VSI at end of an approach
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VSI at end of an approach
Does anyone have any idea as why a standard (read massive delay) VSI will show an increase in descent rate right as you are coming to a hover at the end of an approach. I noticed this when I saw my students doing steep approaches and was waiting to bust them for busting the 300fpm (im a bastard what can I say). Anyway even when they held a perfect descent all the way in and actually as they were applying power to slow the descent rate, the VSI would show an increase of about 100fpm. I know its not something deadly important to flying just wondering the reason.
My only guess is maybe ground effect...?
Thanks,
Cooper
My only guess is maybe ground effect...?
Thanks,
Cooper
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The pressure capsule is not sealed, and the bellow leaks air(at a slow rate). The rate of air leakage is proportional to the rate of climb or descent. The bellows are mechanically connected to the indicating needle. Delays (is it sticky in general ?) might occur due to the instrument being worn out.
But abrubt altitude changes WILL cause reverse sensing.
My guess is, you've got a VSI that needs replacement!
The bellows don't like debris, especially if you fly doors off in dusty areas. Amazing what you can collect !!
Then again, it might be groundeffect. Try it out at altitude. If it behaves the same way as close to the ground, replace the VSI and buy all your students beer :-)
But abrubt altitude changes WILL cause reverse sensing.
My guess is, you've got a VSI that needs replacement!
The bellows don't like debris, especially if you fly doors off in dusty areas. Amazing what you can collect !!
Then again, it might be groundeffect. Try it out at altitude. If it behaves the same way as close to the ground, replace the VSI and buy all your students beer :-)
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Jcooper,
The natural wash in a hover will cause a momentary dip in the VSI, and also in the altimeter. Remember that the static ports are usually in clean air until just about hover speed, when they become immersed in the rotor wash. Some flight manuals have the altimeter calibration curves in them, take a peek and see how speed affects the static ports of helicopters. it is not unusual to see a 30 to 40 foot change in the speed range of the helicopter. remember that about 1" of altimeter (mercury) is 1000 feet, so a 40 foot change is .040" of mercury pressure change. if 30 inches of mercury is 15 pounds per square inch (close enough in my book to 29.92 inches and 14.7 psi!), then 1 inche = 1/2 psi, or .040 inches = .02 psi. Small potatoes!
We covered the whole ground effect thing a while back here on pprune. While there was no consensus (unlike California, where there seems to be consensus.....better not go there!), some of us strongly advocated that there is no pressure bubble under the helicopter in a hover (the belief otherwise is the kind of intuition that helps us think about hover ground effect, but is actually not true aerodynamically.)
The natural wash in a hover will cause a momentary dip in the VSI, and also in the altimeter. Remember that the static ports are usually in clean air until just about hover speed, when they become immersed in the rotor wash. Some flight manuals have the altimeter calibration curves in them, take a peek and see how speed affects the static ports of helicopters. it is not unusual to see a 30 to 40 foot change in the speed range of the helicopter. remember that about 1" of altimeter (mercury) is 1000 feet, so a 40 foot change is .040" of mercury pressure change. if 30 inches of mercury is 15 pounds per square inch (close enough in my book to 29.92 inches and 14.7 psi!), then 1 inche = 1/2 psi, or .040 inches = .02 psi. Small potatoes!
We covered the whole ground effect thing a while back here on pprune. While there was no consensus (unlike California, where there seems to be consensus.....better not go there!), some of us strongly advocated that there is no pressure bubble under the helicopter in a hover (the belief otherwise is the kind of intuition that helps us think about hover ground effect, but is actually not true aerodynamically.)
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Well the reason I asked is that its in a robbie and the static tube is nice hidden in the cowling by the MR gearbox in which case i couldn't really see the reason for the dip unless there was a pressure differential that occured as you went into a hover. Do you still say the static tube is being immersed in the downwash even though it is nicely hidden?
Kind of a side question here but if ground effect has nothing to do with a pressure build up under the airfoil what is the other explanation? Ive heard that the tip vortices are reduced or slowed down but then this explanation wouldn't apply to fixed wing aircraft.
Kind of a side question here but if ground effect has nothing to do with a pressure build up under the airfoil what is the other explanation? Ive heard that the tip vortices are reduced or slowed down but then this explanation wouldn't apply to fixed wing aircraft.
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Next time you lift to the hover take a look at the VSI, it will indicate that you are DESCENDING. Why?
The increase in collective pitch as you raise the collective results in an increase in the total mass flow through the rotor disc, this air flow creates a a divergent duct (between rotor disc and the ground) which results in an increase in pressure. The same phenomena occurs when you come to the hover post transition from forward flight; how marked this is will depend on approach profile, wind strength, all up mass, etc.
Hope that helps.........
The increase in collective pitch as you raise the collective results in an increase in the total mass flow through the rotor disc, this air flow creates a a divergent duct (between rotor disc and the ground) which results in an increase in pressure. The same phenomena occurs when you come to the hover post transition from forward flight; how marked this is will depend on approach profile, wind strength, all up mass, etc.
Hope that helps.........
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This item makes me wonder what instructors are taught and what they discover through attention to detail when flying.
As part of a prestart up check the instruments are checked for damage and serviceability which includes the Altimeter and VSI.
Is it not part of the routine to zero the altimeter? I'm sure it is or ought to be. The engine and rotors are started and if you are attentive with your after start checks you will notice that the reading on the altimeter has changed, it now shows a minus altitude (the pressure under the rotor disc has increased). You are unlikely to see any change in the VSI as the indication of RoD (Rate of Descent) will occur quickly.
Lifting off into a low hover will further increase the pressure under the disc and a further minus altitude will be indicated. Move forward and at about the same time as you loose ground effect and gain translational lift the altimeter will show the actual height and as you climb away the altimeter will operate as you would expect.
In the descent is it not reasonable to expect some reversal of this occurrence?
The VSI reacts to pressure change over time. The time involved in the final part of an approach is short and the pressure change is rapid, hence the VSI indication. The fact that the static port is under a cowling will not change the situation. Pressure change is pressure change!
If you really want to be a bastard, why don't you chase your student for miss setting the altimeter, as it shows an error when hovering.
As part of a prestart up check the instruments are checked for damage and serviceability which includes the Altimeter and VSI.
Is it not part of the routine to zero the altimeter? I'm sure it is or ought to be. The engine and rotors are started and if you are attentive with your after start checks you will notice that the reading on the altimeter has changed, it now shows a minus altitude (the pressure under the rotor disc has increased). You are unlikely to see any change in the VSI as the indication of RoD (Rate of Descent) will occur quickly.
Lifting off into a low hover will further increase the pressure under the disc and a further minus altitude will be indicated. Move forward and at about the same time as you loose ground effect and gain translational lift the altimeter will show the actual height and as you climb away the altimeter will operate as you would expect.
In the descent is it not reasonable to expect some reversal of this occurrence?
The VSI reacts to pressure change over time. The time involved in the final part of an approach is short and the pressure change is rapid, hence the VSI indication. The fact that the static port is under a cowling will not change the situation. Pressure change is pressure change!
If you really want to be a bastard, why don't you chase your student for miss setting the altimeter, as it shows an error when hovering.
