VX or VY on TO, What’s the deal ?

[piperboy84]

I have read in several books on flying that say unless you have an obstruction on your takeoff path it is best to fly to VY after rotation, one reason given is if the engine quits and you are climbing at VX it requires a significant and expeditious nose drop to get to best glide in comparison to being at VY, which all makes sense, the issue I have with this is as follows:

My Maule speeds are as follows VS=47, VX=75, VY=90 ( all speeds at gross and in MPH) and it cruises about 110.

When I do take off VX it just seems to feel better to me for both climbing and speed and that is where my gut instinct positions the attitude, where VY feels like the initial climb lingers. I was taught to “Always Fly the Numbers” but again, VX seems to me to be the more natural and best suits the attitude picture I instinctively am looking for on TO

Any thoughts?

[FlyingStone]

Once you see CHTs in Vy+10 climb on an average Lycoming, you won't even think about climbing at Vy, let alone Vx.

I do it simple: obstacles after TO = Vx, no obstacles = Vy (or whatever required for cooling purposes). I hardly think anyone can object climbing with Vx when there are obstacles present, but there are some reasons why Vy mostly turns out to be better choice for initial climb out.

First of all, as you mentioned, in case of flying at Vx, an engine failure in initial climbout will provide you with a handful of problems. Even with perfect and timely reaction, engine failure at Vx will still require you to push the nose down hard, in order to increase speed to Vg (best glide speed), which is (in most prop aircraft) approximately Vy (give or take couple of knots). Flying with less than Vy brings aircraft to the backside of the drag curve (where drag increases with decreasing speed), which means that you have to be aware of it, otherwise you may easily stall an aircraft while doing memory items during EFATO. You could ask a fellow pilot to pull the throttle to idle at 300ft after takeoff for a simulated EFATO and you'll see the reaction has to be almost instant in order to keep the aircraft flying and accelerating to Vg, while at Vy there is some buffer.

The second problem is lookout. Can you really maintain positive lookout to the front of the aircraft while flying at Vx with some insane pitch attitude or would it be easier to spot an aircraft with smaller pitch (and thus higher airspeed)? Some people try to fly SEPs as jets, but there is too much difference between those type or aircraft to fly them with the same technique. 15° pitch after liftoff will work for most jets, while at the same time is a bad idea for SEP, especially for spam cans.

And the last, as mentioned before - engine cooling. Many engines tend to overheat (CHTs way over 400°F) at sub-Vy speeds due to insufficient airflow through the cooling fins on the cylinders. On hot days, even oil temperature can become a problem at Vy on some installations.

[2high2fastagain]

I fly my C182 at Vx until I am clear of nearby obstacles such as trees and then continue at Vy until cruise height. Never had a problem with CHTs at Vx at full rich - they are always mid to low-upper 300's F. If on a big aerodrome, then I go up at Vy.

Certainly with efato's you have to bang the nose down at Vx, but I've not found that a problem in practice sessions, though i usually decide where I' m going to come down in an efato before I take off - if I can (with a bit of google earthing in the preflight).

[Big Pistons Forever]

I teach Vy to 1000 AGl on takeoff and then climb at the high end of the POH recommended normal climb. If this speed gives you a climb of more then 500 feet/min then increase climb speed until you stabilize at 500 feet/min, assuming terrain/direction of flight/hazards etc are not a factor

For the C 172 P that means Vy 76 kts to 1000 feet then a minimum of 85 kts or higher as necessary to maintain 500 feet min. If 85 won't give you 500 feet min take what you can get. Prolonged climbs at speeds below 85 are to be avoided to ensure good engine cooling

[peterh337]

I think that if everybody had decent engine instruments, they would not climb at Vx or Vy for long :)

I climb at Vx-ish until clear of immediate obstacles and then transition to a decent airspeed (120kt in my case) for engine temperature management reasons, and also one gets a much better lookout.

[Pace]

Also on windy days with wind shear around it it also useful to have a bit of extra energy in hand.

Pace

[Big Pistons Forever]

Peter337

Out of curiosity what speed does the TB20 POH call for, for a "normal" climb ?

[phiggsbroadband]

Hi Piperboy, I think it all depends upon what you want to do...

If you are taking off straight out into a 15 knott wind. Where would you prefer to be in five minutes? Ten miles down-range at 3000ft. Or at 4000ft just above the end of the runway? (using Vy+ or Vx. resp.)

There are so many V speeds, even one for the max speed to open the windows.

Pete

[Steve N]

CLIMB

Mixture FULL RICH
Throttle FULL POWER
Propeller FULL FORWARD (2575 RPM)
Optimum Climb Speed 95 KIAS
Note:Climb can also be carried out at higher speeds and lower
power ratings (better visibility towards front, better engine
cooling, lower noise level

However ours will quickly cook at that speed so we use 120kts as Peter does.

[Mark1234]

Just one thought for the OP - all other considerations aside, remember that Vx is best ANGLE of climb, whereas Vy is best RATE (by definition).

Therefore Vy will get you away from the ground faster - if not there's something odd going on. Possibly it's an illusion that the climb lags? 1 minute after takeoff you'll be closer to the ground with Vx but you'll be closer to your lift off point - if that matters (e.g. when there are obstacles..)

[peterh337]

95kt is Vy but as Steve says you will end up with a high CHT - I would guess 420F at an OAT of +25C.

The only place I have tolerated 420F was coming out of Turkey in +35C, and that was very briefly (10s of seconds).

However the TB20 POH is dated 1988, and this is true for the post-year-2000 GT also, which Socata cunningly call "Mod 151" and keep the POH the same, to reduce the certification work ;)

And 1988 TBs would not have had an EDM700 and if they had a CHT gauge it would prob99 have been on the wrong cylinder ;)

I get a 30F+ difference between #3 (hottest) and say #2 (one of the coolest). It is #3 that I watch during all flight regimes (I stop the EDM700 scanning on it).

This week, having seen slowly rising CHTs over time but seeing absolutely constant oil temperature over the last 10 years, I re-made the upper baffles (which were well shagged after 10 years) using this excellent material (http://www.mcfarlane-aviation.com/Products/?CategoryID=198&). There is a UK disti (http://www.fandh-aircraft.co.uk/). Took a good half a day. The result is an at least 10F drop in #3 CHT; well worth it. I will re-do the lower baffles when I can see a flat calm day with nice wx (not allowed to work inside a hangar) which needs the lower cowling removed. But the lower ones seem OK.

OTOH I know of a very old TB20 with an EDM700 whose CHTs are at least 30F below mine. No idea how it does it....