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power settings

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air.guerner(at)orange.fr
Guest

Posted: Fri Dec 26, 2008 12:54 pm Post subject: power settings

Ira,

  

 I disagree with your assertion that altitude is not  needed in your table. A given setting, i.e. 5000 rpm and 22" will give you  more power at 8000 ft than at 1000 ft. because the temperature being colder  at 8000 ft, the density of the air at a given manifold pressure is  higher.

 As an exemple the following power settings are  extracted from the Cirrus SR20 Flight Manual:  

 2000 ft, 2500 rpm, 23.8"

  4000 ft, 2500 rpm, 22.3" 6000 ft, 2500 rpm, 21.8" 8000 ft, 2500 rpm, 21.2" All these settings produce the same 65% power on  the Continental engine.

  

 So your table as it is today is valid at low altitude only. To be  exhaustive, it would be necessary to compute one table for each density  altitude. If you are prepared to do that work, I am interrested in getting the  results.

 Best regards

 Remi

  

  





  

  

 >>>>>>>>>>>>The table is engine  performance with a constant speed (AirMaster) prop.

All certificated  manufacturers provide a table like mine which shows

the relationship between  throttle, selected RPM, and power output.

Rotax does not.  The graph is  useful to select which combination

works best under specific circumstance,  particularly when fuel flow

and TAS are added to the table. My data  collection is unfinished for the

bigger version of the table.  In this  version of just engine, not airframe

performance, density altitude is  unneeded. No matter what density

altitude, throttle sets manifold pressure,  and obviously without a

turbocharger/supercharger, the maximum MP declines  with altitude.

The desired rpm is set on your prop control.  Green is  the usual 

operating range, the other colors are not important  here.



Example: 75% power is obtained at  27.5" and 4800 rpm  or

26" and 5000 or 24" and 5500 rpm.  The "standard" cruise  setting

on Continental or Lycomings of 25 squared (or 2500 rpm (at) prop  at

25" MP) doesn't work out for Rotax because 2500 rpm would be over

6000  engine rpm,  but by extrapolation would be over 80% power.



Or, if  you set your prop to 5000 rpm for cruise, then you get 75% at

26" (only  possible up 4000' DA), and 65% at 23" MAP.



While there are small  inaccuracies due my simple model of the Otto

cycle, so far as I know, this  table I created is the only published

attempt to provide guidance for 912s  drivers with a constant  speed

prop<<<<<<<<<<<<

rampil

Joined: 04 May 2007
Posts: 870

Posted: Fri Dec 26, 2008 4:10 pm Post subject: Re: power settings

Hi Remi,



Very Interesting!  Your suggestion that a normally aspirated engine

will develop more power at 8000' than at 1000' is quite curious.



The fraction of oxygen is stable in the troposphere at 20.9%

The standard lapse rate of 2°C per 1000' is included in the

rule of thumb that the pressure drop is an inch of Hg per 1000'

Therefore the roughly 7" drop in pressure include the temperature

change of roughly 14°C.  



Look at this page from Wikipedia which has the formulae for pressure

at altitude (with or without temperature lapse):



http://en.wikipedia.org/wiki/Atmospheric_pressure



NB a 7" drop in MP is roughly a 25% drop in power between 1 and 8

k'. The change in temperature is (following the Gay-Lussac's law)

P1/T1 = P2/T2 where temperature is in Kelvin. So P1/P2 = T1/T2

or P1/P2 = (273 + 15)/(273 + 1) = 288/274  which is only a 5% change

in density or partial pressure, so even if temperature was not

accounted for in the pressure drop, the temperature effect is 

completely swamped by the pressure change.



Remi, you might try the calculator for engine power at

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



or look here:

http://www.2-stroke-porting.com/altiden.htm



or here:

http://www.nappf.com/nappf_density_altitude.htm



or better yet read this delightful old book on Aircraft Engines

(I reference it here for you because it is free on the web):

http://books.google.com/books?id=qiBFAAAAMAAJ



and a good book on aerodynamics and gas laws. I like Aerodynamics

for Naval Aviators. See in particular the standard Atmosphere table on

page 5 and pages 100-104, and page 135-149 on Recip engines.



Cheers,

_________________
Ira N224XS

kheindl(at)msn.com
Guest

Posted: Sat Dec 27, 2008 6:09 am Post subject: power settings

do not archive Hi Bud C Your notes are extremely useful and I would like to print them off. But for some reason your emails are double the width of everyone else's and I am scrolling left and right for each line and cannot make to fit on a page C even in landscape format. Is there a simple way of reformatting it ?

 

Cheers C    Karl

From: budyerly(at)msn.comTo: europa-list(at)matronics.comSubject: Re: Europa-List: power settingsDate: Sat C 27 Dec 2008 00:01:52 -0500



Happy Holidays C

Just to throw my hat in...As I have too much time on my hands today...

 

Propeller charts C ram intake effects C cowl mods C prop diameters C etc. all affect our aircraft's performance data.

I agree with all of you that the Rotax Engine Manual is lacking to non existent for use in accurate data measurement for cruise.  God only knows what the propeller efficiency is.  And since we design off the engine charts C which in no way reflect actual engine installation effects C we are best equipped C as experimenters C to follow the age old technique of recorded pilot data from a specific test aircraft.  Namely C Yours. 



 

When I get a chance to flight test an aircraft for a week or so C I log as much data as the owner lets me.  Unfortunately I don't get to do this very often as it is quite time consuming and I leave it to the customer to consider how much he wants me to fly when he has a perfectly safe airplane.  Frankly C most of you want to go fly C and all this data seems dumb.  If you are happy cruising at 5500-6500 ft C 5000 rpm C 26" MP C and 4.5 gph C going as fast as the ground speed says C don't read on.  I have a "simple chart" done up in my flight test book to jog my memory so I don't do something dumb like just fly around during the customers 40 hour fly off.  After all C test flying is supposed to be work.

 

I've read your posts and you guys are somewhat correct C but talking a different language.  What is needed C in my opinion C is a chart/table C derived from data that is painfully boring to fill out and takes about 5 test flights.  This is the data that then translates to Horsepower vs Speed charts to be translated into best cruise and climb performance suited to your aircraft.  I've partially tested RV's (4 and 6) and fully tested a Zenith (Subaru E85) extensively as well as two Europa's C one a with a warp drive fixed prop the other with an Airmaster using variations of these data charts C and you can tweak your plane to get the most performance out of it by proper testing.  It will also make the Rotax engine power curves start to make sense.

 

Here's the gist:

 

Cruise data chart for Aircraft make model C engine type C prop type at a specified gross weight:

 

Density Altitude C RPM C MP C IAS C TAS C Fuel Flow C additional data (see note)

Sea Level

2500 

5000

7500

10 C000

Note: Log for your use items such as; Oil T/P C Cyl. Head C Coolant  C EGT etc. so you can track any future trends.

 

But wait C there's more.  Why climb up to cruise and not record climb also

jimpuglise(at)comcast.net
Guest

Posted: Sat Dec 27, 2008 6:15 am Post subject: power settings

Karl- 

  

No problem with his post here.  Try hilighting the whole thing, copy it and paste it into Word.  You can then set the margins as you please and it will word wrap.  

  

Jim Puglise.  



---

kheindl(at)msn.com
Guest

Posted: Sat Dec 27, 2008 7:47 am Post subject: power settings

Thank you Jim. I am just curious as to why it only happens in this one case. I use Microsoft hotmail.

 

do not archive

Date: Sat C 27 Dec 2008 14:14:57 +0000From: jimpuglise(at)comcast.netTo: europa-list(at)matronics.comSubject: Re: power settings



Karl-

 

No problem with his post here.  Try hilighting the whole thing C copy it and paste it into Word.  You can then set the margins as you please and it will word wrap. 

 

Jim Puglise. 

---

air.guerner(at)orange.fr
Guest

Posted: Sat Dec 27, 2008 9:59 am Post subject: power settings

Ira,

 You misread me.  I said that a  given combination of MP and RPM gives you more power at altitude. I am not  talking about max full throttle power. Of course I agree that full throttle  power decreases with altitude on any normally aspirated engine.

 Go back to the SR20 exemple:  flying at  8000 ft,  2500 rpm and 21.2" MAP  give you 65% power. Now flying at 2000 ft you need 23.8" of MAP to get the  same 65% power at the same 2500 RPM, so obviously if you fly at 2500 rpm and  21.2" MAP at 2000 ft you will get less than 65% power. At 2000 ft the ISA  temperature is 11degC while at 8000ft it is -1 degC. Therefore  the density of the air at the same 21.2" pressure is higher at 8000 ft than it  is at 2000ft. That is why this 2500 rpm and 21.2" MAP setting will give you more  power at 8000 ft than the same setting at 2000 ft.  From my  calculation this setting will give 62% power at 2000 ft. That is just the laws  of physics and all aircraft engine manufacturers seems to agree with  them.

 Cheers

 Remi

  

  

  

  

  

 >>>>>> Your suggestion that a normally aspirated  engine

will develop more power at 8000' than at 1000' is quite  curious.<<<<<<<



    [quote][b]

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sat Dec 27, 2008 3:38 pm Post subject: Re: power settings

Hi Remi,



Remember how I used that fine old French chemist, Gay-Lussac,

to show that an isobaric drop of 14°C gives only a 5% change

in density - scarcely measurable in the performance of a Europa, i.e.,

you would need many test runs to tease the difference from the noise.

It is because that size of temperature change against the background of

standard atmosphere temperatures is quite small in the Kelvin

temperature scale



It's all a matter of magnitude of effect.  



So, if it will make you happy, we can say my chart is only accurate

to 5% from MSL to 8000' DA, and it is still a Christmas gift to

Rotax pilots who otherwise have no guidance at all from the factory in

setting a constant speed prop.



Regards,

_________________
Ira N224XS

budyerly(at)msn.com
Guest

Posted: Sat Dec 27, 2008 6:45 pm Post subject: power settings

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]-->  Ira and Remi,

 Great watching your comments.

 Curiosity got to me on how both of you stick by your guns so I decided to  review the DA20 manual and stick my nose where it doesn't belong.  

  

 You are comparing apples and oranges.

  

 Do not confuse Thrust horsepower and shaft horsepower.

 Ref the DA20 manual figure 5.8(b).  Max power is the thrust horsepower  available for that specific prop and engine combination cross referenced to the  altitude, temperature vs the TAS etc...This clearly shows Ira's point that  the shaft horsepower of the engine, as the altitude is increased, drops  off.  At sea level the engine prop combination produces 130 ktas, but  follow the standard day line up to 6000 feet and the max power the engine is  capable of is 75% and yields 121ktas.  Use the dotted line example and  then look at as temperature decreases, the horsepower increases at a constant  altitude, resulting in an increase in TAS because of an increase in shaft  horsepower due to an increase in air density.  Shaft horsepower is directly  proportional to fuel air mass flow and RPM pumping out the watts of power.

  

 Remi, you are right in saying that you get more power, but the real answer  is, thrust horse power or THp.  THp goes up as speed increases.   >From one of my previous references "Airplane Aerodynamics", I believe the  problem that IRA confuses is he is talking brake horsepower, where as you are  analyzing thrust power required for level unaccellerated flight.  The  equation to derive THp is:  THp available = TV/550.  Thrust is  generated by the relationship of Brake HP times propeller efficiency.  What  this shows is there is no Thrust horsepower when stationary (V=0).  Note on  the chart referenced above how flat the power curve line is.  But when the  speed of the aircraft is considered, an increase in THp is directly proportional  to the speed.  Boy this is boring...  Diamond's charts are to make the  pilots life easy and are a brilliant way to get the most from their prop/engine  combination.  It really is a plot of HP avail vs HP required points  referenced to altitude and speed.  Pretty slick.

  

 Do not confuse MP with horsepower.  Let's face it, MP is just the  pressure measured between the valves and throttle body.  When combined with  the rpm, fuel flow and prop efficiency you get a number that equates to the  THp measure above.  The IO-240 is a fuel injected, mixture  adjustable, ram air induction, engine with a highly refined  Sensenich cruise prop.   (If you think Rotax engine data sucks,  try to get the Continental folks to give you the test material.   Essentially, you must show a need, as they don't want it in general  distribution.) As the speed increases at altitude, the Sensenich prop doesn't  loose efficiency as quickly as a normal prop and with the aid of ram induction  you can achieve 65% thrust horsepower at a higher altitude, but your shaft  horsepower is lower.  Normally a fixed prop unloads significantly with  speed increase, whereas the Sensenich W69EK-63 is a highly refined prop tuned to  the DA20 airframe and IO-240 engine combination.  It has a target sweet  spot of 65 to 75% Thrust Hp from 4000 to 8500 feet at a nominal 120KTAS while  still allowing good takeoff performance.  This is no off the shelf  prop.  The net result is a prop that can allow you lean the mixture to  compensate for air density so as to use the extra MP from the ram air and not  swing the prop too fast.  With a prop pitched like the one used  on the DA20 attached to a Rotax / Bing equipped engine you would lug  the engine down until the plugs fowled shut.  

  

 The Europa 912S, with Airmaster prop and non adjustable mixture is a  different animal.  It behaves like a normally aspirated engine lowering its  MAP as altitude increases.  The fuel injected IO-240 is blessed with a  better intake and fuel metering system.  But alas the shaft horsepower  decreases with altitude just as any other plane.  

  

 So you're both right.

  

 Better to just plot data for your plane, figure out what works best for  your engine/prop/airframe combination and tweak it like you should.  Every  airplane has a sweet spot, you just gotta find it.  I prefer to do it  systematically as IRA does, I just prefer a lot more data points to  plot the HP req vs Velocity curves like Dykins does...  From these curves,  better understanding of max range, max endurance, miles per gallon, speed vs  economy decisions can be made.

  

  

 Have a great holiday.

  

 Bud

  

  

  

  

  

  

 ---

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sat Dec 27, 2008 7:14 pm Post subject: Re: power settings

Hi Bud,



Thank you for your illuminating comments!

Always a pleasure to be enlightened by a pro!

See you at SnF I hope.

_________________
Ira N224XS

garrys(at)tampabay.rr.com
Guest

Posted: Sat Dec 27, 2008 8:12 pm Post subject: power settings

Ira, Remi and Bud,

  

 You guys are taking all the fun out of flying    What happened to the good old days when we simply "kicked the tires,  lit the fires" and went out flying for fun.  All those numbers just confuse  me 

  

 Garry Stout

  

 [quote]   ---

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sun Dec 28, 2008 4:58 pm Post subject: Re: power settings

A small follow-up on the power vs altitude issue.



The power increase that occurs with altitude at constant MAP

is due to the drop in pressure to which exhaust gas is vented.



Lowry, JT: Performance of Light Aircraft (AIAA, 1999) page 129.

See also Gagg & Farrar (1934) Altitude Performance of Aircraft Engines

    equipped with Gear Driven Superchargers.  Soc. Automotive Engineers

    Trans.  29:217-233

_________________
Ira N224XS

air.guerner(at)orange.fr
Guest

Posted: Mon Dec 29, 2008 9:02 am Post subject: power settings

Ira,

 I understand that but I do not have this book here.   How does the effect of the lower pressure on the exhaust vent compare with  the effect of the lower temperature of the intake air?

 Remi

  

 >>>>>>The power increase that occurs with altitude at  constant MAP

is due to the drop in pressure to which exhaust gas is  vented.<<<<<<

    [quote][b]

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