No lift?

[Richard]

In Diana's, California Acro, thread, Lance made the following statement WRT execution of hammerhead stalls:

"I'm not sure what you're hitting on the way down, but it can't be your wake as none is generated on a vertical climb (no lift)."

Something about that just doesn't sit well with me. I'm thinking if the wing is not producing lift because it's past CL max the wing must be in the stalled condition where all airflow has separated from upper wing. Yes, I recognize the maneuver is a stall but the stall doesn't occur until at or nearly at the top of the vertical climb. The stall could not begin before that point otherwise it would prevent the vertical climb in the maneuver. So the a/c is in a vertical climb and still producing lift until the stall.

Therefore, the wing is not stalled until that point and is producing lift. Would not any application of any amount of forward stick in the climb be an indication of lift? (Fwd stick necessary to avoid pitching on her back in the climb.)

There's more. A byproduct of lift is the vertical velocity which produces downwash. Diana stated she is hiting something on her way back down from the hammerhead. How valid is the assumption that it is downwash, a product of lift, which Diana hit on the way back down??

I spend zero time past 30* pitch so I plead ignorance. I greatly desire clarification of Lance's statement. Also, show me my errors in what I have presented here.

 

[mikea]

It seems to me you don't need lift to make a wake. Cars, trains, and boats have a wake. Try standing on the platform when the Amtrak Hiawatha Service goes by at 70 MPH.

Isn't a wake just the result of the air closing in around where your vehicle has been? The air gets disturbed just by the nature of air resistance. It has to get out of your way, whether you're flying or not.

There is turbulence in wind tunnel behind cars.

 

[poadeleted20]

Your wing may not be generating any lift, so you may not be generating any wingtip vortices, but the prop is generating a heck of a turbulent path "behind" you.

 

[gismo]

Your wing may not be generating any lift, so you may not be generating any wingtip vortices, but the prop is generating a heck of a turbulent path "behind" you.

Yes I would expect something from the prop wash, and that's probably what Diana ran into on her way down from a hammerhead turn. OTOH, I don't remember encountering much if anythiing when I tried the HH, maybe I just didn't notice. I would expect that since the downline is displaced from the up line by about a wingspan, that you'd normally miss most if not all the prop wash, but I must admit that I'm not very experienced with hammerheads as the plane I flew most of my Acro in didn't have a very strong tail and we were always a little afraid that a hammerhead attempt would turn into a tail slide.

As to the lift/wake issue, an airplane on a vertical line cannot be generating any lift simply because if it was the line would be curved. The wake of an airplane is definitely a function of lift and when there's no lift there's no "wake". Someone brought up a boat analogy, and that's a good one the chief difference being a boat wake exists in a medium that has different properties in different dimensions because of the air/water interface in the vertical. A boat on plane creates a wake very similar to an airplane's, a displacement hull (trawler) leaves behind a much different disturbance, with the wave crests perpendicular to the boat's path instead of a pair of wave axes forming a 'V' behind the boat. When you encounter your own "wake" you get one or two "thumps" as you cross the shear zone. I expect that behind an airplane generating zero lift (ballistic or vertical flight path) the effect would be more like "chop" or random turbulence than a well defined wake.

 

[gismo]

I'm thinking if the wing is not producing lift because it's past CL max the wing must be in the stalled condition where all airflow has separated from upper wing. Yes, I recognize the maneuver is a stall but the stall doesn't occur until at or nearly at the top of the vertical climb. The stall could not begin before that point otherwise it would prevent the vertical climb in the maneuver. So the a/c is in a vertical climb and still producing lift until the stall.

A hammerhead consists of at least three distinct phases of flight, the upline, the turn, and the downline. During all three, the wings must be at the zero lift AOA or the airplane simply couldn't stay on a vertical line. This is not a stalled condition, in fact on a vertical line, the stall speed is essentially zero. I say essentially because the wings actually must generate differential lift (one lifts "up" the other "down" relative to the pilot's frame of reference) to counteract the engine/prop torque and the amount of this differential lift is fairly small. While a hammerhead is sometimes called a "stall turn", when properly executed there is no stalling involved.

 

[Diana]

A hammerhead consists of at least three distinct phases of flight, the upline, the turn, and the downline.

I'm still trying to figure this all out.

In the Citabria, there is the "dive to pick up speed" phase, so wouldn't there be lift at the transition from diving to obtain 145 mph to the 4 G pull-up? That seems to be about the spot where I kept hitting the bump.

While a hammerhead is sometimes called a "stall turn", when properly executed there is no stalling involved.

Yes, I found out the hard way how things don't work right if you wait too long and stall or come too close to stall (with and without kicking in rudder). Some of those we actually got on video.

 

[Ken Ibold]

In the Citabria, there is the "dive to pick up speed" phase, so wouldn't there be lift at the transition from diving to obtain 145 mph to the 4 G pull-up? That seems to be about the spot where I kept hitting the bump.

That would be the cracked spar flexing.

 

[Diana]

That would be the cracked spar flexing.

 

[Ken Ibold]

You DO know I was joking.

I hope.

 

[Diana]

You DO know I was joking.

Yes.

Although I am curious to see my spar up close in a few weeks. I will feel much, much better about doing acro after it's gone over with a fine tooth comb. Every time I dive for speed or get close to Vne now I think about those spars.

 

[gismo]

I'm still trying to figure this all out.

In the Citabria, there is the "dive to pick up speed" phase, so wouldn't there be lift at the transition from diving to obtain 145 mph to the 4 G pull-up? That seems to be about the spot where I kept hitting the bump.

Sure, when you pull 4 G's you're creating a heck of a wake for a Citabria. I guess I don't consider the dive and pullup to be part of a hammerhead, just preparation. From your original comment I was imagining the "bump" you described as occurring right after you yawed 180 degrees from straight up to straight down.