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Vx or Vy and why?

Posted: Wed Feb 27, 2019 10:48 pm
by Atrosa
I understand that Vx is the best rate of climb and Vy is the best rate. Vx for clearing obstacles and Vy to get up there quicker. However, wouldn't Vx help make the impossible turn a little less impossible?

What does your experience say when climbing out? Even without an obstacle?

-Tony

Re: Vx or Vy and why?

Posted: Wed Feb 27, 2019 10:55 pm
by Wm.Ince
Atrosa wrote:”I understand that Vx is the best rate of climb and Vy is the best rate.”

Actually Tony, Vx is best angle of climb and Vy is best rate of climb.

Re: Vx or Vy and why?

Posted: Wed Feb 27, 2019 11:07 pm
by drseti
If you're trying to get back to the runway after an engine failure, you want to know, and establish, the best glide speed. Typically, in light aircraft, that falls roughly midway between Vx and Vy. The reason you don't want to try the impossible turn at Vx is that it gives you less margin above the stall.

Re: Vx or Vy and why?

Posted: Wed Feb 27, 2019 11:10 pm
by drseti
I discuss Vx, Vy, and Vglide a bit in my EAA webinar on short and soft field operations:

http://www.eaavideo.org/detail/video/5987956462001

Re: Vx or Vy and why?

Posted: Thu Feb 28, 2019 7:32 pm
by Atrosa
Wm.Ince wrote:
Atrosa wrote:”I understand that Vx is the best rate of climb and Vy is the best rate.”

Actually Tony, Vx is best angle of climb and Vy is best rate of climb.


Yeah i kinda fat fingered that sentence.

Re: Vx or Vy and why?

Posted: Fri Mar 01, 2019 3:30 pm
by newsshooter
drseti wrote:I discuss Vx, Vy, and Vglide a bit in my EAA webinar on short and soft field operations:

http://www.eaavideo.org/detail/video/5987956462001


Newbie here, I've been browsing for a couple months as I plan to start working on my PPL soon. Was looking at Sport Pilot too which brought me here to the forum. Any chance these EAA weekly videos are available as a podcast, would be great to listen to while driving and walking the dog each night.

Craig

Re: Vx or Vy and why?

Posted: Fri Mar 01, 2019 4:21 pm
by dstclair
Atrosa wrote:I understand that Vx is the best rate of climb and Vy is the best rate. Vx for clearing obstacles and Vy to get up there quicker. However, wouldn't Vx help make the impossible turn a little less impossible?

What does your experience say when climbing out? Even without an obstacle?

-Tony

I think you know that Vx will get you to a given altitude in a shorter distance while Vy will get you to the same altitude in a shorter time. The key is to determine for your aircraft which speed gets you to an altitude and distance to which you could do the "impossible turn". Keep in mind that it is possible that Vx could get you to a safe gliding altitude that is too near the runway (horizontal direction) when you first reach the desired altitude. Basically, your glide could take you past the other end of the runway, theoretically (yeah, I know one could do a slip but I'm ignoring that for the purpose of the exercise).

This is all good in theory but I follow the PoH (or LSA equivalent). In my current aircraft, I'm supposed to climb at Vx for 400' then Vy to pattern altitude, cruise climb thereafter (absent obstacles). I figure this is a pretty good formula to get you to altitude quickly and safely.

Re: Vx or Vy and why?

Posted: Fri Mar 01, 2019 4:43 pm
by TimTaylor
Unless there is an obstruction ahead, such as tall trees, etc., I prefer to lower the nose to Vy pretty soon after liftoff. Reason being, better able to see over the nose, better engine cooling, and more margin above stall speed. YMMV.

Re: Vx or Vy and why?

Posted: Fri Mar 01, 2019 5:02 pm
by drseti
newsshooter wrote: Any chance these EAA weekly videos are available as a podcast, would be great to listen to while driving and walking the dog each night.

Craig


The webinars are provided in MP4 format. Theoretically, you could set your MP4 default associate to an audio player. But most of us who do webinars subscribe to the multimedia principle that the more sensory inputs we stimulate, the better your comprehension and retention. In other words, without the accompanying images, learning is diminished.

Re: Vx or Vy and why?

Posted: Mon Mar 04, 2019 4:58 am
by bryancobb
TimTaylor wrote:Unless there is an obstruction ahead, such as tall trees, etc., I prefer to lower the nose to Vy pretty soon after liftoff. Reason being, better able to see over the nose, better engine cooling, and more margin above stall speed. YMMV.


I think this may have been what the original post was getting at. He's implicitly asking...Why don't normal takeoffs use Vx?

His thought is...
(hypothetical) If the impossible turn needs 1500 ft AGL to get a dead stick plane back on the runway, wouldn't Vx get you to 1500 ft AGL while putting you that high, much closer to the airport? (safer place?)

Re: Vx or Vy and why?

Posted: Mon Mar 04, 2019 7:31 am
by drseti
bryancobb wrote:I think this may have been what the original post was getting at. He's implicitly asking...Why don't normal takeoffs use Vx?

His thought is...
(hypothetical) If the impossible turn needs 1500 ft AGL to get a dead stick plane back on the runway, wouldn't Vx get you to 1500 ft AGL while putting you that high, much closer to the airport? (safer place?)


Yes, but...

Vx does not get you to 1500 AGL quicker. It will get you there closer. For the impossible turn to work, you don't just want to get back to the airport, you need to actually be able to land. If you climb out at Vx, you may still be over the runway when you get to your turn-around altitude. Unless you're flying a VTOL, you may not be able to get back down from that position if the engine fails.

Climbing out at Vy, OTOH, gets you to turn-around altitude in the minimum time. This reduces your exposure to an accident. The reasoning is, all engines have a lifetime. You don't know what that is until it fails, but eventually, all engines will fail. Getting to altitude quickly reduces the probability that you'll be too low when (not if, but when) the engine fails, since you spend less total time being too low.

Re: Vx or Vy and why?

Posted: Mon Mar 04, 2019 9:42 am
by 3Dreaming
drseti wrote: all engines will fail.


I think all engines can fail would be a better choice of wording. Most engines don't fail, they get overhauled based on time.

Re: Vx or Vy and why?

Posted: Mon Mar 04, 2019 10:07 am
by drseti
I stand by my statement, Tom. All engines will eventually fail, just like all humans will eventually die. It's called entropy. The engine will ideally fail on the ground. Or after an overhaul, or when somebody else owns it. Our hope is that it will not fail in flight, but we're always expecting it to. That's why we teach emergency landings.

Re: Vx or Vy and why?

Posted: Tue Mar 05, 2019 5:22 am
by bryancobb
drseti wrote:Yes, but...

Vx does not get you to 1500 AGL quicker. It will get you there closer. For the impossible turn to work, you don't just want to get back to the airport, you need to actually be able to land. If you climb out at Vx, you may still be over the runway when you get to your turn-around altitude. Unless you're flying a VTOL, you may not be able to get back down from that position if the engine fails.

Climbing out at Vy, OTOH, gets you to turn-around altitude in the minimum time. This reduces your exposure to an accident. The reasoning is, all engines have a lifetime. You don't know what that is until it fails, but eventually, all engines will fail. Getting to altitude quickly reduces the probability that you'll be too low when (not if, but when) the engine fails, since you spend less total time being too low.


I had rather be at 1500 AGl, 1/2 mile from the runway with it in my rear-view mirror that 2-1/2 miles from it at 1500.

Re: Vx or Vy and why?

Posted: Tue Mar 05, 2019 5:28 am
by bryancobb
Pertaining to engines failing, arguments about "every" or not really don't help us. What DOES help us is...Assume YOUR engine may quit at any time and ALWAYS LEAVE YOURSELF AN OUT and you will be a safer pilot and not a dead one...

...Says the guy who flies a helicopter with a 2-stroke Rotax that has this WARNING on page 1 of the Opreating Manual. LOL