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setting prop pitch

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benburb(at)comcast.net
Guest

Posted: Fri Oct 31, 2008 6:57 pm Post subject: setting prop pitch

I have a 912UL with a two-blade Warp Drive prop, and am not getting 

static rpm of  5,200.  I am about to re-set the pitch, and can't 

remember the recommended degrees of pitch.  Does anybody know? 

Thanks.  Ben

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Fri Oct 31, 2008 10:23 pm Post subject: Re: setting prop pitch

Hi Ben,



You don't really care what the recommended pitch is except for maybe the first time to get you in the ballpark for rpm. You want to set your prop so your engine turns 5500 WOT flat and level at the general altitude that you normally fly. I never worry or even bother with static rpm. I know what to add or remove in pitch if you tell me what you are currently running WOT at your average altitude. 5500 WOT is the best all around compromise for your ground adjustable prop and Rotax 912ULS engine combo. I only set it different for special circumstances and flight characteristics. I have a 3 blade Warp Drive.

Go fly your plane and see what RPM you turn flat and level then make an adjustment.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

davidjoyce(at)doctors.org
Guest

Posted: Sat Nov 01, 2008 1:32 am Post subject: setting prop pitch

Ben, For VP/CS props the PFA recommended a max static fine pitch figure of 

5650, which also works pretty well for the go around situation when you slam 

on full throttle from approach configuration at 60 knots. If you are talking 

about a fixed pitch prop then I guess other considerations will come in and 

you may want to compromise some take off performance for better cruise 

performance.

Regards, David Joyce, G-XSDJ

---

daveaustin2(at)primus.ca
Guest

Posted: Sat Nov 01, 2008 4:49 am Post subject: setting prop pitch

Ben,

A static pitch of 5200 rpm will give you great climb but very poor cruise.

Unless you are flying from a short strip I'd suggest 4800 static.

Dave Austin  601HDS - 912

benburb

Joined: 01 Nov 2008
Posts: 7
Location: Jacksonville, FL

Posted: Sat Nov 01, 2008 6:38 am Post subject: Re: setting prop pitch

Guys- thanks for all the responses.  Seems we have many opinions.  I haven't changed my setting for 13 years (912UL and 2-blade Warp Drive) I have always gotten about 4800 max rpm WOT, straight and level. I get great climb rates on my Kitfox III.  I got to thinking that 4800 meant I wasn't getting all 80hp I paid for.  Suggestions ranged from  4800 to 5,650 max static.  Looks like it my not be a critical issue.  Ben

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat Nov 01, 2008 8:04 am Post subject: Re: setting prop pitch

Ben,



Rotax designed the 912's to run optimally between certain rpms for several reasons (4800-5200 for best torque, 5000 peak). Cruise should usually be around 5000 give or take a hundred rpm depending on your plane. Some like to cruise at even higher rpms, 5500 is max continuous , but not preferable. Running your engine to get only 4800 at WOT is lugging your engine too far down and in fact over time won't be good for it. Rotax jumped on a few Mfgs. for running the engine at 5100 WOT or lower. Rotax classes teach setting the max rpm for 5500 WOT unless you have a need for special performance. On my plane 4950 rpm on takeoff puts me right at 5500 rpm flat and level full throttle. Using the ground adjustable prop is a compromise between several flight characteristics.

You are loosing "tremendous" flight performance from your plane with a WOT set at 4800 flat and level. Try 5500 WOT flat and level. Then get back here and tell us your experience. If it is like most people you will think you have a new turbo charged engine.

I guarantee it.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

benburb

Joined: 01 Nov 2008
Posts: 7
Location: Jacksonville, FL

Posted: Sat Nov 01, 2008 6:11 pm Post subject: Re: setting prop pitch

Roger- thanks for the input. I'm excited about the prospect of feeling like I have a turbo-charged engine-  especially after 13 years at 4800.

I am a little puzzled though when you said you run 5500 WOT and recommend the same for me, yet you said "Some like to cruise at even higher rpms, 5500 is max continuous , but not preferable."  If it's not preferable...        Ben

_________________
Kitfox III/IV. Rotax 912UL. Warp Drive 2 blade. On and off Aerocet amphibs.

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat Nov 01, 2008 8:13 pm Post subject: Re: setting prop pitch

Ben,

My prop is set to achieve 5500 WOT flat and level at the altitude range I usually fly. I cruise at 5000-5100. Rotax does recommend a little higher rpms if you burn 100LL all the time. Run 5200-5350. This will help some on the lead build up and use a fuel additive like Decalin if you use 100LL..



 Set the prop to 5500 WOT flat and level, but that is not where you cruise. When the prop is set to 5500 you benefit from the prop pitch during other times such as in takeoff with better climb performance, lower throttle setting for cruise means a little better fuel economy. If it is set for 5500 and lets say you bounce up off the runway and you go "oh no I'm going to drop it in", then when you give it full throttle trying to save yourself the engine will spin up faster to save you, compared to, if you had too big a bite of air with the prop and it had to struggle to spin up. If the prop is pitched too much you will need to give it more throttle/fuel to achieve your cruise speed. When it is set for 5500 you will be able to pull back on the throttle/less fuel and still cruise at the speed you want. You will have a higher top end speed, too.

Mind you we are comparing 5500 to your 4800 here. A 700 rpm gain will make a big difference.

I know some that cruise at 5500 WOT just to cruise. This will cause extra wear over time. My question is why? Do you need to get somewhere a few miles per hour faster all the time? Why wear and tear any extra on the engine than need be. We want it to be reliable for its 1500 hours and more. 



Try it, I know you will be happier.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

daveaustin2(at)primus.ca
Guest

Posted: Sun Nov 02, 2008 4:14 am Post subject: setting prop pitch

Chris Heitz used to recommend cruise at 5500 but I've always used 5300/5400 

as that was where the engine runs the smoothest.  If you set it up as cruise 

at 5500, how do you go any faster without over-revving the engine?

Dave Austin  601HDS - 912

rholder(at)avnet.co.uk
Guest

Posted: Sun Nov 02, 2008 5:51 am Post subject: Setting prop pitch

 	  Quote: 	
		   Set the prop to 5500 WOT flat and level, but that is

 not where you cruise. When the prop is set to 5500 you

 benefit from the prop pitch during other times such as

 in takeoff with better climb performance, lower

 throttle setting for cruise means a little better fuel

 economy. If it is set for 5500 and lets say you bounce

 up off the runway and you go "oh no I'm going to drop

 it in", then when you give it full throttle trying to

 save yourself the engine will spin up faster to save

 you, compared to, if you had too big a bite of air with

 the prop and it had to struggle to spin up. If the prop

 is pitched too much you will need to give it more

 throttle/fuel to achieve your cruise speed. When it is

 set for 5500 you will be able to pull back on the

 throttle/less fuel and still cruise at the speed you

 want. You will have a higher top end speed, too. Mind

 you we are comparing 5500 to your 4800 here. A 700 rpm

 gain will make a big difference. I know some that

 cruise at 5500 WOT just to cruise. This will cause

 extra wear over time. My question is why? Do you need

 to get somewhere a few miles per hour faster all the

 time? Why wear and tear any extra on the engine than

 need be. We want it to be reliable for its 1500 hours

 and more.


	


I think you need to stress (I think this is right) that

the 5500 WOT is static, on the ground. The rpm for WOT in

the cruise will depend on a number of factors.



But i would like to ask if anyone has a 912S/ULS with an

airmaster prop and a MAP guage _AND_ a fuel flow meter.



Could they do an experiment for me ! Chose a specific

airspeed (X) at around your normal cruise speed. I would

use 115 knots on my plane.



Now there are a number of combinations of rpm and MAP that

will achieve that airspeed. I use 25 inches and 4600.



The experiment is to set different rpms; and for each

adjust the MAP so that the airspeed is X (stabilised).

Then for each combination of rpm/MAP record the fuel flow

- ideally to the tenth of a litre but a tenth of a US

gallon would be OK.



As the engine must be producing the same horsepower as it

is travelling at the same speed for each combination, any

variation in the fuel flow is a measure of the efficiency

of the engine. If the efficiency is the same for the whole

rpm range tested then the fuel flow would be the same. So

a "scientific" test will show any variation and maybe

there will be an obvious peak to the efficiency - this

will be where the fuel flow is least.



According to Roger's recent postings the implication is

that this peak efficiency rpm/MAP combination will be at

or around 5000 rpm.



If several people are able to do this, please post results

direct to me so that I can collate independently  The

critical factor here is that the MAP must be adjusted to

get the same airspeed (X) for each rpm figure tested.



So the results might look like the following



at 115 knots

4500   26     15.8 (15.8 would be the litres per hour fuel 

flow)

4600   25      15.5

4700   24.5  15.5

4800   24      15.3

4900   23.5   15.1

5000   23.1   15.0        <------ B E S T    E F F I C I E 

N C Y

5100   22.8   15.2          O N  T H E S E   T E S T  F I 

G U R E S

5200   22.4   15.4

5300   22.1   15.5



It would be something to pass the time on your next two 

hour flight !



And if you had longer, other speeds could also be tried - 

X+10 and X -10 !



I will ask the wear and tear questions in another mail

sometime as many people only answer one question per mail 



Thanks



Richard Holder with an efficiency hat on !

ami(at)MCFADYEAN.FREESERV
Guest

Posted: Sun Nov 02, 2008 8:28 am Post subject: Setting prop pitch

Richard,

To save you the bother, the maximum efficiency of any engine is at maximum 

torque speed and WOT. So that will be somewhere between 4800rpm and 5200 rpm 

for the 'S' (Rotax's figures have varied somewhat over the years on this!).

At less than WOT, efficiency falls off anyway and, within limits, there is 

no point extracting the x% of required power at a higher engine speed than 

necessary, because there is a price to pay for the additional mechanical 

losses of that higher speed.



Duncan McF.

---

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sun Nov 02, 2008 3:06 pm Post subject: Re: setting prop pitch

See the torque chart attached. (Top One)

Max torque is 5000 rpm after that torque goes down and HP climbs.

There is no magic here just the best compromise.

Set your prop to get 5500 engine rpm at WOT in the air flat and level. At this setting you will have the best setting for you average pilot and plane. Most of us do not need special prop settings like maybe a float plane.

This is not hard, don't make it hard. 

I have a Dynon system and have all the info.

Rotax in Word.doc

Description:

Rotax torque curve

Download

Filename:

Rotax in Word.doc

Filesize:

1.49 MB

Downloaded:

592 Time(s)

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

daveaustin2(at)primus.ca
Guest

Posted: Sun Nov 02, 2008 5:49 pm Post subject: setting prop pitch

Roger,

A question remaining is at what altitude do you do the 5500 WOT?

Above 5000 ft I use WOT as the max power is down to 75% at that height.

There is another factor..  What is the optimum rotational speed of a given 

propeller?

Dave Austin  601HDS - 912

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sun Nov 02, 2008 9:43 pm Post subject: Re: setting prop pitch

When I set someone's prop so the engine turns 5500 I always ask at what altitude they fly at the majority of the time. (i.e. some say between 1k-3k, some say 7k-9k). This is the altitude you set the prop for. Whatever they fly at the majority of the time and don't worry about the percentage outside your norms. You will have a slightly flatter pitch prop for the guy that flys at 8k-10k over the guy that flys at 1k-2k. You want your engine to be able to run at it's best HP rating and best torque for your normal everyday flying. Mine is set up for 3k-5k in altitude. Remember a ground adjustable prop setup is a compromise trying to get the best performance at a specific setting for all flights. (it can't be made perfect for all scenarios)

The props on the market and Mfg's that have props for the Rotax 912's know what the Rotax turns for rpm so set your prop for the correct engine rpm for the best combination. This is also a compromise. 

Try it, if you don't like it change it back.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

rholder(at)avnet.co.uk
Guest

Posted: Mon Nov 03, 2008 1:17 am Post subject: setting prop pitch

Roger Lee wrote:



 	  Quote: 	
		   See the torque chart attached. (Top One) Max torque is

 5000 rpm after that torque goes down and HP climbs. 

 There is no magic here just the best compromise. Set

 your prop to get 5500 engine rpm at WOT in the air flat

 and level. At this setting you will have the best

 setting for you average pilot and plane. Most of us do

 not need special prop settings like maybe a float

 plane. This is not hard, don't make it hard. I have a

 Dynon system and have all the info.


	


Gawd. I have a Constant Speed prop. There is no "setting

the blade angle on the ground" !

Everything is different ! I don't

want to run at 80, 90 or 100% power, I want to run at 115

knots at the most efficient (least fuel) combination of

rpm and MAP.



115 knots is about 55 or 60% power. I cannot get that with

the throttle wide open, as the prop won't go that coarse.



I take your point that the more rpm the more friction in

the engine. Also the smaller the opening of the throttle

butterflies the more "throttling" of the air supply there is.



The test question was posed to get ACTUAL test results for

the different combination of rpm and MAP which each give

the same speed. If I had a fuel flow guage i would have

done it long ago.



This will give real figures; I am unsure about your "most

efficient at max torque" statement. And from your .doc

attachment max torque seems to be at 5100 !



Richard

Gtblu

Joined: 18 Jul 2006
Posts: 37
Location: Australia

Posted: Mon Nov 03, 2008 2:52 pm Post subject: Setting Prop pitch

Hi Roger et al  

  

Great thread!   

I’ve just put on a new Bolly Prop 3 blade to a STOL a/c T/O about 38kts and max cruise about 75kts, and am currently straight and level WOT at desired altitude with revs 5600. Because I want the STOL performance rather than speed, does this sound ideal? I’m wondering how much I’ll lose of that initial acceleration on take-off if I increase the pitch another degree.   

 

I have had a borrowed wooden prop 2 blade for a while, and I’m astounded how much softer it is to fly. It seems to absorb the vibrations compared to the compound props. Mind you even the wooden prop resonated at 4000 revs, whereas the new Bolly doesn’t.  

  

It’s a compromise.  

Regards  

Geoff Bell

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Mon Nov 03, 2008 9:05 pm Post subject: Re: setting prop pitch

Hi Geoff,



Balancing your flight characteristic needs. 



You get a little better climb with 5600 and if you fly high (above 8K-11K) you will have a little better performance. Other than that you might not notice a big difference between 5600 and 5500 WOT prop adjustment. Just a little. You loose a tiny bit of your top end too at 5600, but again only a small amount. You are not supposed to run over 5500 for more that 5 min. To drop 100 rpm you might only want to add back in about 

.5-.75 degree. If you had two of your plane's and your plane was set to get 5600 and the other guy was set for 5500 you would most likely have to run a little more rpm to keep up with him at a given flat and level cruise. You could out climb the other guy and at 10K you would probably have a higher top end speed at WOT. 

If you are happy where your performance is right now then leave it alone. Each setting in a props workable pitch range has its pros and cons. It is just a matter of which flight characteristics you want. 5500 rpm just is a good all around number for the best all around performance and keeps you in the Rotax specs without ever giving it another thought.

It's all about balancing out you flight needs.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

Gtblu

Joined: 18 Jul 2006
Posts: 37
Location: Australia

Posted: Mon Nov 03, 2008 11:11 pm Post subject: Re: setting prop pitch

Hi Roger,

Thanks for the info. My curiosity will get the better of me, so I will try a .5 addition, just to see.



Cheers

Geoff Bell

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