R22 POH Hover Charts

[01CelicaGTS]

Can anybody tell me why there is a line on the OGE and IGE hover charts that is marked 12600 DA? What does it mean?

[AStarB3]

referencing the Beta II IGE Chart that line shows you that at that pressure alt (represented on the left side of the chart) and temp. (represented by the diagonal lines) you are at 12600 DA. For example if you are at 8000 ft PA (Pressure altitude) and it is 40 C outside you would be at 12600 DA. Also at 10000 ft PA and about 10 C

[r22butters]

Can anybody tell me why there is a line on the OGE and IGE hover charts that is marked 12600 DA? What does it mean?

 

That line just represents where they had to do the test for FAA certification.

[lelebebbel]

... and here is a question for you all:

 

why is this line not vertical?

 

 

in other words, why is the maximum hover weight greater at lower altitude and higher temp, and less at higher altitude and lower temperature, even though both result in the same density altitude?

 

Aren't we told that DA is what determines aircraft performance? Why is performance different, even though DA is the same?

[kodoz]

why is this line not vertical?

 

I've thought about this long enough that the question isn't even making sense to me any more. If weight, temp, and pressure determine performance, then for a given weight, you should be able to hover at the same density altitude, regardless of pressure altitude. The main rotor and tail rotor see the same air density whether it's a 1000 PA or 10000 PA, right? Likewise, the engine should produce the same power at a given DA.

 

What that chart's saying though is that, as pressure altitude increases, your hover performance decreases even as the density altitude stays the same.

 

Hint??

[lelebebbel]

I don't know either. I've emailed Pat Cox at Robinson about this recently but he hasn't replied yet.

 

My guess is that it has more to do with the engine than the airfoils, and that, for some reason, air pressure is more important than temperature for the engine to work properly. Which is why the engine performance depends more on PA than on temperature. This is only a guess though.

 

Maybe a piston engine expert can help us out here.

[01CelicaGTS]

Well, I'm gonna ask some CFIs at school. Our instructor for CFI ground asked us this question, and nobody knew. My guess was that that was the highest they did the tests.

[C of G]

It's been a while since I looked at that chart, but if I'm thinking of it correctly, it's altitude on one side, temp on another, which would make the DA line at an angle. I also seem to recall that is the limit of the derated engine, ie, it's at full throttle. But, again, I haven't looked at that chart in years, so take that for what it's worth. (Not much)

[lelebebbel]

It's been a while since I looked at that chart, but if I'm thinking of it correctly, it's altitude on one side, temp on another, which would make the DA line at an angle

No, that's not it.

 

This is what the charts look like:

 

This one is an R44 chart, couldn't find a R22 one online - the question remains the same, just the numbers are slightly different.

 

 

So, again:

At approx. 11,000ft PA and 0 C, I'm at 11,800ft DA.

According to this chart, I can hover a R44 Raven II in ground effect at about 2,280 pounds gross weight in those conditions.

 

 

At 8,000ft and about 33 or 34 C, I'm also at 11,800ft DA.

According to the chart, I can now hover at 2,380 pounds gross weight.

 

Why can I suddenly carry 100 pounds more, even though the Density Altitude has stayed the same? Aren't we told that Density Altitude determines aircraft performance?

Edited May 19, 2010 by lelebebbel

[Mikemv]

Dear Lele & All,in my recent Seminar at Colorado Heli-Ops in Denver, I asked the pilots how could they tell what the maximum manifold pressure their normally aspirated recip engine could produce prior to start up? Some looked puzzled, others just no answer but my teaching point was that reading the static(pre-start) MP would let them know that they could only produce approximately 2" less than indicated. Some of the CFIs went flying with students and verified what I had told them. We were covering helo performance and areas of missing or incomplete training!

 

So, from looking at the chart, I am thinking that as pressure altitude(elevation) increases, the engine can not produce as much power before it reaches the FULL THROTTLE position stated at the top of the chart as one of the chart formulation parameters. So at a lesser PA(elevation), the engine can produce more power prior to reaching that full throttle position and carry more weight at the same DA. Once at full throttle, any more upward collective would cause rotor RPM droop.

 

We definitely know that DA is one of the three factors that govern helo performance!

 

Mike

[C of G]

No, that's not it.

 

This is what the charts look like:

 

This one is an R44 chart, couldn't find a R22 one online - the question remains the same, just the numbers are slightly different.

 

 

So, again:

At approx. 11,000ft PA and 0 C, I'm at 11,800ft DA.

According to this chart, I can hover a R44 Raven II in ground effect at about 2,280 pounds gross weight in those conditions.

 

 

At 8,000ft and about 33 or 34 C, I'm also at 11,800ft DA.

According to the chart, I can now hover at 2,380 pounds gross weight.

 

Why can I suddenly carry 100 pounds more, even though the Density Altitude has stayed the same? Aren't we told that Density Altitude determines aircraft performance?

 

 

The simple answer is that performance is based on altitude and temperature, but not necessarily DA. DA is the ultimate limit per the limitations section, but not just it's effect on lift and airfoil performance. There is also the donkey to consider, which will limit out earlier at a higher altitude.

 

Mike has it as correct as I could state it, and a good reference can be found in Shawn Coyle's book.

[iChris]

Why is this line not vertical?

 

Graphs can be arranged to reference data in many different formats along the X-Y axis. However, the accompanying data is just repotted to reflect the actual flight test results. As an example, the 300CB IGE chart has density altitude plotted on the vertical with 8,000 feet DA as a take-off and landing limit. Robinson like most manufactures places the main data on the X-Y. The ISO and DA information is added as an overlay reference.

 

In other words, why is the maximum hover weight greater at lower altitude and higher temp, and less at higher altitude and lower temperature, even though both result in the same density altitude?

 

As with any normally aspirated piston engine helicopter, air density determines performance. As the density of the air increases aircraft performance increases. Conversely, as air density decreases aircraft performance decreases. In both cases, atmospheric pressure change is the major factor over that of temperature (See attached ISO table). Remember, a lapse rate still exist even during non-standard conditions.

 

Also note, the maximum manifold pressure of any normally aspirated piston engine helicopter, can never be more than about 1inches less than the outside air pressure. Once, your normally aspirated helicopter departs sea level conditions, your available power starts decreasing.

 

Aren't we told that DA is what determines aircraft performance?

 

No, that's not technically 100% correct. The air density (mass per unit volume) determines aircraft performance. The atmospheric pressure, temperature, and humidity however, determine air density. In general, we use DA in terms of high-density altitude or low-density altitude as a reference to the actual air density being lower than standard or the actual air density being higher than standard respectively. We've used "Air density" and "Density altitude" as synonymous terms; however, they aren't the exact same in reality.

 

Take another look at your textbook Lift equation and Drag equation. Both calculations are based on “rho” which is the actual air density. Air density is measured in slugs/per cubic feet, which defines the mass of air per unit volume. In contrast, Density Altitudes unit of measure is in feet, a height reference.

 

Density Altitude is a measure of the density of air corrected for ambient temperature. In other words, it is the equivalent pressure altitude with standard temperature. The density altitude gives only general reference to the actual air density. The ISO established a model atmosphere that allows us to estimate performance based on an equation that correlates pressure altitude and temperature into Density altitude (See Attached). Density altitude is an index that tells us if the current conditions are better than, equal to, or worse than the ISO model. The model states as we climb in altitude, the temperature drops off at a rate of 1.98C per thousand and pressure decreases 1 inch per thousand feet, the standard lapse rate. During non-standard conditions we should expect the same type of change, but at different rates.

 

 

Why is performance different, even though DA is the same?

 

 

The performance is different because the actual environment is different. The outside air density is changing. The premise that a given DA calculation yields the same performance at different combinations of pressure altitude (PA) and temperature is only correct if the actual air densities are all the same. The IGE/OGE graph holds true to that fact as it shows actual performance decreasing with altitude. The flight test data reflect actual values of atmospheric pressure, temperature, and humidity. Whereas, PA and DA values are based on the ISO model and yield predictable performance.

 

 

Look at the attached equations for air density and density altitude. First, they are not equal to each other, so they don't produce the same results and secondly, density altitude is computed from ratios of ambient Pressure/temperature vs. ISO model ratios of pressure/temperature. The DA and the ISO model were developed as a simple approximation to allow rapid and easy analysis of aircraft performance. It would be best to say density altitude is used to estimate or predict aircraft performance.

Binder4.pdf

Edited May 21, 2010 by iChris

[r22butters]

...reading the static(pre-start) MP would let them know that they could only produce approximately 2" less than indicated...

 

So when I jump in the helicopter, if the MAP is 25"(before I start it), I will only be able to pull 23" while in flight?

 

I suppose this only works if I stay at a constant altitude?

[Mikemv]

r22butters. You stated that correctly. It will let you know how much MP you can produce for take off at the elevation you are at! Go land at a higher elevation with a different DA and you may not be able to produce that same MP! Lower of course is inverse. Mike

[lelebebbel]

Thank you iChris - this:

 

We've used "Air density" and "Density altitude" as synonymous terms; however, they aren't the exact same in reality. [...] The density altitude gives only general reference to the actual air density.

 

is exactly the part I was missing. I was assuming that DA was simply a reciprocal of rho. The performance chart makes more sense now.

 

So, the Density Altitude model by itself is not by any means an accurate way to predict aircraft performance, and even if you have the same Density Altitude at 2 different locations, the actual AIR DENSITY can be very different.

Edited May 20, 2010 by lelebebbel

[lelebebbel]

So when I jump in the helicopter, if the MAP is 25"(before I start it), I will only be able to pull 23" while in flight?

 

I suppose this only works if I stay at a constant altitude?

 

Yes, approximately 23.5" realistically. Have a look at the Manifold Pressure limitation chart of a R22 Alpha or Beta - if you draw a vertical line at a given PA up to the "full throttle" line, then draw a horizontal line to the left from there, you will end up at the maximum physically possible manifold pressure for that altitude.

 

For example, at 6000ft PA, you get about 22.3".

Your MAP with engine off would be 23.92".

 

Who knows why they changed this chart to that useless table for the Beta 2...

Edited May 20, 2010 by lelebebbel

[r22butters]

...Who knows why they changed this chart to that useless table for the Beta 2...

 

If you want that "good" chart for the Beta II (or Raven II, for that matter) its on the back of the checklists they sell at Robinson.

[iChris]

Thank you iChris - this:

 

 

 

is exactly the part I was missing. I was assuming that DA was simply a reciprocal of rho. The performance chart makes more sense now.

 

So, the Density Altitude model by itself is not by any means an accurate way to predict aircraft performance, and even if you have the same Density Altitude at 2 different locations, the actual AIR DENSITY can be very different.

 

 

Knowledge of the differences between "Air Density" and "Density Altitude" and how they are sometime misused is the key.

 

Please understand, Density Altitude along with its ISO model, are still useful tools in predicting general aircraft performance. They also are the base reference for our altimeter system and altitude reporting equipment, transponder mode C.

 

As long as we use them along side and in compliance to the RFM performance data, we'll be assured accurate results.

[tattooed]

Interesting that you referenced the HP. We have 2 at the school I am working for. And after flying them for over a year (in almost all wx conditions), I have yet to come up with any max MP setting other than 23.5 to 23.8 (ish). I am not exactly sure why the HP is so consistent with it's MAP setting compared to the Beta and Beta II's I flew at another school, who's MAP was all over the place depending on the day.

 

Anyone?

[Tom_Heli_AV8R]

I was told by my instructor that the 12,600' DA was the Maximum DA required by the FAA to be verified for certification, and thats why it is depicted. Just to show that the data has been established and verified up to that specific requirement.

[rodrop]

http://wahiduddin.net/calc/density_altitude.htm

This tries to explain density altitude and air density

[cascadeAlex]

Hello everyone,

 

First time posting and recently started training for PPL.

 

Back to a early post and to the OGE hover charts.

 

Lets use the R44 image form before:

 

 

 

 

My question is what if when you follow the Gross weight (low value say 2200 lb) vertical and you do not hit the corresponding OAT(say -10C). Instead you run directly into the 11,800 ft DA line around 13,200 pressure altitude. What does this mean?

 

I assume this means that for the given temperature and PA results in Density altitude greater than 11,800ft.

 

Second question is why do these charts stop at 11,800 DA when the limitation portion of the POH says max operation is up to 14,000DA. I am not sure it this is true for the R44 POH and Charts but in the R22 POH the charts stop at 12,600 DA but the limitation say max up to 14,000DA.

 

I have read this topic closely but still had these question. Please let me know if you can help out. Sounds like it may be how the carts were created during certification but I am unsure.

[ridethisbike]

My question is what if when you follow the Gross weight (low value say 2200 lb) vertical and you do not hit the corresponding OAT(say -10C). Instead you run directly into the 11,800 ft DA line around 13,200 pressure altitude. What does this mean?

 

I assume this means that for the given temperature and PA results in Density altitude greater than 11,800ft.

 

Second question is why do these charts stop at 11,800 DA when the limitation portion of the POH says max operation is up to 14,000DA. I am not sure it this is true for the R44 POH and Charts but in the R22 POH the charts stop at 12,600 DA but the limitation say max up to 14,000DA.

 

I have read this topic closely but still had these question. Please let me know if you can help out. Sounds like it may be how the carts were created during certification but I am unsure.

 

This is how they are teaching us...

 

When you fall to the left of that line at -10C you have to interpolate the max pressure altitude you'll be abe to hover at. Realistically, you could probably hover higher than that, but since testing wasn't completed at those specific conditions, you get limited on how high you can go.

 

Example: 2300 lbs, +20C. If you notice, you don't hit the 20C line with that particular weight. This is how they limit you. What you have to do in this case is draw a line STRAIGHT ACROSS (horizontally) to the PA as shown in the image below. So when you hit the line you've now drawn across the chart, that's the max PA that you're allowed to hover at. So no matter if you weigh 2300, 2200 or 2100, you're limited to an implied hover capability of 9200 PA at +20C.

 

+10C outside? weigh 2200? 10100 PA limit.

 

As I said, you could most likely sustain a hover at higher altitudes with a reduced weight at the same temp, but it gives a cushion level so you KNOW you can hold the hover instead finding out the hard way that you can't when you start sliding across your landing area.

 

 

Like I said, this is the way we are being taught right now. That we can't just continue the temp line in the direction it's going. Reason? look at a HOGE chart... the temp lines at the heavy weights rise dramatically and the taper off. What if the temp lines did the same thing at the lighter weights? Since it's not shown, and you're not a test pilot, you don't get to find out...

[lelebebbel]

Good explanation of how to read the chart, but I'm also curious where the 11,800ft DA limit for IGE hover comes from. As far as I know, there is no such limit listed in the "Limitations" section? The only altitude limitation there is a max. operating altitude of 14,000ft DA (or 9,000ft AGL), so why don't the lines in the IGE chart continue to 14,000ft?

 

The performance chart itself is not a limitation, and doesn't legally stop you from hovering higher than what the chart tells you.

Edited June 6, 2012 by lelebebbel

[ridethisbike]

Good explanation of how to read the chart, but I'm also curious where the 11,800ft DA limit for IGE hover comes from. As far as I know, there is no such limit listed in the "Limitations" section? The only altitude limitation there is a max. operating altitude of 14,000ft DA (or 9,000ft AGL), so why don't the lines in the IGE chart continue to 14,000ft?

 

The performance chart itself is not a limitation, and doesn't legally stop you from hovering higher than what the chart tells you.

 

The 11,800' thing I have no idea, which is why I didn't comment on it...

 

I view those hover charts as implied operational limits and give you a buffer from the ACTUAL operational limit. I understand they are not legal ones, such as the 14,000' DA limit.