wrong" - seriously??
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.
For aircraft where the Vx is lower than Vtoss, the manufacturer states you should fly at Vtoss, then adopt Vx for obstacle clearance. Maybe this bloke should just read his POH.
Vy is there for a reason in singles, maximum height in minimum time. If you have an engine failure after 5 minutes, you got as high as you possibly could to give you maximum gliding range and maximum gliding time.
Has anyone got a link to the report about the Caravan out of Townsville that had that happen to him? VH-PSQ I think it was. I'm sure he was glad he had time!
Also if you're engine is getting hot at Vy, get it sorted out by the engineers. Leave your mixture at full rich and if it's still too hot, get them to tweak it so it gives you more fuel when the lever is fully in, lean back once you adopt a cruise climb.
I could go on all night and pick this rubbish to pieces, but I've got better things to do.
There are very good reasons for Vx, Vy and especially Vtoss.
Instructors just don't make it up or teach it because they were taught it. They read the POH and research credible information and don't listen to pub talk...well the good ones do anyway.
Cheers.
Last edited by CaptainInsaneO; 3rd Apr 2014 at 11:52. Reason: Steve Jobs
1) If you are operating an aircraft into and out of a strip that a few knots is so critical that you may or may not collect the terrain, trees or obstacles at the end without being familiar with that aircrafts performance... then you are a much braver man than I.
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Insaneo
Perhaps in the morning you will read it again. And realise what you think you read was not actually what was written.
If you want to be constructively critical, go right ahead, but use a little science with it.
Just your comment regarding engine temps and reading the article shows you need to read again with better comprehension.
As for the C208 out of Townsville, just doing some quick back of beer coaster maths, there is no way he was doing a Vy climb. Had he done so he would have been at about 7000' by 8 miles when you allow for the takeoff on typical runway heading and departure from TL, either visual or SID.
So with all due respect to whatever your flying experience is, and quite likely far more than mine, I think you owe it to yourself to at least read with the view this guy might know some stuff, and do not try to apply it to B777 ops....coz it was not written for that. You might find it contains a few gems
By the way Dr Rogers research work was just a tad more detailed than a few jollies in a 172. I have read the whole research paper for Embry Riddle, so I guess I have an unfair advantage.
Cheers
Perhaps in the morning you will read it again. And realise what you think you read was not actually what was written.
If you want to be constructively critical, go right ahead, but use a little science with it.
Just your comment regarding engine temps and reading the article shows you need to read again with better comprehension.
As for the C208 out of Townsville, just doing some quick back of beer coaster maths, there is no way he was doing a Vy climb. Had he done so he would have been at about 7000' by 8 miles when you allow for the takeoff on typical runway heading and departure from TL, either visual or SID.
So with all due respect to whatever your flying experience is, and quite likely far more than mine, I think you owe it to yourself to at least read with the view this guy might know some stuff, and do not try to apply it to B777 ops....coz it was not written for that. You might find it contains a few gems
By the way Dr Rogers research work was just a tad more detailed than a few jollies in a 172. I have read the whole research paper for Embry Riddle, so I guess I have an unfair advantage.
Cheers
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"I went and did 3 flights with a doctor (to help me record the time and draw 3 graphs) and I have proved all the engineers and test pilots in Cessna and Beechcraft wrong" - seriously??
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.
For aircraft where the Vx is lower than Vtoss, the manufacturer states you should fly at Vtoss, then adopt Vx for obstacle clearance. Maybe this bloke should just read his POH.
Vy is there for a reason in singles, maximum height in minimum time. If you have an engine failure after 5 minutes, you got as high as you possibly could to give you maximum gliding range and maximum gliding time.
Also if you're engine is getting hot at Vy, get it sorted out by the engineers. Leave your mixture at full rich and if it's still too hot, get them to tweak it so it gives you more fuel when the lever is fully in, lean back once you adopt a cruise climb.
I could go on all night and pick this rubbish to pieces, but I've got better things to do.
Instructors just don't make it up or teach it because they were taught it. They read the POH and research credible information and don't listen to pub talk...well the good ones do anyway.
Best...
John Deakin