Vx and Vy

Dana said:
Vy is where power required is a minimum, regardless of what's available.
Click to expand...

Definitely not correct. The power required is a curve and the power available is a curve. Max excess power does not occur at min power required.
download.jpeg
 
Last edited:
Salty said:
Can a 150 even lift off from a standing start with full flaps? lol
Click to expand...

actually they can! Only reason I know is I picked one up this fall, wanted to get it into an annual tho it wasn’t overdue… the plane hadn’t flown a lot over the summer, shop had a surprise opening. I wanted to run it a bit before a compression check, however ceilings were very low. So I just did some rejected take off practice. I wanted as long of a ground roll as I could, cranked in all 40 degrees and was surprised when I floated up as quick as I did! Granted I was solo and it was cold- but I was still rather surprised! :)
 
Huckster79 said:
actually they can! Only reason I know is I picked one up this fall, wanted to get it into an annual tho it wasn’t overdue… the plane hadn’t flown a lot over the summer, shop had a surprise opening. I wanted to run it a bit before a compression check, however ceilings were very low. So I just did some rejected take off practice. I wanted as long of a ground roll as I could, cranked in all 40 degrees and was surprised when I floated up as quick as I did! Granted I was solo and it was cold- but I was still rather surprised! :)
Click to expand...

MauleSkinner said:
Why would you think more flaps would lengthen your ground roll?
Click to expand...

In general flaps on takeoff shortens the takeoff roll but hampers climb rate once in the air. The opposite is true as well no flaps on takeoff results in a longer ground roll but better climb once off the runway.
 
Bill said:
In general flaps on takeoff shortens the takeoff roll but hampers climb rate once in the air. The opposite is true as well no flaps on takeoff results in a longer ground roll but better climb once off the runway.
Click to expand...
Exactly...with the "in general." If excess power as a result of the additional drag reduces acceleration enough the ground roll will be longer with flaps extended. I do have to go back and correct the part of my post where I said the takeoff distance would definitely be longer :cool:

Nauga,
who posts in haste
 
Last edited:
midlifeflyer said:
But if you think about it, it's actually obvious. What's your forward speed when you can go straight up?
Click to expand...

DMS wrote airspeed in his post but you seem to be referring to ground (forward) speed. It may be less obvious than one may initially think. You can have zero forward speed while still indicating a fair amount of airspeed if it is a windy day. Performance will also change. But, at the end of the day, zero forward speed should translate into the best angle one could climb at.

Jim K said:
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.
Click to expand...

I'm far from an aerodynamic expert but a better correlation might be looking at it like a propeller. You'll typically get better performance (more rpm) out of the engine and propeller on the takeoff roll or in the air than you would if you were doing a static runup.

http://www.copters.com/aero/translational.html
 
Bill said:
In general flaps on takeoff shortens the takeoff roll but hampers climb rate once in the air. The opposite is true as well no flaps on takeoff results in a longer ground roll but better climb once off the runway.
Click to expand...

correct, I just thought from a full stop with all 40degrees in that that extra drag to get rolling would be noticeable, and probably would have been more so in the heat of summer
 
MauleSkinner said:
Why would you think more flaps would lengthen your ground roll?
Click to expand...

full 40 degrees in a 150, I figured all that extra drag to get rolling would have slowed the acceleration more noticeably. 10 or 20 absolutely would shorten it, but I figured with those big barn doors all the way down and a plane not known for it’s gusto that fully deployed they would work against me due to delayed acceleration.
 
Huckster79 said:
correct, I just thought from a full stop with all 40degrees in that that extra drag to get rolling would be noticeable, and probably would have been more so in the heat of summer
Click to expand...

Not that I've tried it, but I'd imagine you'd get off the runway right quick, but getting out of ground effect and climbing away might be more difficult.
 
Bill said:
Not that I've tried it, but I'd imagine you'd get off the runway right quick, but getting out of ground effect and climbing away might be more difficult.
Click to expand...

that’s exactly what happened, my assumption was wrong that the delayed acceleration would hinder it. Though it probably did compared to leaving them up and pulling em full on once rolling but they didn’t slow the acceleration like I figured they would… It didn’t matter that day as as soon as I was off I’d chop power and land- was great fun and great practice.
 
dtuuri said:
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.
Click to expand...
As I was reading through all of the answers and argumentative posts leading up to yours, I was wondering when someone might point this out to the OP. Took awhile.
 
SkyDog58 said:
As I was reading through all of the answers and argumentative posts leading up to yours, I was wondering when someone might point this out to the OP. Took awhile.
Click to expand...
On what basis? It was a good catch by dtuuri but not really obvious from the OP.
 
It seems no matter any explanation, there’s someone to take issue with it.

With that in mind, be ready with the book answer, pass the test.
 
VX best angle, VY best rate, book answer. Oh, VX is slower.


My comments were a vague reference to the article linked above, good reviews. Then you read the comments below the article, at least one says it’s all wrong.

My son has moved on from CFI-ing, a few times he mentioned the circular discussions about various topics. Yeah, put 8 CFI’s in the room then wait for agreement in an issue. Pilots can be substituted for the CFI’s.
 
Last edited:
sourdough44 said:
What controls VSI on the glide slope? Is it power or attitude?
Click to expand...
Neither. Vertical Speed controls the VSI. ;)

The real answer to "what controls" is both. What's the best way to control VS? power, most of the time, but dependent on the circumstances attitude may be better.
 
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...

Maybe this video will help:
 
dmspilot said:
Definitely not correct. The power required is a curve and the power available is a curve. Max excess power does not occur at min power required.
View attachment 104635
Click to expand...
Yes, and also there's a difference for jets. The power available is a straight line, angling upward from the origin. On a thrust chart, the thrust available is a (more or less) straight horizontal line compared to a propeller driven thrust available curve. The thrust and power "required" curves would be the same for a given airframe. FWIW.
 
dmspilot said:
Definitely not correct. The power required is a curve and the power available is a curve. Max excess power does not occur at min power required.
Click to expand...
Mmmm, yes, you're correct. What I should have said was that for any specific power setting, the best rate of climb will be at the minimum power required airspeed... which may not be the same as the published full power Vy. Though on the lighter end (typical small plane with a climb prop), the power curve may be flat enough that it's pretty close.

The draggier the plane (like the biplanes I fly), the closer together Vx and Vy are... and the closer they get to stall speed.
 
dmspilot said:
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.
Click to expand...
Define power and thrust please
 
You must log in or register to reply here.
Back
Top