flyrotary@lancaironline.net Mailing List Archive

Dave,

Reading Corky Bell, I got the impression that adjusting for density altitude is really all you need to do. The most accurate maps are measured in pounds of air per minute and that allows perfect conversion to any altitude you want. You just can't take the compressor map and assume it's going to be valid at Colorado Springs (6800' elevation). Like takeoff distances, your airspeed, and everything else, you've got to adjust for density altitude.

Corky Bell just runs all his calcs through the same density altitude calculations that pilots are accustomed to. After all, after the air intake charge is run through the turbo, it is pretty hot. Bell's calculations show how the increase in pressure is partially offset by the increase in heat. That heat is what makes the turbo more susceptible to detonation and it's why reducing the heat is key to getting the most out of a turbo. It's also the reason that you want to operate in the middle of the efficiency island because that's where the heat pickup is the least.

The good news is that, unless you're flying through a temperature inversion, atmospheric lapse rates demonstrate that we're sucking in colder air when we climb. We're helped a little by colder air at altitude. However, if you run a sample calculation on the heat of compression, you'll be amazed at how much heat is picked up in the charge.

While Rajay, Airesearch, and other OEM aircraft turbo manufactures might not be forthcoming about what specs their turbos put out, some of that is due to the fact that it doesn't matter to pilots: We've got to stick to FAA approved OEM replacement parts no matter what.

Among manufacturers, Turbonetics is said to be helpful to pilots. However, other experimenters have gotten data and advice from turbo manufacturers under the guise that they are running a car in the Pike's Peak hill climb, where the summit is over 14,000'.

I believe you're correct in pointing out the dangers of overspeeding the turbo. That would be consistent with the thesis that a turbo isn't working against so much resistance because it's starting with low density air up high.

Barry Gardner
Wheaton, IL

DaveLeonard wrote:

David, like you I am working on the ultimate turbo installation and am trying to better understand these compressor maps. I think that extrapolating the maps to altitude is going to be inaccurate.

First. theses maps were all created by testing at or near sea level. While the turbo RPM required to achieve a particular pressure & air flow is likely going to remain relatively unchanged, other info gleaned from the turbo map might be different at altitude.

I do to know the particular qualities of a compressor that determine the choke and surge limits of that particular compressor, so this are just a random guesses on my part. One thing is for sure, the force (and/or work) required to produce a PR of say 2.5 will be less at altitude than at sea level. How does this play into the shape of the compressor maps? I'm not sure.    I suspect that the choke curve (seen in my mind as the airflow limit) is a function of inlet and outlet sizes, and turbine size and shape. Since the density of the air up high is thinner and the resistance of (turbulent) air through a tube depends on its density, I suspect that the choke curve of a give compressor will shift to the right at altitude.

I also suspect that the surge portion of the serve may be based partly on the force (torque) the compressor is able to handle. This is because that is the high torque portion of the curve. Qualities such as axle thickness, blade shape and thickness, blade angle of attack, and clearance to the compressor housing may play a part. Specifically, blade 'stall' is a highly suspect in my mind as the cause of 'surge'. I believe that the gas turbine guys even call them compressor stalls. That being said, the lower torque required at altitude will decrease the tendency of the the blades to stall at a given RPM and air flow (just like unloading the G's will allow a wing to fly at a lower airspeed).   This will shift the surge line to the left.

Thus, my guess is that compressor maps have more area at altitude. It is also probably easier to get into over speed at altitude because the choke limit will move up some into the over speed area.    Regardless of my guess on which direction the curves move one thing is for sure, they will change at least a little. Now is when we could use all the good research probably done by Rayjay etc.. when they came out with all the certified turbos. Notice they never mention the A/R or give a compression Map for any of the certified turbos. Just what airplane to put it on and where to send the money.

JMHO,

Dave Leonard

----------------------

Ed,

    I am trying to work my way through the compressor maps and formulae that I have stumbled across in the past few days, and looking at your formula, I am able to make sense of it.. and I am using a website with a calculator built in that reproduces your values...but the value of 277 cfm.. thats how much air the engine is pumping normally aspirated at wide open throttle.. right?

So plotting this at 1.0 atm and 277 cfm wouldnt event be on the "map" at all.. If I am understanding this at all.

Using the TD06H-20G map listed at http://cybrina.mine.nu/MR2_Docs/compressor_flow_maps.htm (and attached to this mail)
if I wanted to maintain 23 PSIA (8.3 PSIG) boost (48" map). which comes out to a presure ratio of 1.6..
. then I would need to calculate the CFM required at 1.6 PR..

At 3000 rpm I am getting 222 CFM and
At 6000 rpm I am getting 444 CFM... (1.6 PR),
which falls in the map on that site....

To take it further.. if I wanted to maintain that amount of boost (23 PSIA/8.3 PSIG) to say.. 12,000 feet..
Ambient pressure at 12k is a ballpark value 18"/9psi.. the pressure ratio there is 2.5...

3000 rpm, PR 2.5, comes out to 347 CFM.. appears to fall just left of where I'm told the "surge line" is
6000 rpm, PR 2.5 equals 697 CFM and falls to the right of the 65% efficiency island..

If I am interpreting this correctly that means that this turbo would be able to give the desired performance but would NOT be able to maintain 23" absolute boost to 12k feet....

5000 rpm at 2.5 PR gives 578 CFM and DOES fall within the map though...

I am throwing all this out there to see if I am grasping the concept clearly or not.. and to see what I need to adjust conceptually..

I am also guessing that the .55 value listed below by Mike, is BSFC.. correct?

I am trying to comprehend the "volumetric efficiency" concept as it relates to pressurized/charged intakes.. are we assuming a value of 1.0/100% or is this not realistic? What about porting... will that improve the VE? (I'm wanting to say YES) Iis it realistic or even possible to have a VE > 1.0/100%?

The reason for all the head scratching on my end is that I have several turbo 13b cores.. but none of them have turbo's attached.. so I get to pick what I want on there.. but I want to make sure I have a grasp of the concept before I start turbo shopping..

I must admit that this listserv group has been a literal gold mine of information. I have exposed to a truckload of data in the past 2 weeks and am just trying to organize it at this point.

Dave Staten
2nd Gen 13B rebuild in progress..

Ed Anderson wrote:
Mike makes a good point.  While the official displacement of the 13B is
1.308 liters (actually 1308 cc), for airflow/power purposes it acts the same
as a 4 cylinder engine of 40 CID (each cylinder) or 160 CID or 2.6 Liters.

CFM = (4*40)RPM/(1728*2) so for 6000 rpm, CFM = 160*6000/(1728*2) = 277.77
CFM at 100 Ve

Some compressor maps use CFM on the X axis and some use mass flow usually
Lbs/min.  My spreadsheet automatically calculates the lbs/min of air flow
for every rpm and power situation.  Or its easy enough to calculate.

Just multiply your air flow volume in CFM by 0.076 for sea level density.
So in this case 277.77*0.76 = 21.1052 lbm/min



Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message ----- 
From: "Mike Robert" <pmrobert@bellsouth.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, August 18, 2004 11:14 AM
Subject: [FlyRotary] Re: Compressor maps


  
John Slade wrote:

    
John, here is a URL to a pretty good discussion on reading compressor
maps with examples
 http://cybrina.mine.nu/MR2_Docs/compressor_flow_maps.htm

That's great, Ed. And you did it in one sentence. :)
Do you happen to know the engine capacity (in litres) and the
volumetric efficiency of an REW engine?
Regards,
John

      
John, the auto people use 2.6 litres and .55 for those turbo map calcs.

HTH, Mike

    
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