constant speed or variable speed approach
Avoid imitations
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I guess this comes from the offshore world teaching inexperienced pilots how to be gentle on the controls but what else do people think of the following type of approach? More a Heli deck type of landing I suppose.
0.5 miles, 50kts, 500’, 0.4 miles, 40kts, 400’, 0.3 miles, 30kts, 300’ which pretty much brings you to your decision point at around 30kts for moving across and down to a landing on the deck. No big collective movements required, no big speed changes required. But I guess it can often be very type specific.
0.5 miles, 50kts, 500’, 0.4 miles, 40kts, 400’, 0.3 miles, 30kts, 300’ which pretty much brings you to your decision point at around 30kts for moving across and down to a landing on the deck. No big collective movements required, no big speed changes required. But I guess it can often be very type specific.
OK, stipulating that this has nothing to do with transverse flow, nevertheless per your own statement you don't get flapback until the vibration clears. That would mean that during the condition of vibration it is not a fully developed state of ETL, or perhaps not a state of ETL at all, which was my original point despite the aerodynamic arguments I was referencing from various manuals. Also, the figure out of Prouty is not relevant because we were originally discussing an approach, not forward flight in ground effect.
The vibration is not transverse flow - it is the boundary of ETL (not incipient, not fully developed just ETL) and is due to the vortices clearly shown in the diagram. It happens too quickly to separate in minute detail - the vibration happens and almost immediately the nose pitches up (flapback) and the aircraft climbs (ETL).
On the way down the approach you can ride the burble (vibration), as Robbie describes, and are remaining just above the boundary of what you could call non-ETL. You can feel if you lose that ETL as the burble disappears and your power required to maintain the angle of descent increases - you have lost ETL.
We weren't discussing a speed reducing descent. Robbie said he liked to ride that vibration down, so it's a constant speed descent.
Also, the figure out of Prouty is not relevant because we were originally discussing an approach, not forward flight in ground effect.
I'm not an aerodynamicist either but I do know what a helicopter does and doesn't do.
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Had to turn on few more processor cores to comprehend those last few posts. I think I will dig out the Prouty book and give it a second read.
Still in denial aa777888?? Do you think the rotor vortices disappear when you are further from the ground??
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Seems to me that the air gets more complicated every time I read PPRuNe.
This technique when power limited or VIPs onboard or if it's a tricky to judge approach such as a confined area or a raised pad.
Variable speed: you establish your approach angle and decrease your speed gradually in proportion with your distance to the pad, all the way working the collective to stay on the glide slope. Finish with very gentle leveling off. [This allows you to climb on the back of power curve super progressively]
This technique, or similar most other times: (my advice is to do it not pointing at the pad, leave a 90deg turn-on for the latter stages, that way your judgement of the decel is not critical, you just decel, and turn the last 90deg when you have the crafts speed at a manageable pace)
Constant speed: establish your approach angle and keep your speed 60 knots or above as long as practicable (collective hardly requires any work because all things remain equal) finish with a solid flare and leveling off. [The later part requires you to climb on the back of power curve very quickly]
passenger comfort, risk profile, landing situation, weather, type of aircraft. what factors come into play?[/QUOTE]
When you have hours under the belt, you can blend the techniques seamlessly, varying approach style according to the situation.
Variable speed: you establish your approach angle and decrease your speed gradually in proportion with your distance to the pad, all the way working the collective to stay on the glide slope. Finish with very gentle leveling off. [This allows you to climb on the back of power curve super progressively]
This technique, or similar most other times: (my advice is to do it not pointing at the pad, leave a 90deg turn-on for the latter stages, that way your judgement of the decel is not critical, you just decel, and turn the last 90deg when you have the crafts speed at a manageable pace)
Constant speed: establish your approach angle and keep your speed 60 knots or above as long as practicable (collective hardly requires any work because all things remain equal) finish with a solid flare and leveling off. [The later part requires you to climb on the back of power curve very quickly]
passenger comfort, risk profile, landing situation, weather, type of aircraft. what factors come into play?[/QUOTE]
When you have hours under the belt, you can blend the techniques seamlessly, varying approach style according to the situation.
It is better to do a constant speed approach when practicing autos. It is better to do a variable speed approach when a check pilot is sitting next to you and you want him to see that you know what you're doing.
Outside of that, you're the pilot, you decide what works best for you.
Outside of that, you're the pilot, you decide what works best for you.
I always liked the v low level, v max, downwind approach, followed by a pull up to a wingover around 250ft, spiral descent down to the low hover or 0/0 on the spot. Always surprised the marshaller that one!
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Originally Posted by [email protected]
Still in denial aa777888?? Do you think the rotor vortices disappear when you are further from the ground??
I'm sure you can find a figure in Prouty that shows rotor vortices, but that certainly isn't the one!
For someone who demands strict accuracy and precision from others, crab, or at least appears to, I would expect better.
Absolutely, positively not. But that figure does not show rotor vortices. Not only is entirely clear from the drawings in the figure, the damn figure is labeled "Effect of Ground Vortex on Inflow Patterns" (emphasis mine).
The ground vortex in the diagrams is caused by the rotor and the point I am still trying to make is that you have to get through those vortices to get to ETL - it is just more noticeable nearer the ground because the rotor wake doesn't contract as it would in a free air hover. And then as you reduce speed, those same vortices impinge on the rotor. In both cases they cause vibration. Remember what I said about a boundary for ETL.
As I said before - just go out and try my slow and steady transition to forward flight and tell me if it rolls BEFORE the vibration or not. Perhaps you will then acknowledge that transverse flow just produces roll, not vibration.
And before you say we are supposed to be talking about an approach - the same rotor vortices are what the blades are bumping into to cause the vibration when you make an approach just above ETL.
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Originally Posted by [email protected]
So you think the ground has a vortex????
This describes the effect in the Prouty figure in much more detail.
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The flow around a helicopter is very complex; it becomes much more complex when it comes close to the ground. The presence of the ground changes the aerodynamic characteristics of the rotor and the flow environment becomes much more complex compared with that of flight out-of-ground effect (OGE) and hence the behavior of the rotor wake in the vicinity of the ground is challenging to predict. Under in-ground-effect(IGE) conditions, the wake collides with the ground and causes a significant perturbation to the flow near the blade. Significant interactions between the main rotor wake and the ground have been associated with the formation and passage of the ground vortex in forward flight. The presence of a ground vortex affects the handling qualities of the helicopter.
This describes the effect in the Prouty figure in much more detail.
The behavior of the rotor wake in the vicinity of the ground is challenging to predict. Under in ground effect(IGE) conditions, the wake collides with the ground and causes a significant perturbation to the flow near the blade. Significant interactions between the main rotor wake and the ground have been associated with the formation and passage of the ground vortex in forward flight. When the helicopter encounters a ground vortex, the stability of the the aircraft is degraded.