Vx and Vy

plongson

plongson

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I'm having a hard time getting my head around Vx and Vy. After research and study, I still don't understand why Vx is actually at a lower speed than Vy. My intuition is it should be a faster speed.

Any help understating this is appreciated...
 
plongson said:
I'm having a hard time getting my head around Vx and Vy. After research and study, I still don't understand why Vx is actually at a lower speed than Vy. My intuition is it should be a faster speed.

Any help understating this is appreciated...
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In order to cover the least amount of ground and gain the most altitude, you’re going to have to climb at a greater angle. Therefore the airspeed will be slower than what’s needed to achieve Vy.
 
I think one way to help visualize this is to look at an extreme case. Although not true for the planes that we here mostly fly, some planes have powerful engines that provide thrust larger than the weight of the airplane. In such a case, Vx is somewhat obviously achieved by pitching the airplane up to 90 degrees (straight up, and you don't even need lift then, kind of like a rocket). However, if the engine is just barely able to overcome weight, the airplane may climb very slowly. Climbing vertically at 100 feet per minute is probably not going to get you up to pattern altitude the quickest. In such a situation, you'd be better off lowering the nose to pick up a tremendous amount of speed, even though you are aiming the airplane lower towards the horizon. You'd likely pick up hundreds of knots such that the vertical component of that velocity exceeds 100 feet per minute by a lot. Does that help?
 
Vx is best angle. Vy is best rate. Look at the letter "x". There's more angles in it than the letter "y"
 
plongson said:
I'm having a hard time getting my head around Vx and Vy. After research and study, I still don't understand why Vx is actually at a lower speed than Vy. My intuition is it should be a faster speed.

Any help understating this is appreciated...
Click to expand...

A helicopter's Vx airspeed is 0. Its Vy is significantly greater than 0.

Screen Shot 2022-02-13 at 5.40.38 PM.png

(Vx is not even listed, since it's obvious a helicopter's steepest climb angle is straight up. It can't go up very fast though.)
 
dmspilot said:
A helicopter's Vx airspeed is 0. Its Vy is significantly greater than 0.

View attachment 104614

(Vx is not even listed, since it's obvious a helicopter's steepest climb angle is straight up. It can't go up very fast though.)
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I actually didn't know that. Interesting.
 
How could you climb in a shorter horizontal distance by going faster?
 
Clip4 said:
How could you climb in a shorter horizontal distance by going faster?
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I was actually thinking about this. For the Warrior I trained in, Vx was 63 knots. Suppose after liftoff near mountains I was climbing for a while at 58 knots, and then my instructor told me, "pitch for Vx, you're too slow." Wouldn't that be a case where going faster would allow me to climb in a shorter horizontal distance? Otherwise, why was Vx 63 knots instead of 58 knots?
 
Clip4 said:
How could you climb in a shorter horizontal distance by going faster?
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By reducing induced drag. Which you actually do when you increase your speed from Vs0 to Vx, and get a better angle of climb at the faster speed.
 
What happens to your airspeed when you pull up the nose? What happens when you keep your nose level (to achieve cruise speed). Play around with it. Add some flaps too (keep in mind your Vfe flaps extended max speed).
 
Salty said:
By reducing induced drag. Which you actually do when you increase your speed from Vs0 to Vx, and get a better angle of climb at the faster speed.
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Vso is the plane in the landing configuration. Why are to attempting any takeoff full flaps?
 
Clip4 said:
Vso is the plane in the landing configuration. Why are to attempting any takeoff full flaps?
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short/soft field takeoff, maybe 10 or 25 degrees flaps. What happens if you do full flaps?
 
Clip4 said:
Vso is the plane in the landing configuration. Why are to attempting any takeoff full flaps?
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willfully ignoring my point. Vx is a better angle of climb than stall speed. Which is faster?
 
mandm said:
I never did a full flap takeoff. Am wondering. I think it would be fine and give more lift.
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You’re working towards a commercial rating and believe a full flap takeoff would provide more lift? Surely you can’t be serious.
 
Picture walking up a flight of stairs vs running up a wheelchair ramp..

Shortest distance vs shortest time
 
RyanB said:
You’re working towards a commercial rating and believe a full flap takeoff would provide more lift? Surely you can’t be serious.
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Flaps *decrease* lift? Full flaps add a ****-ton of drag and thus reduce excess power considerably, which is going to make the takeoff roll acceleration longer, potentially by a large margin; but they don't really decrease *lift* at takeoff conditions.

ETA: Wrote "roll", meant acceleration. The distance may increase if excess power is reduced too much but it's not guaranteed.

Nauga,
unstuck
 
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nauga said:
Flaps *decrease* lift? Full flaps add a ****-ton of drag and thus reduce excess power considerably, which is going to make the takeoff roll longer, potentially by a large margin; but they don't really decrease *lift* at takeoff conditions.

Nauga,
unstuck
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Do flaps increase lift? Absolutely. The problem that I see is, flap deflection beyond 15deg produces a large increase in drag, so any (as theorized) increase in lift by using full flaps, is really negated by the increase in drag. The notion that a full flap takeoff would provide *more* lift is really moot.
 
Hi Plongson. If you can find a Lift/Drag curve in one of your study books it might help.
 
martym said:
Don't feel bad. AOPA has shown in at least 2 articles over the years that they don't understand the differences either.
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Source so I can laugh at them?

RyanB said:
Has nothing to do with his question.
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True, but I had a student that expressed the same sentiment as OP and it turned out he got the two mixed up.

Salty said:
Can a 150 even lift off from a standing start with full flaps? lol
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Yes, pretty well actually. The hard part is climbing out of ground effect.
 
Pilots shouldn’t think too hard…just step on the ball, follow the needle, etc.
 
plongson said:
I'm having a hard time getting my head around Vx and Vy. After research and study, I still don't understand why Vx is actually at a lower speed than Vy. My intuition is it should be a faster speed.

Any help understating this is appreciated...
Click to expand...
Shot in the dark here. Do you realize both speeds are with max power? The throttle is pushed all the way forward for takeoff and climb in either case. Your pitch attitude establishes the speed not the throttle setting.
 
Jim K said:
I actually didn't know that. Interesting.
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But if you think about it, it's actually obvious. What's your forward speed when you can go straight up?
texasclouds said:
Pilots shouldn’t think too hard…just step on the ball, follow the needle, etc.
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...just follow the magenta...
 
nauga said:
Flaps *decrease* lift? Full flaps add a ****-ton of drag and thus reduce excess power considerably, which is going to make the takeoff roll longer, potentially by a large margin; but they don't really decrease *lift* at takeoff conditions.

Nauga,
unstuck
Click to expand...

Reminds me of my old crusty CFI's discussion on flaps when I was doing my initial training. He said "Flaps increase lift and drag. Small amounts of flaps add more lift than drag, large amount of flaps add more drag than lift."

Yes, it's a generalization, but makes sense on how and when we use flaps on our little planes.
 
midlifeflyer said:
But if you think about it, it's actually obvious. What's your forward speed when you can go straight up?
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What's not obvious (to me at least) is that the helo can climb faster moving forward than it can going straight up. I would've assumed putting all the horsepower into going straight up would be fastest, but apparently that rotor disc acts more like a wing than I realized.
 
I think I'm starting to get it...THANKS for all the feedback. I think where I was getting confused (and I'm REALLY new at this!) I didn't realize that Vx/Vy ratings are with the throttle pinned full open and pitching up for those speeds...thus establishing a rate of climb.

I was thinking, and where I went wrong, is you had enough power to throttle back to (arbitrary numbers here) 75mph for Vy from 85mph Vx...My thought was "Why would you throttle back if you have more power available"

It's all about matching pitch angle to air speed with the throttle pinned.

I think I got it now!!! THANKS for all the discussion!

Paul
 
plongson said:
I think I'm starting to get it...THANKS for all the feedback. I think where I was getting confused (and I'm REALLY new at this!) I didn't realize that Vx/Vy ratings are with the throttle pinned full open and pitching up for those speeds...thus establishing a rate of climb.
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Yes and no. Yes, you will almost always be using full power when you want a maximum performance climb, whether it's angle or rate you're looking for. But, Vx and Vy don't change depending on the power setting. Even at a reduced power setting, the same Vx or Vy will give you the best angle or rate of climb that you can get at that power setting, though naturally that rate or angle will be less than it'd be at full throttle.
 
Dana said:
Yes and no. Yes, you will almost always be using full power when you want a maximum performance climb, whether it's angle or rate you're looking for. But, Vx and Vy don't change depending on the power setting. Even at a reduced power setting, the same Vx or Vy will give you the best angle or rate of climb that you can get at that power setting, though naturally that rate or angle will be less than it'd be at full throttle.
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Isn’t the fact that power/thrust decrease with altitude the reason that Vx and Vy converge as you get higher?
 
MauleSkinner said:
Isn’t the fact that power/thrust decrease with altitude the reason that Vx and Vy converge as you get higher?
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This^. I just logged on to make the same point, but you beat me to it. Maybe a jet is different?
 
dtuuri said:
This^. I just logged on to make the same point, but you beat me to it. Maybe a jet is different?
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I’ve never seen Vx or Vy published for a jet, but while jets are “thrust” based and props are “power” based, Vx and Vy should still converge with altitude.
 
Dana said:
Yes and no. Yes, you will almost always be using full power when you want a maximum performance climb, whether it's angle or rate you're looking for. But, Vx and Vy don't change depending on the power setting. Even at a reduced power setting, the same Vx or Vy will give you the best angle or rate of climb that you can get at that power setting, though naturally that rate or angle will be less than it'd be at full throttle.
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That is not correct. Vx is max(thrust available minus thrust required), Vy is max(power available minus power required). Changing the power will shift the thrust and power available curves. For example, when thrust available is 0, the least thrust required is at best glide speed, not Vx. Furthermore, the minimum power required is equal to minimum sink speed which is even slower than that.
 
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Vy is where power required is a minimum, regardless of what's available.
 
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