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MD500 RoD in Autorotation

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MD500 RoD in Autorotation

Old 4th May 2018, 11:30
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MD500 RoD in Autorotation

Chaps,

Could any of our regular MD500 pilots give me an idea of the aircrafts 'typical' rate of decent in a standard autorotation please and an account of its behaviour in Auto? (General handling, range and inertia)

Many thanks,
CRAN
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Old 4th May 2018, 21:58
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well really depends if you are talking c model d/e model or Notar

You are looking at around 1800 ft a min for a c model and around 2200 for a D. Does depend on how heavy and how the auto revs are set.
I now teach in a D model at 125 kts at around 1000ft shut the throttle hold the lever up at the cruise and haul back on the cylic. The aircraft will climb at about 500 ft a minute for around 5 o 10 seconds. As she reaches 80 kts ( best range speed )and the bottom of the green arc lower the lever, this drops the nose and allows speed to build up again and will allow you to haul back the cylic which will increase rrpm back to around 480 and a rate of descent will build up to around 2000ft a minute as you get back to 60 kts. Range with this technique is a good 1 km plus.
Hope that helps
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Old 5th May 2018, 07:01
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Angel

Originally Posted by Hughes500 View Post
well really depends if you are talking c model d/e model or Notar

You are looking at around 1800 ft a min for a c model and around 2200 for a D. Does depend on how heavy and how the auto revs are set.
I now teach in a D model at 125 kts at around 1000ft shut the throttle hold the lever up at the cruise and haul back on the cylic. The aircraft will climb at about 500 ft a minute for around 5 o 10 seconds. As she reaches 80 kts ( best range speed )and the bottom of the green arc lower the lever, this drops the nose and allows speed to build up again and will allow you to haul back the cylic which will increase rrpm back to around 480 and a rate of descent will build up to around 2000ft a minute as you get back to 60 kts. Range with this technique is a good 1 km plus.
Hope that helps
Hey H500...very interesting, thanks! Why is the C have lower ROD compared to the D when the D has 1 extra MR blade?? & how about the E & the 530FF? Amazing machines, the sexiest machines by far!
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Old 5th May 2018, 10:39
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Originally Posted by Vertical Freedom View Post
Why is the C have lower ROD compared to the D when the D has 1 extra MR blade?? & how about the E & the 530FF? Amazing machines, the sexiest machines by far!
I dont know anything specific about the 500’s, but as aerodynamics and kinetics decide things it mainly works like this:
-For the same type, heavier makes rate of descent decrease. There is more potential energy stored in altitude, which can be used to drive the rotor/ produce lift. And the other way, Lower weights means higher rate of descend.
-Between aircraft types, a few number of long blades makes the power needed go down, and many short blades makes power needed go up. For same weight the helo with fewer and longer blades will be conservative of the stored potential energy from altitude, and need less rate of descend to drive the rotor. Same weight but higher number of blades and shorter, more power needed will make rate of descent higher to get the power to drive the rotor.
”Solidity” is a term for how much the blade area cover from the total disc area. High solidity = high rate of descent, low solidity low rate of descent.
Rate of descent will often be in the 1500-2500fpm range. If I recall the Bell 206, it was around 1500fpm?
As a guess, R22/R44 should also be in the low range.

Also rate of descend (ROD) is not a fixed value. Changing speed and rpm in autorotation changes ROD. At the Vy speed the ROD will be lowest, and att high rpm and VNE/ for auto, it will be highest.

The values given, and also the answer from H500, would be in ”normal” auto, normal IAS and normal Rotor RPM.

Last edited by AAKEE; 5th May 2018 at 10:43. Reason: Spelling
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Old 5th May 2018, 13:17
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To establish a steady state, ie constant RoD in auto, the mass must be balanced by the rotor thrust - therefore the higher the mass, the higher the RoD required to maintain Nr.

You are trading potential energy for kinetic energy as you descend.
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Old 5th May 2018, 14:18
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If I read it correctly, then aakee and crab saying the exact opposite of each other.

Like the age old saying goes: You go see two doctors, and get three different diagnoses :-)
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Old 5th May 2018, 14:34
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Stick with Crab. You should have been demonstrated this during basic instruction. Start the sortie enter auto record the ROD and RRPM. At the end of the sortie when fuel burned off repeat and observe that ROD and RRPM are less. Similarly you should have been warned never to fly at less than minimum specified weight as you may not generate sufficient RRPM to establish autorotation, hence potential disaster.
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Old 5th May 2018, 15:58
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I think Crab and Pof need to do some research to establish what you most sure did know, a long time ago.
It will be very easy to show that my post is correct.

Just do some googling, im sure it will be easy to find.

I know R.W.Prouty says the same thing as I do, I have all the books. If I remember it right, most of his knowledge is(was?) online via some rotorcraft magazine?
I also can show some maintenance test charts for verifying rigging of NR during autorotation. They also tell the same thing.

I hope you do a check before I have to prove the case :-)
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Old 5th May 2018, 18:20
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Just an aside and I think I have talked about this before:
In Olden Daze.
Springtime in Canada is training season for bush pilots.

We trained with 205- 206- 206L, As 350.
Firstly we made sure the aircraft used were adjusted for proper RPM regime in the now, warm WX. (+15-20C vs winter -35, -20C ) All autos were full ons.
We noticed that some pilots were not totaly comfortable in autos initiated from below 1000AGL....heck they only got to do this once a year so competely understandable.
So we started, when someone was uncomfortable to climb to 4-5000 AGL , make a very easy entry into the auto and just let the fellow fly the aircraft in automode at various speeds and RPMs for Min Rate, Max range ect. while making turns and concentrating only on Speed and RPM control ( as IP you took over responsibility of making sure you were over a suitable area in case the donkey resigned. LOL)
This worked very well and improved their confidence level very quickly. Seldom had to do it more than once or twice.
Just a thought.
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Old 5th May 2018, 19:40
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Sounds like Pofdude might need a new handle...
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Old 5th May 2018, 20:02
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Aakee and Crab,

If you google the UH-60 MTF ( Maintenance Test Flight Manual ) and look at chart 5-1 in the back, it will answer your discussion.The one that is online is a 1997 chart, but it illustrates the underlying principles you are after.It reflects a check done at flat pitch and is therefore a crosscheck on low pitch rigging. Data base is the same used for the production flight test aircraft.
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Old 5th May 2018, 20:38
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Aakee:
It will be very easy to show that my post is correct.
So you are saying that a helicopter at minimum all up mass will descend in autorotation at a slower rate than one at max all up mass?
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Old 5th May 2018, 20:47
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Look at this test flight report on UH1: Investigation...steady state autorotation

What we talk about can be seen in a number of charts. For example these two:
-Figure 2 /page16: ROD 6700lbs /DA 6000 / 300RPM
-Figure 3/page 17: ROD 8800lbs /DA6000 / 300RPM

At 6700lbs the minimum rate of descent is 1874fpm, and at 90kts 2200fpm.
At 8800lbs the minimum rate of descent is 1866fpm, and at 90 kts 2100fpm.
At Vy, not a big difference but it is a difference. At 90knots, clearly visible.

Red lines in charts compensated for chart not level by creation on the X- and Y-axes.
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Old 5th May 2018, 21:39
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Thanks for digging up that old report Aakee. The pilot, John ( Jay ) Shapley became good friends over the years. First met him when he was a Boeing Test Pilot in 1964, then later on when he worked for the FAA with Sikorsky on various programs. Sadly, Jay is no longer with us. He was a really good test pilot, appreciated by all who were privileged to work with him.

As to the charts, certainly your observation as to the data is correct. BUT, the data was flown to a target NR, or at least it appears to be. Nowhere is the collective position plotted or noted and that I think is the underlying misunderstanding about the subject being discussed.
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Old 5th May 2018, 22:26
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John - looked at the chart you mentioned - just like the one from the Sea King - higher AUM equals higher Nr in autorotation because you are falling faster to extract energy to spin the rotor - then you raise the lever to contain the Nr which helps reduce the RoD - if you didn't the RoD would be higher for a higher AUM
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Old 6th May 2018, 00:05
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Crab just beat me to it.

A heavy machine will initially fall faster, making the rotor RPMl go over the upper limit, so the pilot pulls some pitch to control the revs - slowing down the ROD.

At the bottom of the auto, a light machine will have the lever on the floor, before the flare, with ALL the pitch available to cushion on, but a heavy bird will have the lever much higher with less in reserve - making the flare very important to ease the lever down and recover that ability to cushion on. However, he will also use more lever for the cushion, being heavier.
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Old 6th May 2018, 06:45
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Interestingly just been doing some autos in an OH 6 which has an empty weight of just under 1400lbs, it had a ROD of 1400ft a min, never ever seen 500 with such a low ROD and with rrpm bang in the middle of the green !
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Old 6th May 2018, 07:23
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Aakee - I believe the charts and study you refer to are meant to prove that in order to achieve minimum RoD in autorotation, you must vary the speed depending on AUM.

It seems to show that min RoD is achieved at a slightly higher (than bucket speed) forward speed at high AUM and slightly lower speed at lower AUM.

The rigging charts show that for a given rigging setting for minimum collective pitch, the lower the AUM is, the lower the acceptable Nr in auto is - conversely, the higher the AUM, the higher must be the minimum Nr achieved.

In a vertical autorotation at steady Nr, the only way to further increase that Nr is to increase the RoD and that is what would happen if you suddenly increased the AUM.

Higher AUM = fall faster = higher RoD and higher Nr. Because we have Nr limits, you will contain the Nr by raising the lever. is there any change in steady state RoD at the same Nr between high and low AUM? - never seen a graph to show it but happy for you to prove it is lower at High AUM.

And my copy of Prouty doesn't have the answer - even if there is one it would be a generalisation and not necessarily true for all helicopters given the variation in blade designs and aerofoil sections, even down to the cleanliness of the blades as that markedly affects your profile drag.
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Old 6th May 2018, 07:26
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AUM versus rate of weight

Newbies to big jet flying also find it interesting/confusing that a jet aircraft close to max landing weight must start its descent earlier (by up to 30nm) than one close to empty. A smaller rate of secent at the same Mach .85/340kts descent speed.
Using the 3 times rule with adds/subtracts for wind and weight were standard with us many years ago - don't see them much anymore :/
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Old 6th May 2018, 12:06
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Originally Posted by JohnDixson View Post
Thanks for digging up that old report Aakee. The pilot, John ( Jay ) Shapley became good friends over the years. First met him when he was a Boeing Test Pilot in 1964, then later on when he worked for the FAA with Sikorsky on various programs. Sadly, Jay is no longer with us. He was a really good test pilot, appreciated by all who were privileged to work with him.


Interreseting to read about John Shapley and your friendship, sad to hear he isn’t still here. A part of the helicopter history !

Yes, I also glad that you found the issue why I had the opposite oponion from Crab and Pof. Different references.

Crab and Pof, then we probably agree?

Cran: I guess H500 could give you the numbers for ”normal” autorotation values straight ahead: IAS (would be in the ball park of 60(-70kts) and the ROD coupled to this. From this you can calculate the ”normal” range( I take the range question as glide ratio). I guess the glide ratio would be around 3.5:1 a bit depending where in the envelope this is( some faster than Vy and 100% Nr I guess).
Increasing speed towards best range will stretch the glide about 15-20% I guess. And if decreasing the Nr towards min allowed in auto will add around the same numbers again. With Nr min the L/d curve is changed to more effeicient and you could increase the speed further for max glide.


Last edited by AAKEE; 6th May 2018 at 12:11. Reason: iPhone spells....
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