Prop Pitch

I know that this query will raise some eyebrows regarding my actions and goal in making this prop pitch change, but it's a rainy day, and I needed a project.

History: I have 70 plus hours on the build now, and I have a pretty good feel for the present performance. On my first flight following the sign off, I found the prop to be a bit flat, and I was concerned about over-revving, so I increased the pitch, and it turned out a bit course. I now have a full throttle cruise of 5250, and although the speed are good, it doesn't take long for the oil temp to start climbing. Also, I would like to shorten my takeoff roll a bit, particularly when i have full fuel, and a passenger. All things point to the need to be a bit flatter. How much is the question.

Since I can't do it the suggested way of, making an adjustment, followed by a static performance test, and then to go fly, I plan to make a stab at it, and see just how close I can get. I'm thinking that one full degree flatter would do the trick. I did a search and found very little in actual documented experience, with real numbers, but from what I did find, it sounds like one degree flatter should get me close to a 200 to 250 rpm increase in full throttle performance. I'm just looking for another educated guess for now, because it looks like it will be several days before I can get an opportunity to test fly.

Tom

Hi Tom,
On the 1500 hr. TBO engines setting the prop pitch too course like this put cracks on the top of the case. In about mid 2006 Rotax changed the case.
You should be pitched to get over 5500 WOT in level flight at your average cruise altitude.
Too course a pitch cause increased oil and engine temps, major loss of fuel economy, worse take-off performance (a big concern when the density altitude changes on you or you need to clear an obstacle), poor WOT top speed. You have to have more throttle (more fuel usage) to maintain the same speed that one of your fellow RV12 pilots have that is set to get around 5600 WOT in level flight.
This was such a concern years ago Rotax talked to aircraft MFG's about the excessive stress that ta WOT of only 5200 rpm can cause.

As an example:
I had to pick up a Flight Design CT at one airport. I flew mine over to get it. A friend was in my plane. My prop is pitched to get 5650 rpm. he took-off and left me standing in the dust. He lifted off the runway way before I did and proceeded to out climb me really bad. I had to radio him to slow down. I finally caught him. I had to fly WOT at 5150 and he was only flying at 4650. My fuel usage was 6.2 and his was 4.2.

Your engine was deisgned to run at 5500 rpm all day long and between 5500 and 5800 for up to 5 minutes. That has a pretty good safety margin built in. I t actually handle more. (factory liability and all that)

If you were to set your prop up to let's say get 5650 WOT at your average altitude (that part is important) then throttle back to some where between 5100-5400 rpm you will fly faster, save more fuel and climb better. Here is a small extra benefit. If at landing you have an incident where you need full power and rpm right this second to go around or save yourself from a stall the 5650 rpm setup will give a much faster response and better climb than one set at 5250 which will be slower to spin up and have less saving climb.

Vans has it right about having an WOT rpm that goes over 5500 rpm.
I have re-set at least 75-90 aircraft's prop pitch because someone or a MFG set them too low. To this day not a single person has ever gone back and two MFG's changed their prop pitch from the factory.

I tested 10 props 2 years ago for a couple MFG's. These settings held true for all of them.

Static rpm will get you in the ball park before a first flight with a new prop. After that you need to fly it to fine tune it. If you have an in flight WOT rpm and want some help on how many degrees to go one way or another you can call me for help. 520-574-1080

p.s
If you want to go from 5250 to around 5650 try about 2 degrees flatter. It will feel like someone put a new engine in.

Here is a good article on the Rotax website about prop pitch for the Rotax.

http://www.rotax-owner.com/rotax-blog/item/9-understanding-the-ground-adjustable-prop

This is the data I have:

RPM Blade angle (measured with my setup)
4400 43,8
4600 45,7
4680 47,5
5000 48,2
5170 49,1

My WOT on the ground is now just below 5000RPM.
WOT at 1500ft 5500RPM 128kts.

Gagarin737 said:

This is the data I have:

RPM Blade angle (measured with my setup)
4400 43,8
4600 45,7
4680 47,5
5000 48,2
5170 49,1

My WOT on the ground is now just below 5000RPM.
WOT at 1500ft 5500RPM 128kts.

Click to expand...

Good job Joeri, the angle is tool dependent but interestingly the angles I get are close to yours. What is not tool dependent is the variation of RPM per angle variation so your publication above is a useful reference for everyone using the Sensenitch propeller on a Rotax powered RV-12 (you may want to confirm it is the case as you are not under US E-LSA regulations in Europe).
Personally I prefer to be a tad above 5500 WOT at altitude and throttle back a bit to get 5500 but I usually throttle back to 5200 to stay at the top of the green.

My RV-12 is build according E-LSA with wheel pants, all kits bought from Van's. I normally cruise with 5000RPM at 1500'->110kts IAS. Fuelflow (average last 70 hours) 17,5l/hr=4,6USG/hr.
I guess I can improve take-off performance a bit but I'm quite happy now with the performance. Take-off roll 1320lbs/59*F/sea level/average pilot is about 850ft, 100ft more than given in chapter 5-3.

roger lee said:

Hi Tom,

Static rpm will get you in the ball park before a first flight with a new prop. After that you need to fly it to fine tune it. If you have an in flight WOT rpm and want some help on how many degrees to go one way or another you can call me for help. 520-574-1080

p.s
If you want to go from 5250 to around 5650 try about 2 degrees flatter. It will feel like someone put a new engine in.

Here is a good article on the Rotax website about prop pitch for the Rotax.

http://www.rotax-owner.com/rotax-blog/item/9-understanding-the-ground-adjustable-prop

Click to expand...

Thanks Roger, for the great explanation. Your p.s. gave me the tip that I was looking for. So the bottom line is that one degree change should net about a 200 RPM WOT difference. That info alone should reduce the number of re-pitch tasks. I will do what I need to do to get to 5650 WOT.

Tom

Hi Tom,

Glad I could help. I think you'll like the 5650 area. Because we only have a ground adjustable prop we need to balance our flight characteristics with a balanced WOT rpm choice and still stay within the Rotax parameters. There are special considerations at times like flying with floats or living in Leadville, CO the highest airport in the US.
I always advise to to set the pitch for your average cruise altitude. If you live in Florida and never go about 2K that prop pitch would be different than a friend on mine in Mammoth, CA that lives above 7K and always flys because of Mnts. at 11K-13K. We loose 3% of our engine efficiency per 1K in altitude. So at 10K we have lost 30%. This means we need to set the prop so it can develop as much HP and torque at our cruise altitude as we can and help the engine to not get overloaded with too course a pitch prop.


Have fun. If you need any help give me a call.

Thanks again Roger. I can't wait to see the results of my latest prop pitch tweaking. Just waiting for the weather to break, but I will post it.................Tom

I kept a record of my "pitch and fly" settings

and plotted them to more quickly home in on the desired performance. I get to 5500 rpm at WOT in 45-60 seconds and climb at about 5100 rpm. 200 rpm gets me about 108 kt TAS cruise solo or about 103-104 with 225 lb passenger/copilot. Weight (thus angle of attack) does make a difference in such a light airplane!

Wayne

yankee-flyer said:

and plotted them to more quickly home in on the desired performance. I get to 5500 rpm at WOT in 45-60 seconds and climb at about 5100 rpm. 200 rpm gets me about 108 kt TAS cruise solo or about 103-104 with 225 lb passenger/copilot. Weight (thus angle of attack) does make a difference in such a light airplane!

Wayne

Click to expand...

What static RPM can you pull? Looks like your prop is a little over-pitched. I usually get 112-115 KIAS and can run over 5500 all the way up to about 9000 MSL. If you can pull 4950 static, that is the sweet spot.

This Service Letter from Rotax says that you shouldn't run below 5200 WOT at takeoff/continuous operation:
http://www.flyrotax.com/portaldata/5/dokus/d04645.pdf

It specifically says that fixed/ground adjustable propellers "should be set so that take-off RPM is above 5200 WOT". Presumably this applies to all phases of the takeoff before reducing the throttle to cruise, including being stationary at the start of the take-off roll, or equally, being stationary at WOT while doing the initial prop adjustment. I don't understand how you can set the prop for 4950 on the ground and comply with this SL.

Good question.

It doesn't say static RPM.

It doesn't say climb RPM.

To me, based on nothing, it means that RPM you attain just before lift off.

Maybe Roger has an answer??

rgmwa said:

It specifically says that fixed/ground adjustable propellers "should be set so that take-off RPM is above 5200 WOT". Presumably this applies to all phases of the takeoff before reducing the throttle to cruise, including being stationary at the start of the take-off roll, or equally, being stationary at WOT while doing the initial prop adjustment. I don't understand how you can set the prop for 4950 on the ground and comply with this SL.

Click to expand...

I don't read it the way you do and I don't think any other LSA manufacturers do either.
If the prop was set for 5200 RPM static on the runway before movement begins, all of the faster performing LSA's would be rockets on climb, but turtle slow in cruise because you would have to throttle back a lot.

If the prop is set as recommended on an RV-12, a cruise climb will result in about 5100 RPM (which is not all that far off of 5200)

Keep in mind what this Service Letter (with Recommended, compliance... it is not an operational requirement) was issued for in the context of take-off engine RPM. It was to assure a safe margin from the possibility of detonation and/or pre-ignition (their words). If you are careful to use the recommended fuel octane, and keep all of the recommended maint, and adjustments up to date, you should not have any problems using the RPM recommended for the RV-12. If you owned an airplane that was often running near red line for CHT and coolant temps, it might be prudent to comply.

BTW, I have always found it interesting that Rotax says in this document that a manifold pressure gauge (which all ELSA RV-12's have) should be installed for a fixed pitch prop because it is necessary to use when setting the pitch on the prop. but it never says anything about what to use the instrument for (no MP values are given, etc.).

In my opinion, some of the other issues highlighted in the Service Letter should be considered of higher importance to RV-12 owners. Such as prop balance, maintaining good carb. synchronization, etc.
The tip on shut down procedure is also a good procedure to follow, to reduce stress on the gear box during shut down.

Thanks for the reply Scott. Maybe it was just the way I was reading the Rotax SL, but it seemed to contradict what I understood about the usual way to set up the RV-12 prop.

You will find that many of Rotax's writings consider the in flight adjustable prop that the rest of the world gets to use, but we don't. Some interperlation needs to be used. We need to consider the fixed pitch prop, the ground adjustable and the in flight adjustable. Unfortunately the manual doesn't seem to seperate these very well.

Update

Okay, I finally had the opportunity to take a test flight after making that "one degree flatter" change in prop pitch. To reiterate, before making the change my WOT rpm was about 5250. The change in performance is quite noticeable. I always fly 2 up and lots of fuel This test flight was at about 1250 lbs. It was a cool morning, around 50 deg. clear, and light winds. The take off roll was stellar, and appreciably shorter. Climbing out at 75 kts showed 5350 rpm, and over 1,000 fpm climb, all the way from 380 ft, to 3,500 ft. Leveling off, throttling back to 5200, and trimming up, netted me 110Kts. So, what's not to like..................Tom

Curious.... What was your WOT static rpm after going 1 degree flatter? Should be 4950-5000 rpm. Did you check the WOT rpm after accelerating to maximum speed when flying?

MartySantic said:

Curious.... What was your WOT static rpm after going 1 degree flatter? Should be 4950-5000 rpm. Did you check the WOT rpm after accelerating to maximum speed when flying?

Click to expand...

Marty, I did not do the static or flat out tests. I'll get there, but I was so interested in testing the takeoff and climb performance, that I skipped these items. I just didn't think that it was necessary at the time, but with the numbers that I have during climb, I would think that my static would be very close to 5,000. It is just so hard to come up with zero wind, in order to do an accurate static test. Perhaps, on the next test flight, and I will re-post..

Tom

todehnal said:

Marty, I did not do the static or flat out tests. I'll get there, but I was so interested in testing the takeoff and climb performance, that I skipped these items. I just didn't think that it was necessary at the time, but with the numbers that I have during climb, I would think that my static would be very close to 5,000. It is just so hard to come up with zero wind, in order to do an accurate static test. Perhaps, on the next test flight, and I will re-post..

Tom

Click to expand...

If you are getting 5350 climbing at 75 kts you are now way under pitched. The simplest way to get close is adjust to get 4950 - 5000 WOT on the ground. You would probably be happy and not change it again.

rvbuilder2002 said:

If you are getting 5350 climbing at 75 kts you are now way under pitched. The simplest way to get close is adjust to get 4950 - 5000 WOT on the ground. You would probably be happy and not change it again.

Click to expand...

Agreed. Closer to 4950, the better.

Okay, I had a chance to check my static this morning and it was 5170. A little high from what is being suggested. So, I repitched again ( added .7 degrees of pitch) and I now have 4970 static. Both blades are right on the money, and it runs very smooth, so I plan to leave things alone for now. I will check my climb RPM, and level flight WOT RPM, but it sounds like I am finally in the ball park.

Two issues caused me to flatten the prop in the first place. During hot weather, 100 degree climb, my oil temp was approaching 225 degrees, so I pulled back the power and flattened out the climb, stair stepping up to altitude. I was not real happy with that. I know that even at 230, I am still in the green, but I would rather be a little conservative with the engine, as I am not sure what others are running in a hot weather climb. Hot weather cruises run around 207 degree oil temp. Secondly, I read a post that Scott made that stated that it was very easy in level flight to exceed the 5500 RPM redline, and I was only getting about 5460, level with WOT. These two items made me think that I had too much pitch, and decided to flatten it 1 degree. It is all starting to run together, but as I recall, my static was about 4920 before I started all of this. My hope is that going from 4920, to 4970 will bring it all together. I guess I could make another tweak, and try to hit that magic 4950, but I plan to try it as is for awhile.

Tom

Glad to hear Tom
I was skepticle about the 1 deg change orig. recommended to you but figured you would eventually work it out.
BTW 4950 is just a starting point recommendation, so you may be very happy with it just a bit higher in your warmer OAT's.

Hi Tom,

Sounds like your there or really close.

Static is a good place to start for an unknown like a new prop on a plane or a first start. It is just for a ball park setting. The real proof of the pudding is actual flight at WOT in level flight at your average altitude. Someone in Florida at sea level that never flys over 2K' should have a different prop pitch than someone that lives in Mammoth, CA and always flys at 12K or higher. The 4950 bench mark is a fair place to start. The 225F you mention is nothing. The yellow starts at approx. 230F. Many see around 240F on a sustained climb. Even that isn't a big deal. If you have a higher oil temps of 245F+ on take off and a max WOT rpm lower than 5500 (i.e. 5100-5400) then this usually contributes to higher oil temps. Flattening the prop pitch to get over 5500 rpm WOT in level flight at your average altitude usually results in lower temps because you unload and
un-stress the engine. 5500 isn't max redline for the engine. It is 5800. You can run 5500-5800 for up to 5 minutes which is where many with in flight adjustable props run during take off then adjust back after climb out. A WOT max rpm in level flight is best around 5650+/-. The engine continuous run rpm is 5500 rpm that can run like that for the life of the engine. Most of us set up the prop to get over 5500 rpm, but throttle back between 5000-5400 rpm for cruise. Those rpm settings are just a personal preference for cruise.
Each plane's different prop and fueslage design can affect the end WOT prop pitch and rpm setting.

P.s.
207F is a normal cruise temp for this time of year. (Depends on altitude, OAT and prop pitch)

Loki said:

Hi Tom,

Sounds like your there or really close.

Static is a good place to start for an unknown like a new prop on a plane or a first start. It is just for a ball park setting.
Each plane's different prop and fueslage design can affect the end WOT prop pitch and rpm setting.

Click to expand...

That is the great thing about this forum.
Since it is specifically for the RV-12, we can talk specifics.
There is now a lot of data to show that they all are very close to identical, right down to a specific amount of bend on the rudder trim tab.

Thanks to all of you folks for hanging in there with me. I think that I now have 4 attempts at getting the pitch right and I feel that we are just about there.

Guaranteed there is not a more talented and supportive builders group out there. It all supports one great decision that I made in selecting and RV to build, and I am so glad that I did, including the RV-12 selection, which fits my desires to a T.

Tom

Just 4 attempts? That's perdy good.

WOT with warm engine

I've settled on 4,950 and its been ideal for all the reasons previously posted. Don't forget to check the static ground RPM when the engine is fully warmed up. (i.e. after a flight) Just warming it up on the ground and then throttling up to WOT will give you different readings than what you will measure after the engine is fully warm after a real flight.