Tx all
 

Long time i've been here. First of all tx for your answers. But in the mean time i decided to quit with aviation. I'm moved back to my home country and found there another regular job (9 to 5) and having back a normal social life. Anyway, i stick around here because i like to keep on learning.

But surely with your backward logic you would want to teach them to be aerodynamicists first!

.....:ugh:

Vx, Vy
 

...aaaactualy, it's quite simple:
climb at a constant airspeed, lets say the advertised Vx for your A/C at a given altitude, climb for 1,000' noting the elapsed time to gain the 1,000'
Now climb at a constant airspeed. lets say the advertised A/C's Vy speed for your A/C at a given altitude (and the same starting altitude you started at for Vx), climb for 1,000' noting the elapsed time.

You now have 4 numbers; the Vx IAS, the Vx time to gain 1,000', the Vy IAS and its time to gain 1,000'
correct the IAS to CAS then into TAS for the temp and pressure altitude for both Vx & Vy then convert the Kts or mph as the case may be into fpm.
Take the recorded times (in minutes) and divide them into the 1,000', yielding the climb rate for each of Vx & Vy
Noe take the ARCSIN (or use Mr. Pythogoras's Theorem if you prefer) of the climb rates divided by their respective TAS in fpm.
The resulting 2 angles will be the angle of climb for Vx and Vy.

You will find the Vx angle to be greater than the Vy angle.
The reason for that is there was a trade off between Vx's lower airspeed than Vy's shortening its horizontal (ground) distance traveled while not reducing it's climb rate proportionally.

This is of course performed in a homogeneous, stable inertial air mass and totally divorced from earth, short of gravity.

Some of the confusion over this is when attempting to clear an obstacle the pilot is totally focused on the obstacle and therefore in a subjective earth relative reference frame while the A/C remains in that air mass reference frame although close to an obstacle/ground the air mass is probably not an inertial one any more.

...aaaactually maybe this is not so simple.

What is confounding thou is none of the POHs give the climb rate at Vx, and none give the ground roll and total distance to clear a 50' obstacle at the Vy speed, probably because that data is not specified to be included in the GAMA Specification No.1 for POHs.
So angle of climb for Vx and Vy cannnot be obtained from POHs directly, it has to be derived.

When it is the Vy climb angle is always greater than the Vx climb angle.

There, I hope I've cleared all that up.

Look at some thrust horsepower graphs.

Tangential line through origin is best angle climb speed. Highest point is best climb speed.

Speed is true airspeed.

Thing is... its all a bit of certification nonsense isnt it? I cant imagine anyone ever sits at the threshold with an obstacle ahead that they will clear if they fly at Vx but hit if they fly at Vy and then goes ahead with the take off. That would be taking fine margins to a ridiculous extreme!

Right!

For a representative example of ALL POHs consider;

The "805' of ground roll and 1440' Total distance to clear a 50' obstacle" (distances interpolated to 15 degrees C from the '78 172N POH specifying Vx TO distances at 10 then 20 degrees C) using a "Lift Off" speed of 52 KIAS and speed at 50' of 59 KIAS.

1) I have difficulty eye balling correctly 750' vs 1,000'. Am I supposed to go out and tape of the distances to the nearest foot??
2) If you subtract the ground roll distance from the total distance you would think you had the distance from "Lift Off" to the obstacle, but if you take the arc Tangent of the 50' divided by that distance you find the angle is always LESS than the Vy arc SINE angle of the Vy climb rate over the Vy KCAS - so Vy has a steeper flight path angle than Vx??
3) Of course not, so ALL POHs leave out the technique supposedly used to clear 50', most likely: lift off at just above power on stall (which incidentally is not specified in any POH), stay in ground effect until reaching 52 KIAS, then pull up to 59 KIAS.
4) If by some miracle you actually duplicate whatever the technique was to clear 50' (assuming there actually was one) do the wheels roll over the top of the 50' obstacle? or is there some unstated safety factor inherent but unstated in the numbers, e.g. is it really 60', or maybe 55' ?

There are many other omissions and commissions in POHs, this is just one glaring example of ; "If you want to know the actual performance of that commercially manufactured airplane, plan on spending 20 to 50 hours flight testing it yourself."

Still needs to be taught and tested, in case it's ever needed. If you know the pitchure (spelling error intended) you can set it and get Vx in a flash...............

Wrong way around?

What a load of crap. I hope no early PPL or student is reading this thread and thinking any of it matters. Heres a top tip - if you ever find yourself thinking that flying a couple of knots slower will mean that the certain crash you are about to have will now become a close, but somehow heroic, shave you honestly need to get out of the plane and go and have a beer to make sure you dont get back in the plane for a bit. Its a load of flight testing nonsense. Let me explain how ridiculously stupid it all it. Nobody ever bothers to wind the gradient. In a rubber band powered single, the wind component will very significantly improve or degrade your climb gradient and it will do it by orders of magnitude more than messing about with Vx and Vy. Forget all about it because it just doesnt matter.

Where's Genghis when we need him?

Pick an average single. Do the sums climbing still air to 1000 feet at Vx and then at Vy. I bet you find that you reach 1000 feet within 10 or 20 seconds of each other and I bet the difference over the ground is a few hundred feet. For both of those climbs you have been hammering your engine with the highest possible power demand with poor cooling due to the low airflow which significantly increases your chance of an engine failure. You are operating the prop in a speed range where it is less efficient and the engine won’t make full power anyway because the RPM will be lower. So you are a dillon if you do either of them. I totally disagree with Mr Average above - it doesnt need demonstrating and is of no use at all when you go flying. Its interesting and its a good idea to understand it so go and watch a video and spend your money in the aeroplane learning useful stuff. Learn some simple aerobatics, become really confident at crosswind landings, get really proficient at flying on the AH maybe, there are loads of things you can do in the plane that make your flight safer. I have an image of Alex W with his head in his hands saying, “Other way round, numbnuts”

Whether you agree or not is insignificant. It is a requirement to teach it and to test it. You'll need to convince the CAA of your opinion in order to remove the requirement.

Well I am more than happy to take it up with them and I will let you chaps know the result. Incidentally, when have you ever selected a Vx or a Vy climb other than to demonstrate them? What departure have you ever undertaken which required their use? (I say you, its the royal you and open to the group, obviously). When you demo it do you discuss with the student when you would use it? Because I would venture to suggest that it would be highly irresponsible to suggest that using a Vx climb in order to clear obstacles is a good idea. I would think that if a departure is that limiting you should get out of the plane and do something else for the afternoon..

Also, do you actively calculate the effect of wind on the gradient and if not, whats the point of messing about with flying such a limiting departure anyway? My co-pilot looked at me like I was from mars when I showed him my calculations for a departure a few weeks ago. I explained that although APG said we could make the SID gradient we actually couldnt because we were taking off with a tailwind and the tailwind increased with height which would flatten our gradient. He asked why I had done that when there is no requirement. The gradient is predicated on missing the mountains..

GQ, I beg to differ.

Say you need to climb to FL430 before the FIR boundary (200nm). Are you telling me it doesn't matter if I elect to choose best rate (say 300/M0.8) or best angle (Vfto)? Okay, not doing this in a single piston, but rest assured that those in the know understand and do have a use for max angle of climb!

If you chose to climb at Vfto you would be a complete dillon. Vfto is min clean after slat retraction. Other than fourth segment following an engine failure I would never climb at Vfto and even in hat scenario I would probably climb at a higher speed unless I was gradient limited - what would be the point? I doubt 300/M0.8 best rate either - thats almost certainly a cruise climb or high speed climb schedule and nothing to do with Vy. I commonly use 300/M0.8 or 0.82 but if I was trying to climb more quickly i may slow to 250 and use 78 on conversion but the calculations are not easy as it involves he mass of the aircraft, ISA, and the amount of power you get changes a bit with airspeed. So the last time I needed to get to FL450 in a hurry, which was for a NAT entry point out of Goose, I requested an orbit rather than fanny about trying to zoom climb just to get up more quickly. If you try to get too high a climb rate and the airspeed is reducing, the ITT rises and you arent doing the engines any good and I dont really want to cane a few million dollars worth of engines simply to reduce the track miles to an altitude.

Hmmm, well if you can't manage it without increasing the air-track you could do an orbit, but then if you've also wrongly elected to to go for the higher climb speed your turn is going to be fairly large, even more at altitude as the TAS increases. Therefore your rate of climb will suffer for a significant period. Of course the orbit can cause ATC a headache; not fair to give them half of my problem. Close but no cigar, when you can climb at a better speed in the first place.

Now, as for Vfto being for dillon's, you mention something about it only ever being of use for a 4th segment climb. I wonder how they came up with the Vfto recommended speed? Best rate?? Cruise climb??? Nope, its approx min drag and therefore best angle. Thats the whole point of Perf A OEI using a V2/Vfto climb, its best angle (gradient limited as you put it) to avoid the granite! So in my scenario, Vfto is just what you need when you want to achieve, er, best angle!

I think you're confusing yourself GQ. You refer to getting to FL450 in a hurry, = time and therefore best-rate is what you need. My scenario was clearly stated as limited ground-track therefore has to be solved with best angle.

Best rate in the Global is achieved somewhere around max turb penetration speed but I hardly ever do that. I recently climbed to FL450 in 19 mins. Vfto is more likely to be best L/D than min drag but the engines dont produce maximum power at 160-190 kts. Even when you are OEI you still may get a better rate at a higher speed because he remaining engine produces a bit more power and you use less rudder so there is less drag. There are no climb figures for it, however, because here is no reason for a manufacturer to certify it. I am a current Glex and 6000 training captain and have flown the aircraft all over the world. I have had plenty of time to sit and look at the AFM and CCM and am pretty up to speed with what a 50 tonne swept wing jet aircraft can do. But hey, if you want to climb your global at Vfto, go for it....

GQ. My first post on this thread was to counter your statement that Vy & Vx and anything to do with them was "crap" and meaningless; I maintain you're wrong.

My scenario was based entirely on needing best angle for a climb, and that by using an appropriate min-drag speed (effectively Vx in a jet) I could achieve my goal. Same scenario, but flying at any other speed used a longer ground track. My scenario was extreme; any noteable tailwind during the 25 minute climb had an adverse effect.

You keep referring to rate and power - if you knew and understood the theory you'd know that these are not relevant to best angle climb, they are for best rate.

You state:
So you acknowledge a speed/configuration that will give you a greater RoC than flying at Vfto; correct, Vfto is best angle - fly faster and you probably will get a better rate, but not the correct procedure when terrain is still an issue.

If you are indeed a Global Express expert I doubt you've ever looked and studied the Vfto graph. Vfto is not best L/D, it is approximately min-drag! Ask your instructor on your next Recurrent to explain why you must climb at V2/Vfto to get above safety alt when OEI.

If you've time to spare in your Global in the cruise why not refresh yourself on the theory and hence differences between climbing at Vx and Vy, you may well learn something that you presently don't appear to understand. And if you choose not to believe they are relevant, then kindly refrain from telling other aviators to ignore them until you demonstrate that you fully understand the theory.

Climbing at V1.35+10 in the VC10K at Incirlik meant that we could make the correct level for the K-town transit corridor quite easily - whereas climbing at SOP best rate speed meant we couldn't. Not that many crews bothered - they were happy to ignore the Turkish limit...:rolleyes:

At Abingdon (before it was squaddified), on RW 36 if you taxyed as close to the RW threshold as possible, then took off and climbed at best angle, you were reasonably well placed in the event of EFATO as the fields at the end of the RW ran east-west. If you climbed at best rate, you would be too low to position for a field - but at best angle you could either jink round to RW 08 or make a descending steep turn to position for a field.

There are good reasons to climb at best angle, but to do so requires thought and quite careful handling.