Thanks @Pilot_DAR for the Vx discussion. I have always been very vocal about AoA awareness and it’s humbling to learn that I had some rather large blind spots in my own flying!
Our original Cessna 175 POH lists Vx (clean) 60mph with 20 degrees of flaps. Stall in this configuration is 54mph. Even worse is the other note in the climb checklist that says to clear obstacles after takeoff use 61mph in clean configuration. Clean stall speed is 62mph. I can only imagine that the airplane doesn’t stall in this case because of the prop wash, if it isn’t a mistake in the POH. Useful when absolutely necessary, but any other case rather dangerous.
Your posts made me realize that this is really like the dead-mans-curve for helicopters, where there are low altitude/speed combinations where a flare/landing is impossible (that’s why helicopters accelerate over the runway and don’t go vertically up). I never applied this thinking to airplanes before. Due to the rather short runways and the terrain in Switzerland we thought it would be safer to always initially climb Vx, thinking it would be a good habit. Looks like we should only do this when it’s necessary and normally use Vy. (@Bleriot, this is interesting for you as well)
Edit: Just wanted to add a dead-mans-curve diagram for illustration.
Even worse is the other note in the climb checklist that says to clear obstacles after takeoff use 61mph in clean configuration. Clean stall speed is 62mph. I can only imagine that the airplane doesn’t stall in this case because of the prop wash, if it isn’t a mistake in the POH.
DA40 & DA42 had very low Vx (2kts above stall) that the manufacturer decided not to publish it, they only publish V50 (to be used bellow 50ft on liftoff)
The DA42 I fly have an identical Vx and Vy in the POH.
Are you sure it’s POH? or club V-speed card?
VX is moot in twins as you will opt for climb above the blue line at Vyse right after liftoff to avoid rolling back to Vmca or or hitting trees with one engine at Vxse..the DA40 has the same: V50 after liftoff under 50ft and VY above 50ft 
ArcticChiller wrote:
Even worse is the other note in the climb checklist that says to clear obstacles after takeoff use 61mph in clean configuration. Clean stall speed is 62mph. I can only imagine that the airplane doesn’t stall in this case because of the prop wash, if it isn’t a mistake in the POH.
When the aircraft is in a climb, the wings don’t have to provide full lift as the thrust vector will have a vertical component. As Vx by definition gives maximum climb angle, this effect will be most pronounced at that speed. I would say this, rather than propwash, is what could make Vx < Vs possible.
Theoretically, a 10% climb gradient will reduce the stall speed by 5%.
On the other hand, the obstacle clearance speed could be lower than Vx if accelerating to Vx would give a lower net initial climb gradient.
I notice the POH page you quote is for the NG. It certainly is different from the 2004/2005?.
The concept of V50 doesn’t appear in the POH I have.
From memory, I do not have one to hand, there is Vyse and Vxse which are the same speeds. You wouldn’t want to climb after take off, any slower than that speed.
And yes it is in POH not on club check list.
Does anybody else on here have an early DA42?
When I get a chance I will also look up the Vy v Vx on the DA40D which is 2004.
I admit that it’s a long time since I have had to consider the Vx as most airfields I use have no obstacles which need it.
That might or to be more precise will change when I try out the ULM, some of their airfields are less than 300m and have trees close by.
But I am told that at MTOW that the Super Guepard (the French for Cheetah) although cruising at less than 100kts will take off with flaps in a.ground roll of 130m and get to 50ft in 200 metres. It will climb at around 1300fpm.
The POH has no Vx or Vy speeds quoted.
Although it does have the finesse (Vbg) quoted at a lowly 10km per hour. I await my first lesson.😁
Snoopy wrote:
Hats off to John Deakin.
Indeed. He is one guy to listen to anytime. I know him since the days of the avsig forum on compuserve. Class act. His book about flying (a summary of the pelican perch articles) is always in reach if I need a good flying tale.
The Vr discussion is long and often quite spirited when it comes to small airplanes. I think this has a few reasons. John mentions that Vr was introduced with jets. He is right, as usual. It was, amongst others, a consequence of the Rome crash on take off of a Comet1, which never got airborne while the crew was waiting for it to “fly itself off” at the speed they expected it to. It was soon found that jets, particularly with swept wings, simply don’t do that the same way as it was taught all the years before. However, rotation speed is not a bad concept even for small planes, IF it is used properly.
First of all, V1 has no relevance on a single as in case of engine failure the option to continue does not exist. Some schools still train it, mostly with the vision of becoming airline pilots, a bit like the fake gear leaver I found on a Beech Musketeer, and they define it as identical with Vr, so the call out would be V1-Rotate. I find this a bit of deformation rather than information, particularly if it is not even taught what they mean.
Rotation speed, for one, Vr is most of the time not constant, but dependent on weight first and temperature 2nd. This is where most GA use of Vr already is off. Vr for a fully loaded airplane is significantly different than for a plane massively under MTOM.
Some manufacturers actually give Vr in their POH’s as a fixed entity, some take the trouble to define it for maybe 2-3 weights.
In one book I used when doing my ATPL course years and years ago, I found a definition which makes quite a bit of sense: Vr being 1.1 x Vs for the configuration, with V2 being 1.2 x Vs.
Looking at actual figures, this works out quite well. Almost every airplane has Vs tables for various configurations.
The problem @PilotDar and John mention happens exactly when Vr is introduced as a fixed figure and when people are taught to keep the nose wheel down until Vr. This almost always results in Vr being too high for the configuration, hence over unstick speed. That is rubbish. However, if it is calculated properly (or at least the ballpark is known, it’s mostly within 3-5 kts in range), it will mostly work well, provided that the nose gear is not held on the ground by force, which is nonsense in almost all prop airplanes and outright dangerous on grass.
One other factor is, to a much lesser extent, that “unloading the nose gear” appears to mean for quite a few people that they start pulling prematurely, which will increase drag and prolong the take off ground run. I came across this in the venable Cessna 150. Mine had a cruise prop, which made it a ground hugger. Take off distances were quite longer than expected. When I changed to the Seneca, it had a Vr and it was interesting to me why the Seneca jumped off the runway while the C150 did not.
Interestingly, the POH of the 150 does define Vr without naming it as such. It states under normal operations: “Lift Nose Wheel at 55 MPH”. There you go. Vr. If you calculate it out, stall speed at take off flaps is 50 MPH x 1.1, voila. 55 MPH.
I found that if I let the airplane accelerate to this speed with flaps 10° while maintaining the yoke light and then rotated positively by about 3-5°, the airplane would become unstuck almost immediately and kept accelerating through 70 kts within a few seconds of being airborne. It started using the kind of ground run it was supposed to, while before, holding the nose wheel off, had accelerated slowlier and became airborne more sluggish. I think the original way it was shown to me did exactly what happened to that Comet, it increased drag and slowed down acceleration.
Mooney also defined Vr in its POH, even though as a range of speed.
Apply back pressure to the control wheel at about 65-75 mph airspeed.
Vs with take off flaps and MTOW is given as 64 mph. 1.1 x Vs therefore 70 mph, right in the middle. We use the procedure as Mooney indicates starting rotation at about 65 when light and 70 when at MTOW, which is practically always. It works well indeed and uses book figures on take off roll, I am curious to see what it will do with the new 3 blade prop. After unstick, the Mooney always needs a good push on the controls to get into acceleration attitude (also mentioned in the POH) but then accelerates quite briskly through 100 mph once the gear is up. Cruise climb speed is 106 mph (which incidently is Vbg as well). In case of an engine failure, that would be the speed to keep.
Personally I think the use of rotation with small airplanes can be very useful, provided it is done right. “unloading” the nose gear and actually lifting it off are two different things. Some airplanes have tendencies to develop shimmy, which particularly on grass gets people rightly to take the weight of the nose gear. But overdoing it, which I’ve seen a lot, will do exactly what killed that Comet.
Mooney_Driver wrote:
Personally I think the use of rotation with small airplanes can be very useful, provided it is done right.
I agree with everything you wrote. I have recurring arguments with the instructions of my club about this who teach student pilots to let the aircraft lift off by itself. It works well with the C172, but not so much with a PA28.
gallois wrote:
I admit that it’s a long time since I have had to consider the Vx as most airfields I use have no obstacles which need it.
Taking Diamonds to grass with obstacles is a real challenge, I recall in Membury, the main problem was not Vx but the huge wing span, southern sailplane does composite maintenance for DA40/D42
gallois wrote:
Does anybody else on here have an early DA42?
There it is 
And the note :)
