Let's talk about Vx

[fiveoboy01]

Luckily I have a CFI-I with a lot of gray hair and ratings to match. I was a Vx chicken. My thought "this is a high performance airplane and the stalls are eye openers!" His thought was likely "you dumb newbie - with a few 4-letter words that were filtered out before he spoke."

You can't be afraid of Vx and you need to know how the aircraft behaves at Vx. Sounds to me like it is time for a few stalls at 5000' agl. Went through a series of power off and (gasp) power on stalls a few months ago in the doctor killer. I like knowing where the edge of the envelop really lives. At least for me, Vx is 80 MPH and the aircraft won't stall until around 60 MPH. Momentum is less than at Vy, but I'm convinced that if the engine suddenly stopped I'm not falling out of the sky. (Very quickly pushing the column forward is something you have to consider.)

(Just not sure how to test that rotor comment. Then again, I don't usually fly when the winds are a cracking like that..)

I'm not afraid of Vx, I just don't find any real reason to use it when I depart a 7000 foot long runway. Less engine cooling, reduced forward visibility, pax don't like it, etc etc... I'll use it if necessary or if I want to demonstrate a steeper takeoff to someone.

If I want to climb steep, I just accelerate to 100+ knots in ground effect and then zoom climb

 

[brian]]

I'm not afraid of Vx, I just don't find any real reason to use it when I depart a 7000 foot long runway. Less engine cooling, reduced forward visibility, pax don't like it, etc etc... I'll use it if necessary or if I want to demonstrate a steeper takeoff to someone.

If I want to climb steep, I just accelerate to 100+ knots in ground effect and then zoom climb

7000'. Shish, wait until Vno!

 

[fiveoboy01]

I need A JATO STC. That will settle the issue.

 

[Hank S]

Ok, I'm not an aerodynamic expert.

All I know is the deck angle is pretty steep on a Vx climb and I also usually have two notches of flaps(drag) out. Engine quits, you're losing a lot of energy.

You takeoff with two notches of flaps? I thought the discussion was climbing away from the runway at Vx.

Learning and being based at a 3000' field with trees at both ends, initial climb at Vx became habit. I generally transition to Vy pretty soon, after I clear obstacles. Now that I'm based at a 5000' field, I'm getting lazy with landing, although I rarely miss the midfield turnoff, but I still hold Vx for a little while on departure.

Get high near the airport, then get high quickly. The 30 seconds or so spent at Vx will not have a measurable effect on my enroute flight time.

 

[fiveoboy01]

My POH specifies for a short field or obstacle clearance takeoff, two notches of flaps and Vx. But it does say to retract the flaps whilst climbing away at Vx(specified as best angle with flaps retracted), then pitch for Vy after clearing obstacles.

I guess one could take off flaps retracted and climb at Vx too. Don't know how it would effect the takeoff though.

 

[Kritchlow]

My POH specifies for a short field or obstacle clearance takeoff, two notches of flaps and Vx. But it does say to retract the flaps whilst climbing away at Vx(specified as best angle with flaps retracted), then pitch for Vy after clearing obstacles.

I guess one could take off flaps retracted and climb at Vx too. Don't know how it would effect the takeoff though.

If Vx is published with flaps, that number will be different without flaps. Further more it is likely not a true Vx. IOW, if your manual says the best angle of climb is flaps X at xx kts, that is the best angle of climb scenario. Period.

 

[MAKG1]

Archers definitely do take off from short fields with 25 flaps (second notch). That minimizes the distance to 50 feet and improves the angle of climb over no flaps, as a somewhat slower speed.

There aren't too many airplanes that take off clean on short fields. Some 172s do, but that's because the extra drag in the ground roll increases the 50 foot distance; if FAA trees were higher than that, the answer would be different.

 

[Hank S]

I never considered 3000' to be a short field. The nearby 2000' grass strip, sure. Most of my takeoffs are clean, but from 3000' I will use Takeoff flaps if I'm loaded near gross. Trees and all, no problem clean.

Thinking back to my student pilot days, I may have regularly used 10° in the 172, again not following short field procedures except when specifically practicing those.

 

[fiveoboy01]

If Vx is published with flaps, that number will be different without flaps. Further more it is likely not a true Vx. IOW, if your manual says the best angle of climb is flaps X at xx kts, that is the best angle of climb scenario. Period.

There's no Vx published with flaps out. The short field procedure says accelerate to 41-49 kias, after breaking ground accelerate to 45-51 kias, then accelerate to 64 kias(which is the published Vx), and slowly retract the flaps.

So judging from that, best angle with 25 degrees of flaps is likely a little less than 64 indicated.

 

[flyingron]

Ok, I'm not an aerodynamic expert.

All I know is the deck angle is pretty steep on a Vx climb and I also usually have two notches of flaps(drag) out. Engine quits, you're losing a lot of energy.

Available energy controls where you're going to hit the ground. It has nothing to do with stalling. Stalling is purely determined by angle of attack. Speed doesn't matter, energy doesn't matter.

 

[MAKG1]

There's no Vx published with flaps out. The short field procedure says accelerate to 41-49 kias, after breaking ground accelerate to 45-51 kias, then accelerate to 64 kias(which is the published Vx), and slowly retract the flaps.

So judging from that, best angle with 25 degrees of flaps is likely a little less than 64 indicated.

On the other hand, 182s call for retracting flaps after all obstacles are cleared (the Archer POH is a bit vague on that point), and publishes Vx at flaps 20. Flap retraction is called for at a significantly higher speed. So, if you want best angle of climb, do it with flaps 20.

 

[drotto]

I was told with short field to set the flaps, and use Vx to obstacle clearance or at least 200 AGL. Then pitch down to Vy, and begin retracting flaps.

 

[flyingron]

I was told with short field to set the flaps, and use Vx to obstacle clearance or at least 200 AGL. Then pitch down to Vy, and begin retracting flaps.

"What you were told" is a cop out. What is in the POH for the aircraft should be the default, followed by what you've determined by reasonable demonstration in your aircraft.

What to do with flaps depends on what you are flying. Flap design is different in every aircraft. I've seen more bs old wive's tails about flaps than just about anything else in aerodynamics.

"Begin retracting flaps?" How about retracting them? What are you flying? I've seen more nonsense about "milking the flaps up." First off, the book doesn't call for that in things like my plane or a 172; it just says to retract them upon reaching a given speed (which is roughly the same as Vx). Most people's attempts at retracting them slowly in the later 172's doesn't do anything different than if you just put the switch all the way up.

Why on earth Vx if you're not trying to avoid obstacles? That's it's the only advantage.

 

[jsstevens]

Hey - I'm not the guy that will be turning back to the runway if my nearly 70 year old engine decides its time to retire. (Might, but not my first choice - it's the insurance company's airplane at that point and I'm not going to kill myself trying to save their airplane.)

Still, being "afraid" of Vx isn't smart. It's like the Bonanza guys you see driving their aircraft onto the pavement at over 100 MPH because they don't like the pitch attitude. If you need Vx - point the nose up and go. The more I get into this IMC stuff the more I like to being "up there" before I'm "over the trees". Vx is the answer. Running into a tree (or a hill) trying to keep the engine cool or keep a passenger comfy is pretty dumb. (A local guy killed himself a few months ago because he didn't climb fast enough to clear a hill. Likely a mechanical issue - but lesson learned - get "up there" before you run into something.)

I guess my CFI-I is rubbing off on me. Don't be a one trick pony - Vx to clear obstacles. Vy to get to altitude. Cruise climb to keep the engine happy. All depends upon what you see out the windscreen just before takeoff.

It's all energy management appropriate to the conditions and risk.

Airspeed is one kind of energy (kinetic) and is used to counter some sorts of risks (gust factor for example). Altitude is another kind of energy (potential) and helps with other things (like having choices as to where to land if the fan gets quiet).

Short field with obstructions? Better be able to climb at Vx. Hills to avoid? Better be able to climb at Vx. Longer runway, no obstructions? Vy is the ticket. Flying a high performance airplane with cooling concerns? Use a shallower climb.

Fill your tool box with as many tools as you can use effectively. Learn which tool to choose based on risks and conditions. Cruise climb into the side of a mountain and nobody will care how much cooler the engine was running. Vx out of a remote old airbase (10,000' runways) in the middle of Kansas farmland and burn your engine up and you'l probably live to realize it wasn't the best choice.

Get comfortable as close to the edges of the envelope as you can so you have ALL the tools.

John

 

[drotto]

"What you were told" is a cop out. What is in the POH for the aircraft should be the default, followed by what you've determined by reasonable demonstration in your aircraft.

What to do with flaps depends on what you are flying. Flap design is different in every aircraft. I've seen more bs old wive's tails about flaps than just about anything else in aerodynamics.

"Begin retracting flaps?" How about retracting them? What are you flying? I've seen more nonsense about "milking the flaps up." First off, the book doesn't call for that in things like my plane or a 172; it just says to retract them upon reaching a given speed (which is roughly the same as Vx). Most people's attempts at retracting them slowly in the later 172's doesn't do anything different than if you just put the switch all the way up.

Why on earth Vx if you're not trying to avoid obstacles? That's it's the only advantage.

Fully aware of what Vx is for, but it also comes down to instructors giving guidelines that will work for the majority of the aircraft, not just the one you are training in. I do not fault CFI's for making some generalizations, and what I was told is basically out of the POH, which reads 25 degrees of flaps, climb out at Vx till obstacle clearance then continue out at Vy, and retract flaps. I actually think the sensation of a small plane lightly loaded doing short field is fun. I like that sensation of "jumping" off the runway.

The flap worries that everyone seems to focus on is people dumping all the flaps at low air speed. Thus in theory reducing lift to the point that could give you a stall at the steeper Vx climb, and low air speeds. Now from experience, I have felt a drop in the airplane with rapid flap retraction (using manual flaps not electric). It certainly did not cause a stall, and the altitude drop was maybe 20 feet, but it was there. So there is likely more concern than validity, but it is clear where the mentality comes from. So yes most students are instructed to retract the flaps one notch at a time, regardless of what is in the POH.

 

[fiveoboy01]

On the other hand, 182s call for retracting flaps after all obstacles are cleared (the Archer POH is a bit vague on that point), and publishes Vx at flaps 20. Flap retraction is called for at a significantly higher speed. So, if you want best angle of climb, do it with flaps 20.

But at what speed? A best angle with flaps out speed isn't published to my knowledge. I agree the POH is vague in this regard.

 

[fiveoboy01]

Available energy controls where you're going to hit the ground. It has nothing to do with stalling. Stalling is purely determined by angle of attack. Speed doesn't matter, energy doesn't matter.

I never said otherwise. They were two seperate statements, and I was saying that having as much energy available is a plus if you lose the engine.

 

[kgruber]

On the other hand, 182s call for retracting flaps after all obstacles are cleared (the Archer POH is a bit vague on that point), and publishes Vx at flaps 20. Flap retraction is called for at a significantly higher speed. So, if you want best angle of climb, do it with flaps 20.

PA-28-181 POH
SHORT FIELD, OBSTACLE
CLEARANCE
Flaps………………….25 degrees (second notch)
Accelerate to 41 to 49 KIAS depending on a/c wt.
Control wheel………....back pressure to rotate
to climb attitude
After breaking ground, accelerate to 45 to 54
KIAS depending on aircraft weight
Accelerate to best flaps up angle of climb speed –
64 KIAS, slowly retract the flaps and
climb past the obstacle.
Accelerate to best flaps up rate of climb speed –
76 KIAS

 

[flyingron]

QUOTE=drotto;2012435]
The flap worries that everyone seems to focus on is people dumping all the flaps at low air speed.
[/QUOTE]
Once you're above Vs1 by more than a couple of knots it's no big thing.

Thus in theory reducing lift to the point that could give you a stall at the steeper Vx climb, and low air speeds. Now from experience, I have felt a drop in the airplane with rapid flap retraction (using manual flaps not electric).

You don't adjust your attitude for a configuration change you're in trouble no matter what the regime is. I've flown airplanes (Cessna 170's and older 172's) with the manual flaps that could move effectively as fast as I could move my arm. It's even less of an issue than the slow motorized ones. You can FEEL the change in the yoke when you're moving the flap lever.

It certainly did not cause a stall, and the altitude drop was maybe 20 feet, but it was there.

Again ALTITUDE DROP is not the same thing as a stall.

 

[MAKG1]

PA-28-181 POH
SHORT FIELD, OBSTACLE
CLEARANCE
Flaps………………….25 degrees (second notch)
Accelerate to 41 to 49 KIAS depending on a/c wt.
Control wheel………....back pressure to rotate
to climb attitude
After breaking ground, accelerate to 45 to 54
KIAS depending on aircraft weight
Accelerate to best flaps up angle of climb speed –
64 KIAS, slowly retract the flaps and
climb past the obstacle.
Accelerate to best flaps up rate of climb speed –
76 KIAS

Don't read that as absolutely literal. That checklist isn't enough to say whether one should accelerate and lose flaps before or after clearing all the obstacles. From having tried it, you get better angle slow and flaps 25, but it's not comfortable -- you can't see the obstacles with the nose that high.

For instance, have you MEASURED the best flaps up rate of climb speed, in utility category? It's closer to 70 knots than 76. It drops real fast with weight.

 

[fiveoboy01]

The POH almost implies that you retract the flaps and THEN climb over the obstacle.

I've read it a million times and it still messes with my head.

It's not an issue really. I never use the procedure unless I am practicing it. I think they could have been better with wording the procedure though.

 

[MAKG1]

The POH almost implies that you retract the flaps and THEN climb over the obstacle.

I've read it a million times and it still messes with my head.

It's not an issue really. I never use the procedure unless I am practicing it. I think they could have been better with wording the procedure though.

Oh yes, absolutely.

It's not the only example.

That Vy beef I mentioned earlier is a real hassle for IFR in a Warrior. The performance tables make it look like 500 FPM is not possible above 4000 in standard conditions. In fact, it is, just not at 90 knots and not at what the manual calls Vy, unless you're loaded to max gross. I've yanked 500 FPM up to 8000 solo in a Warrior (with full tanks and 10 lbs of stuff), on a warmer than standard day. But at 70 KIAS. I thought I was going to have to climb in a hold to clear a minimum crossing altitude, but it turns out I didn't. Gained 5000+ feet in 10 miles. Not bad, but the POH makes it look a lot worse than that: 18 miles.

 

[exncsurfer]

The POH almost implies that you retract the flaps and THEN climb over the obstacle.

I've read it a million times and it still messes with my head.

It's not an issue really. I never use the procedure unless I am practicing it. I think they could have been better with wording the procedure though.

I would assume that it means to leave flaps in until you reach that speed, then slowly(whatever that means) retract (regardless of obstacle clearance) to maintain best distance to climb ratio(which is the point of the procedure). ??

 

[3 in the green]

Stirring the pot some more, courtesy of my old flight school...

http://www.planeandpilotmag.com/pil...st-speaker-safe-turnarounds.html#.VrlYp_lVhBc

 

[exncsurfer]

 

[uncreative]

Intuitively, that article makes sense to me, and that is the way I fly. The airports that I frequent are in the California foothills and don't have friendly terrain surrounding them.

 

[jaymark6655]

Only works on runways that are over 5000', assuming a C172S at gross. Anything shorter, by the time you get to 500'agl (1 mile horizontal distance) you are too far to glide back to the runway. 300' lost in the turn, that leaves you 200' till ground impact and at most 1800' glide until you hit the ground.

Why risk a steep turn where your stall speed is increased to your best glide speed?

 

[uncreative]

I just ran the numbers on the Archer I usually fly, at the field I normally fly out of (3600' rwy) and you're right, at gross with an engine out at 500 feet at Vx climb, no way I'm making it back to the field, by the book anyway. At 1000 feet it looks possible to make it back over the fence. I think I'll grab my CFI and see what kind of performance we get trying out the practice scenario outlined in that article.