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Weight VS Descent Rate-Autorotation


heliflyknow

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I have a question that I hope some enlightened person can answer. During an autorotation will your descent rate increase with more weight or decrease?

 

If you performed an auto at minimum weight, and then picked up enough pax/cargo to put you at maximum weight would you have an increase in descent rate or a decrease, or is weight not a factor?

 

In the Robbie POH it says that at low weight full down collective may not keep RRPM's in the green(low), so conversely it seems that high weight would require more collective.

 

I know gravity pulls any object down at the same rate regardless of weight, but it seems if you have more pitch in the blades you would descend slower.

 

An auto requires upward airflow/descent, so does that mean you have to counteract the increase in downward force(weight) with more collective? Does that mean you have the same descent rate regardless of weight?

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You will find that once you have the RRPM controlled at top of the green, the stabilised descent rate is fairly consistent, but the lever position will differ - as you surmised, a heavy machine has the lever under your armpit to control the revs, particularly at altitude. It just means you have to milk the lever down in the flare to regain some cushioning potential, otherwise a big fall-through and an OUCH at the bottom.

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Great question. I've been asking this for a long time and can't get a good answer. In the R44, heavy is a fairly steep angle in autorotation with high rate of descent. Then mid-weight is a lower rate of descent. Then light weight goes back to a high rate of decent and steep angle. AS350 does the same.

 

I believe it has to do with collective position. When the heli is very light, collective has to stay full down to keep the RPM at the bottom of the green.

 

Interested to hear some ideas...

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Ray Prouty addresses this in his book "Helicopter Aerodynamics vol. 2"

 

He states the heavier helicopter will have a slower descent rate in autorotation. Even Frank Robinson will confirm this in the R-22.

 

Rate of descent should depend on how much change in potential energy per second is needed to generate enough power to produce rotor thrust which is equal to gross weight in unaccelerated flight.

 

We've all seen the HP required curve with varying weight which is very similar to HP required in autorotation.

 

The approximate ROD in FPM can be found by multiplying required HP by 33,000 and then dividing by gross weight. Any time an increase in HP required is less than proportional to the increase in gross weight, it will result in a smaller ROD at high gross weights than at low gross weights.

 

Flight tests also confirm this. This is very counter-intuitive at first!

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I have a question that I hope some enlightened person can answer. During an autorotation will your descent rate increase with more weight or decrease?

 

An auto requires upward airflow/descent, so does that mean you have to counteract the increase in downward force(weight) with more collective? Does that mean you have the same descent rate regardless of weight?

 

 

  • Rate of descent should depend on how much change in potential energy per second is needed to generate enough power to produce rotor thrust which is equal to gross weight in unaccelerated flight.

 

  • Any time an increase in HP required is less than proportional to the increase in gross weight, it will result in a smaller ROD at high gross weights than at low gross weights.

 

 

In a stable autorotation an energy balance must exist where the potential energy (m x g x h) feeding the rotor just balances the sum of the induced, profile, and parasite losses of the rotor. In other words, potential energy must balance gross weight.

 

Don't forget, the additional weight also increased the potential energy.

 

Scan-1_zps3d309c8e.jpg

Edited by iChris
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For once I'm not exactly sure what iChris is illustrating.

Manhattan explained how Ray Prouty sees it, to wit: increases in gross weight slow your (autorotative) descent.

As Manhattan states, this is very counterintuitive. Fixed wing increases in gross weight decrease (engine out) glide performance.

Intuition tells me the same should apply to rotary wing.

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For once I'm not exactly sure what iChris is illustrating.

Manhattan explained how Ray Prouty sees it, to wit: increases in gross weight slow your (autorotative) descent.

As Manhattan states, this is very counterintuitive. Fixed wing increases in gross weight decrease (engine out) glide performance.

Intuition tells me the same should apply to rotary wing.

 

It's exactly the same as Ray Prouty. However, Manhattan's post did not accounts for the boundary point or crossover, were the reverse takes place.

 

Remember the Ray Prouty statement from Manhattan's post (it's conditional):

 

 

 

  • Any time an increase in HP required is less than proportional to the increase in gross weight, it will result in a smaller ROD at high gross weights than at low gross weights.

 

Not until an airplane reach its optimum cruise speed will the induced drag ceases to dominant. Airplanes usually glide at lower airspeeds than cruise and will not see a reduction in rate of descent with an increase in weight.

 

Again, In a stable autorotation an energy balance must exist where the potential energy (m x g x h) feeding the rotor just balances the sum of the induced, profile, and parasite losses of the rotor. In other words, potential energy must balance gross weight.

Edited by iChris
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It's exactly the same as Ray Prouty. However, Manhattan's post did not accounts for the boundary point or crossover, were the reverse takes place.

 

Remember the Ray Prouty statement from Manhattan's post (it's conditional):

 

 

Not until an airplane reach its optimum cruise speed will the induced drag ceases to dominant. Airplanes usually glide at lower airspeeds than cruise and will not see a reduction in rate of descent with an increase in weight.

 

Again, In a stable autorotation an energy balance balance must exist where the potential energy (m x g x h) feeding the rotor just balances the sum of the induced, profile, and parasite losses of the rotor. In other words, potential energy must balance gross weight.

You're absolutely correct, Chris. It is conditional. The relationship between weight and ROD will not approach zero and is nonlinear of course. I'd upload Ray's flight test data points if I were near a scanner. Also, where did you find that graphic?

 

Aeroscout, I would be careful trying to compare FW as the engine out aerodynamics are quite different. In fixed wing your AOA is (hopefully) fixed at best L/D and indicated airspeed will vary with weight while glide angle will remain constant. Not so with Rotorcraft.

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Fixed wing increases in gross weight decrease (engine out) glide performance.

Intuition tells me the same should apply to rotary wing.

Although counterintuitive, this is not true which is why in competition sailplanes they load up with water ballast to increase gross weight. The heavier weight increases not only the speed of L/D max but also the glide ratio.

 

Had this discussion many times flying wide body jets as well. Bring the thrust back to idle at top of descent at light weights versus heavy and the you could demonstrate very clearly that rate of descent and glide angle were significantly higher and steeper when light than at heavy weights.

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What the graphic iChris posted is saying is that rate of descent varies with airspeed. Duh!

If I am reading iChris's chart correctly at 70 knots the 16k pound helo has the same descent rate as the 20k pound helo.

Below that airspeed the heavier helo has a higher ROD. This appeals to me.

Above 70 knots the heavier helo has a slower ROD. I don't like that part of the chart. Not one bit.

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Done the comparison in a Black Hawk several times and it conflicts with the -10 autorotational chart. At approx. 14,500 lbs and 80kts the aircraft's ROD was just under 2,300 FPM. That's around a 500 ft DA (Rucker). At approx. 16,500 lbs and 80 kts the aircraft's ROD was around 2,000 FPM. That was approx a 5,000 ft DA (Afghanistan). Both rotors were kept at 105 % with the heavier aircraft needing collective to maintain it.

 

Doesn't seem logical but Ray Prouty is correct. The thrust required in an auto is not proportional to it's potential energy. Seen it first hand.

 

I was just reading an old thread online from a guy who did the same thing with an S-61 in the late 90s. Got basically the same results that I got.

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  • 2 weeks later...

If I am reading iChris's chart correctly at 70 knots the 16k pound helo has the same descent rate as the 20k pound helo.

 

Below that airspeed the heavier helo has a higher ROD. This appeals to me.

 

Above 70 knots the heavier helo has a slower ROD. I don't like that part of the chart. Not one bit.

 

Again, in a stable autorotation an energy balance must exist where the potential energy (mass x gravity x height) feeding the rotor must balances the sum of the induced, profile, and parasite losses of the rotor. In other words, potential energy must balance gross weight.

 

Out of the three components (induced power, profile power, and parasite power) only induced power is a main function of gross weight, and as you can see from the power-required curve in forward flight, its proportion of the totally power required becomes smaller as forward speed increases.

 

You can see the proportional miss-match form the table below when you compare cases #3 and #4 against cases #1.

 

Case #1 is a 100% increase in gross weight; however, it’s also yields 100% increase in potential energy.

 

Moreover, that 100% increase in gross weight in case #1 only required a 37% increase in power required over case #3 and 68.5% increase in power required over case #4.

 

However, things can swing the opposite direction under high altitudes/high gross weight conditions

 

CLICK PHOTO TO ENLARGE

 

PowerRequiredCurve_zps719f4728.jpg

 

RateofDesentvsgrossWeight_zps4c98aed5.jp

 

Scan-1_zps3d309c8e.jpg

Chart references:

Helicopter Performance Stability & Control

pg. 130 & pg. 139 – Ray Prouty

 

Principles of Helicopter Aerodynamics 2nd Edition

pg. 247 – J. Gordon Leishman

Edited by iChris
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