constant speed or variable speed approach
AC - I know where you are coming from but you can't disregard the H-V curve just because you aren't in level flight - you are slightly better off being in a descent as the power is a bit lower but not in the latter stages as ETL is lost.
aa777888
have to disagree with that - the vibration is ETL not transverse flow and you can tell by the lower power riding that vibration that you are not below ETL. You can show a demo of transverse flow in a transition into forward flight - it starts much earlier than ETL and there is no vibration until just before ETL (you are powering through the vortices to get through ETL) and then the nose pitches up due to flapback.
As for single vs twin - it doesn't matter unless you are doing PC1 profiles in a twin.
aa777888
Except that when you feel that vibration you are already below ETL and what you are feeling is the vibration from the transverse flow effect.
As for single vs twin - it doesn't matter unless you are doing PC1 profiles in a twin.
Join Date: Apr 2010
Location: USA
Posts: 850
Likes: 0
Received 0 Likes
on
0 Posts
Crab: according to the FAA Helicopter Flying Handbook, page 2-23:
This vibration happens at an airspeed just below ETL on takeoff and after passing through ETL during landing. The vibration happens close to the same airspeed as ETL because that is when the greatest lift differential exists between the front and rear portions of the rotor system. As such, some pilots confuse the vibration felt by transverse flow effect with passing through ETL.
This vibration happens at an airspeed just below ETL on takeoff and after passing through ETL during landing. The vibration happens close to the same airspeed as ETL because that is when the greatest lift differential exists between the front and rear portions of the rotor system. As such, some pilots confuse the vibration felt by transverse flow effect with passing through ETL.
Thread Starter
Great feedback, the point related in the answers have concentrated what has been running in my head for a long time.
Good point I concur from Ascend Charlie: Let the collective-up increase your flap back and kill your speed as you keep apparent speed above your toes constant, keeping the apparent speed constant is effectively a gradual deceleration because the closer you get to the ground the faster is the apparent speed is. I got that well practiced and it looks gorgeous, so when I want to play it super safe (family is on board or whatever) I request the runway and make it to the number in variable speed, I can abort at any time by a gentle push forward. But there are those few second where the speed is below the green arc and the height is too high for an engine misfortune at that speed (single).
Good point I got form FH1100 Pilot, So I have been practicing hours of variable speed approach and yes the timing has to be exquisite, my interpretation is that when it is perfectly balanced it is indeed very close to an autorotative approach and could bring you to touch down safely more or less regardless of power reserve. But then there had been time when the wind changes direction or the tarmac on a hot summer day keeps a few meters of high temperature air by radiation of the black surface. And then your great timing yet sends you sinking and clenching your teeth.
Robbiee: I had that figured out too, check pilot always nod at a fine variable speed approach, the other always prefer to save 2 min on the odometer.
Good point I concur from Ascend Charlie: Let the collective-up increase your flap back and kill your speed as you keep apparent speed above your toes constant, keeping the apparent speed constant is effectively a gradual deceleration because the closer you get to the ground the faster is the apparent speed is. I got that well practiced and it looks gorgeous, so when I want to play it super safe (family is on board or whatever) I request the runway and make it to the number in variable speed, I can abort at any time by a gentle push forward. But there are those few second where the speed is below the green arc and the height is too high for an engine misfortune at that speed (single).
Good point I got form FH1100 Pilot, So I have been practicing hours of variable speed approach and yes the timing has to be exquisite, my interpretation is that when it is perfectly balanced it is indeed very close to an autorotative approach and could bring you to touch down safely more or less regardless of power reserve. But then there had been time when the wind changes direction or the tarmac on a hot summer day keeps a few meters of high temperature air by radiation of the black surface. And then your great timing yet sends you sinking and clenching your teeth.
Robbiee: I had that figured out too, check pilot always nod at a fine variable speed approach, the other always prefer to save 2 min on the odometer.
aa777888 - is that the same FAA handbook that called settling with power and VRS the same thing?
Go and try a transition to forward flight on a still wind day from a 15' hover - don't change the power but move the cyclic very slightly forward. You will start to descend as you have tilted the lift vector, then as the speed starts to increase you get inflow roll (transverse flow) which makes the aircraft roll towards the advancing side - correct that with lateral cyclic and you will next feel the vibration of ETL as the rotor passes through the roll-up vortices of the downwash - then the nose pitches up and the aircraft climbs as the rotor experiences cleaner air.
Transvers flow is a flapping to equality in roll that exists throughout the speed range but is most noticeable just after the disc is tilted forwards to initiate a transition. Not associated with vibration.
Go and try a transition to forward flight on a still wind day from a 15' hover - don't change the power but move the cyclic very slightly forward. You will start to descend as you have tilted the lift vector, then as the speed starts to increase you get inflow roll (transverse flow) which makes the aircraft roll towards the advancing side - correct that with lateral cyclic and you will next feel the vibration of ETL as the rotor passes through the roll-up vortices of the downwash - then the nose pitches up and the aircraft climbs as the rotor experiences cleaner air.
Transvers flow is a flapping to equality in roll that exists throughout the speed range but is most noticeable just after the disc is tilted forwards to initiate a transition. Not associated with vibration.
Agile - if you have your lift vector tilted backwards - a decelerative attitude - raising the collective increases that vector and slows you down as well as reducing your RoD. As you have probably discovered, if you get it right, you can hold the same decelerative attitude all the way from the top of descent to just before the hover and control the angle of descent with lever.
Thread Starter
don't change the power but move the cyclic very slightly forward. You will start to descend as you have tilted the lift vector, then as the speed starts to increase you get inflow roll (transverse flow) which makes the aircraft roll towards the advancing side - correct that with lateral cyclic and you will next feel the vibration of ETL as the rotor passes through the roll-up vortices of the downwash - then the nose pitches up and the aircraft climbs as the rotor experiences cleaner air.
raising the collective increases that vector and slows you down as well as reducing your RoD. As you have probably discovered, if you get it right, you can hold the same decelerative attitude all the way from the top of descent to just before the hover and control the angle of descent with lever
best is to manage to couple collective movement with longitudinal cyclic twick, not there yet.
Agile - the nose pitch up is flapback which has to be overcome with forward cyclic to continue the acceleration.
Controlling angle of descent with collective while reducing speed gradually is one of the more difficult things to get students to do properly - there is a strong temptation to keep the landing point in the same place by moving the cyclic which just changes the speed.
As you say, in anything but smooth conditions, it requires constant small adjustments of cyclic and collective to fly the approach accurately.
Controlling angle of descent with collective while reducing speed gradually is one of the more difficult things to get students to do properly - there is a strong temptation to keep the landing point in the same place by moving the cyclic which just changes the speed.
As you say, in anything but smooth conditions, it requires constant small adjustments of cyclic and collective to fly the approach accurately.
To answer the original question: It depends on where you are, what you're trying to achieve and who's watching!
Flying a variable speed approach to the numbers of a runway, then taxiing 1000m at a walking pace is pointless and expensive (unless your instructor asked you to) so a constant speed approach/air taxi/quick stop might be more fun and more appropriate.
Flying over forest into a small confined area requiring a near vertical descent from a high hover would obviously be much safer with a variable speed approach.
I'm guessing most flights are somewhere in between. It's a magic carpet so put it where you need it?
Flying a variable speed approach to the numbers of a runway, then taxiing 1000m at a walking pace is pointless and expensive (unless your instructor asked you to) so a constant speed approach/air taxi/quick stop might be more fun and more appropriate.
Flying over forest into a small confined area requiring a near vertical descent from a high hover would obviously be much safer with a variable speed approach.
I'm guessing most flights are somewhere in between. It's a magic carpet so put it where you need it?
I think the Attitude=Airspeed/power=ROD simplification stems from the fact that we can operate on both sides of the power curve, but it sure is a whole lot easier to level-off with cyclic, then set the speed with collective!
Interestingly in a Jet, speed is throttle and pitch maintains altitude, but climbing or descending speed is pitch and throttle is ROC. Unless you select V/S when speed is throttle again, and ROC is pitch!
Originally Posted by [email protected]
aa777888
Go and try a transition to forward flight on a still wind day from a 15' hover - don't change the power but move the cyclic very slightly forward. You will start to descend as you have tilted the lift vector, then as the speed starts to increase you get inflow roll (transverse flow) which makes the aircraft roll towards the advancing side - correct that with lateral cyclic and you will next feel the vibration of ETL as the rotor passes through the roll-up vortices of the downwash - then the nose pitches up and the aircraft climbs as the rotor experiences cleaner air.
Go and try a transition to forward flight on a still wind day from a 15' hover - don't change the power but move the cyclic very slightly forward. You will start to descend as you have tilted the lift vector, then as the speed starts to increase you get inflow roll (transverse flow) which makes the aircraft roll towards the advancing side - correct that with lateral cyclic and you will next feel the vibration of ETL as the rotor passes through the roll-up vortices of the downwash - then the nose pitches up and the aircraft climbs as the rotor experiences cleaner air.
Ah Crab - I'm straight back to Shawbury about 30 years ago!
You can have full climb power, but if you're pointing down you will still go down.
If you set constant power for level flight you can still climb and descend just using the cyclic.
For any airspeed, there is one power setting that gives zero ROD = level flight. But for one power setting, there are usually 2 airspeeds for level flight. White man magic, ain't it?
Join Date: Apr 1998
Location: Mesopotamos
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
Attitude = airspeed
Power = Rate of descent.
Power = Rate of descent.
Also works quite well in a helicopter on long finals for a decelerating speed landing, same principle, just the vectors are pointing differently.
Used to be quite good at setting an attitude and selecting power levels from miles out and getting close enough to the pad with barely any input. Sadly those skills are now not what they used to be.
Ah Crab - I'm straight back to Shawbury about 30 years ago!
Join Date: Apr 2010
Location: USA
Posts: 850
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by [email protected]
aa777888 - is that the same FAA handbook that called settling with power and VRS the same thing?
Go and try a transition to forward flight on a still wind day from a 15' hover - don't change the power but move the cyclic very slightly forward. You will start to descend as you have tilted the lift vector, then as the speed starts to increase you get inflow roll (transverse flow) which makes the aircraft roll towards the advancing side - correct that with lateral cyclic and you will next feel the vibration of ETL as the rotor passes through the roll-up vortices of the downwash - then the nose pitches up and the aircraft climbs as the rotor experiences cleaner air.
Transvers flow is a flapping to equality in roll that exists throughout the speed range but is most noticeable just after the disc is tilted forwards to initiate a transition. Not associated with vibration.
Transvers flow is a flapping to equality in roll that exists throughout the speed range but is most noticeable just after the disc is tilted forwards to initiate a transition. Not associated with vibration.
Every reference I've read to date says the vibration is due to transverse flow and not ETL. Therefore they are ALL wrong and you are right, or you are wrong and all of them are correct.
Of course I am by no means an aerodynamicist, therefore I must rely on what these references tell me is so. Can you provide a written reference (other than your own!) that does not say that these vibrations are due to transverse flow prior to accelerating into ETL, or the same but after decelerating out of ETL?
Just go and try what I suggested. If you do it smoothly you will encounter transverse flow (inflow roll) towards the advancing side with NO vibration. It is due to the difference in the inflow angles at the front compared to the rear of the rotor disc.
You know the rotor produces vortices at the tips right? Those vortices are what cause the vibration as you transition to and from the hover because the rotor has to battle through them into clear air - that marks the onset of ETL and as the vibration clears the aircraft pitches nose up and climbs due to flapback and the increase in lift. The vibration marks the boundary of ETL and that is why, once you are through it - it doesn't keep vibrating.
If transverse flow was the reason for the vibration, you WOULD feel that throughout the flight envelope whenever you tilt the disc and create an inequailty of lift.
Don't confuse flapback (blowback) with transverse flow (inflow roll).
You know the rotor produces vortices at the tips right? Those vortices are what cause the vibration as you transition to and from the hover because the rotor has to battle through them into clear air - that marks the onset of ETL and as the vibration clears the aircraft pitches nose up and climbs due to flapback and the increase in lift. The vibration marks the boundary of ETL and that is why, once you are through it - it doesn't keep vibrating.
If transverse flow was the reason for the vibration, you WOULD feel that throughout the flight envelope whenever you tilt the disc and create an inequailty of lift.
Don't confuse flapback (blowback) with transverse flow (inflow roll).
To get back to the question, I would suggest the approach you use will depend on
1. Experience level of pilot, seen far too often people hacking into the airfield , flash quick stop with tail wagging al over the place
2. Situation eg wind, temp weight of aircraft power margin
Go back to your last lesson when being taught the recce of a confined area ie the 5 s that will then dictate the correct approach for each situation. Basically there is no right or wrong, just degrees !!!
1. Experience level of pilot, seen far too often people hacking into the airfield , flash quick stop with tail wagging al over the place
2. Situation eg wind, temp weight of aircraft power margin
Go back to your last lesson when being taught the recce of a confined area ie the 5 s that will then dictate the correct approach for each situation. Basically there is no right or wrong, just degrees !!!
aa777888 - this is from Prouty - note where the vortex is at 15 to 20 kts (ie the onset of the vibration and the boundary of ETL) and then where the vortex is when you experience transverse flow (5 to 10 kts)
Apologies for the large size of the picture - don't know how to resize it on pprune.
Apologies for the large size of the picture - don't know how to resize it on pprune.
And now consider your previous comment about how the vibration is transverse flow as you reduce speed on the approach.
You perhaps will agree that in the transition to forward flight, the disc is tilted forward and the difference in inflow angle (small angle at the front and large at the back) is the cause of the transverse flow effect (I say it rolls, you say it vibrates).
If so, what causes the change in inflow angle in a steady, speed reducing descent when the disc attitude doesn't change and the inflow angles are much the same (but from underneath the disc).
The vibration is the boundary of ETL - there isn't 'fully developed ETL' it is either there or it isn't.
You perhaps will agree that in the transition to forward flight, the disc is tilted forward and the difference in inflow angle (small angle at the front and large at the back) is the cause of the transverse flow effect (I say it rolls, you say it vibrates).
If so, what causes the change in inflow angle in a steady, speed reducing descent when the disc attitude doesn't change and the inflow angles are much the same (but from underneath the disc).
The vibration is the boundary of ETL - there isn't 'fully developed ETL' it is either there or it isn't.
Join Date: Apr 2010
Location: USA
Posts: 850
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by [email protected]
that marks the onset of ETL and as the vibration clears the aircraft pitches nose up and climbs due to flapback and the increase in lift.
If so, what causes the change in inflow angle in a steady, speed reducing descent when the disc attitude doesn't change and the inflow angles are much the same (but from underneath the disc).
The vibration is the boundary of ETL - there isn't 'fully developed ETL' it is either there or it isn't.