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Is my prop *too* flat?


FlyingMonkey

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I know that most CTs come from the factory with their props set very coarse, which hurts performance.  When I got my airplane from the original owner, it had the factory prop settings and was a poor performer, only getting about 800fpm climb max and and max level speed at 3000MSL was 108mph @ 5150rpm.

 

Based on advice from Roger and others, I had the prop re-pitched.  I don't recall the exact settings, but I think it was flattened by 1 - 1.25 degrees.  The difference was like night and day.  My plane now climbs at 1100-1200fpm solo on a cool day, and not less than than ~750fpm even at full gross on a hot Georgia afternoon.  I can now get the following repeatable and reliable speeds when flying solo at 3000MSL:

 

5000rpm: 106kt

5200rpm: 112kt

5400rpm: 121kt

5500rpm: 126kt

 

I think this puts my 2007 CTSW near the top of CT performance.  Having non-tundra (i.e. "roller skate") wheels helps as well.  ;)

 

I know that in general, "flatter is better than coarser" to get good performance from the 912ULS.  My max rpm is on the high side, about 5750rpm or even a hair higher at WOT at the 3000-4000MSL I typically fly at.  This is even beyond Roger's recommendation of having 5600rpm or better in level flight at your typical cruise altitude.

 

When flying to/from Oshkosh, I was flying at altitudes well above my norms, mostly 7500-9500ft.  At those altitudes I expected to run out of rpm, but I really didn't.  Even at 9500ft on the way home, I could still do slightly better than 5500rpm at WOT.  We cruised at 5500rpm coming back to try to beat the weather building in the south, and I was seeing 118kt/136mph GPS ground speed with neutral winds or possibly a slight headwind.  9500ft is above optimum power vs. drag altitude of 7000-8000ft for naturally aspirated piston engines, so this seemed like a great number.  

 

I still had to manage rpm because it was possible to get into the yellow arc, maybe up to 5550-5600rpm or so at WOT at 9500.  The engine seemed fine running 5500rpm all day long, as Rotax advertises...we did it pretty much all the way to OSH to keep up with our faster companion planes, and all the way home to make sure we avoided weather.  Not a hiccup.  It does suck fuel though, I was probably burning 6gph or a little higher.  Seems we burned right at 24-25 gallons in our last 4 hour leg (hard to know exact numbers with only sight tubes and dipsticks to go by).  

 

I'm very happy with my airplane's performance, but the question lingers...is my prop pitched *too* flat?  Am I leaving anything on the table or causing any problems at my current settings, or did I just get lucky and happen to hit the exact sweet spot with my setup?

 

Opinion, speculation, and recrimination welcomed.   :D 

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Andy,

 

I'm not an expert on prop pitch but I've been tweeking my prop from time to time to get the best climb/cruise performance.

I beleive that 5750 is on the high end and that you could benefit in cruise by adding 1/2 degree to your pitch.

 

The dealer had my prop pitched at 23.5*. At that pitch I only had 4900 RPM at take off. After posting on this forum and receiving good advice I re-pitched to 22* which increased the climb RPM to 5250 and red line easy to reach in cruise. However my cruise performance was not as great.

 

I reset the pitch to 22.5 and am very happy with the outcome. I get 5100-5150 at climb out and max cruise RPM of 5450. At this point going either way 0.5* on my aircraft, with the DUC inconel prop on the Sting S4, would make one aspect better at the cost of the other.

 

I'd try adding 1/2* in your situation. I think your cruise would improve while giving up a little on the take off performance. Find the :) place.

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Thanks guys.  I agree that coarsening it a small amount might help me be a little faster in cruise.  But I'm happy with my cruise numbers and I've never heard of anybody complaining about having too much climb.  :)

 

I think unless my settings are way off or there is some compelling reason to change things (such as engine wear problems or something), I'll accept that it's not "perfect" and be content with it being "pretty damn good."  I like that I can be reasonably efficient at any altitude, even if it's not totally optimal at any particular altitude.

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At your highest cruise altitude if you have to throttle back to limit to 5,500 then you are leaving some performance on the table.

 

You may be too flat but not by very much.

 

 

I still had to manage rpm because it was possible to get into the yellow arc, maybe up to 5550-5600rpm or so at WOT at 9500.  The engine seemed fine running 5500rpm all day long, as Rotax advertises

 

2 things, you were well above the 75% max power altitude and you were not WOT so you were cruising at 70% power or less, yes your Rotax can do this all day long.

 

What if you were cruising the beach at 5,500 WOT?  80% plus?  This is why it makes sense to optimize for 7,500 and not lower even if you mostly fly lower.  At 80% plus you will throttle back due to noise.  This is why you guys report cruising at 5,200

 

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With a ground adjustable prop it's all about balance. Balancing fuel economy, climb and cruise performance.  Some of us have special needs like flying in and out of short backcountry fields where climb may be more important than cruise and fuel.

We don't want to be too flat or too coarse. Too course is worse.

 

Some owners may live at sea level and never fly over 3K' and some like Ed live at 7K' and fly from 10K-13K all the time. Both owners have different needs and prop pitch should be considered for your best BALANCE.

 

You can set it for anything you want like 5800 rpm at sea level, but just know there is price for that climb performance on the other end of fuel economy and cruise rpm. If you have a prop set too flat like 5750 rpm at 3K' and your buddy in the exact same plane is set for 5600 rpm at WOT your buddy will be faster, save more fuel and you will be cruising at 5300-5350 to keep up with his 5200 rpm.

 

You should set your prop to either achieve the special needs operation and for the huge majority for your AVERAGE ALTITUDE. The sweet spot for rpm is around 5600-5650 RPM at WOT at your average altitude knowing that sometimes we fly lower and sometimes we fly higher. This particular rpm setup gives the best balance between fuel economy, climb and cruise performance.

 

 

I've done all this research and testing years ago because of Sensenich. We won't get into different props.

 

Some wise Chinese person must have said:    Balance grasshopper, BALANCE.

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Before you fine tune ask yourself if you want to find a compromise or adjust twice a year, summer setting and a winter setting.

 

I found to perfect my pitch I needed to re-pitch for the seasons.  If you get yours perfect in August come January you might want it a bit flatter.

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Also the engine matters.  The 912iS with sport package has higher torque and faster climbs than the other engines.  Since no one knows what assumptions the factory makes when it delivers planes the only way to tell if the prop is a problem is the Roger test....

 

"sweet spot for rpm is around 5600-5650 RPM at WOT at your average altitude"  altitude being density altitude.  The climb rate varies widely dependent on density altitude at field level.

 

We prefer optimal cruise rather than optimal climb...also a personal choice.  Our fuel burn is 4.5 gpm or less at 5450 or so at 10k density altitude and we get about 120ktas in that scenario...

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 It does suck fuel though, I was probably burning 6gph or a little higher.  

 

It's funny how relativism sets in.  I too have started feeling profligate at 6 gph burn and often reduce rpm to achieve a fuel burn below 5.  That's $3 an hour I'm trying to save....in the most fuel efficient plane I have ever flown.    

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Also the engine matters.  The 912iS with sport package has higher torque and faster climbs than the other engines.  Since no one knows what assumptions the factory makes when it delivers planes the only way to tell if the prop is a problem is the Roger test....

 

"sweet spot for rpm is around 5600-5650 RPM at WOT at your average altitude"  altitude being density altitude.  The climb rate varies widely dependent on density altitude at field level.

 

We prefer optimal cruise rather than optimal climb...also a personal choice.  Our fuel burn is 4.5 gpm or less at 5450 or so at 10k density altitude and we get about 120ktas in that scenario...

 

I'm not changing my engine, so let's assume for purposes of my question it's a 912ULS.  I would sure like to see your data showing faster climb rates for 912iS vs. 912ULS in same model aircraft at identical weights...I don't think that data exists.

 

Wait...climb performance varies with density altitude?!?  That sure explains a lot, thanks!   :thinking-1376:

 

In the climb vs. cruise debate, I would favor climb.  I don't think anybody ever died because of lowered cruise performance, but insufficient climb performance kills pilots every single year.  

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Sounds pretty close to what I would consider ideal.  The only way I would add pitch (less flat) is if the current configuration had more take-off and climb performance than I wanted or needed.  At this point, I think that any change is a matter of optimizing the balance of climb versus cruise that best suits your needs.  

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Andy,

 

If you want to make an adjustment then .25-.5 degrees would work. I'm guessing the .5

I tried 5750 and for my average altitude it was just a tad too flat. Slightly better climb, but I used more fuel and used more rpm to keep up with the guys setup at 5600.

I rarely fly at anything over 7K' and usually around 3K-5K Ft. Do I ever get over 7K, yes and up to 10K, but it is 1-2 a year so a prop adjustment isn't worth it for an hour flight and I fly heavier than the majority of you guys. 

 

 

I never re-pitch for seasons. It's close enough.

This is all just advice and owners have to make up their own mind.

What most owners never have is 4 2006 CTSW's with different props all taking off together and flying side by side at the same time with only one person setting prop pitch for all of them. This is where some good research info comes from. 

 

If you want balance then the 5600-5650 at average cruise altitude works very well. If you  as an owner want to tweak it a tad higher or lower then that's up to the owner and their personal preference. 

I never worry about fuel economy just engine health. Fuel is cheap, engines and parts aren't. Too flat a pitch and too corse a pitch usually burns extra fuel.

 

Last year when I was playing around with pitch I tried 5725-5750 against anther CT with only 5620. He saved more fuel and had better cruise performance. I did have better personal aircraft take off for me, but suffered in cruise and fuel. When he was only 5150-5200 I was at 5300 and burning more fuel.

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RIch -- just curious.  What altitude do you target for best performance?  As an example I target 55-5600 rpm at 5,500'.

Dan,

 

I fly between 4500 and 5500 on trips up and down the east coast. That's where I set by prop for best performance. I set my RPM's at 5200 to 5300 for best economy and see an airspeed of 112-115 Kts.

 

On my trip from FL to PA I flew at 5400-5450 RPM's to try and beat the wx and arrive in PA at a reasonable time. If I remember correctly, I burned 6.1 GPH at that setting at 3500 MSL, where the favorable winds were. The extra speed cost approx 7.5 gallons of fuel.

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For the sake of accuracy, when posting airspeed numbers, please designate if those speeds are true airspeed or indicated airspeed.

 

Wmince,

 

My post was indicated. I pretty sure we went over this topic before and found that in most cases there was ony a knot or two difference. I believe it was my post on the x/c from North Tampa to PA that got the true/indicated topic going. I'm sure it's in the Trip Report section of the forum

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Does putting more hours on an LSA to save fuel actually reduce cost?  All things considered, I would think not much if at all.  In 50+ years of flying, I don't think I have ever flown at less than max allowable rpm.  It just doesn't seem right.

 

Say over a ten year period of flying your LSA at reduced RPM's to save fuel, you save a hypothetical $1,200.00.

You've put more hours on the plane saving fuel. If you fly frequently, that could add up to a lot of hours on the hobbs.

At re-sale I wonder if it's worth the savings?

 

If you're keeping time on your engine using a tach, then I believe it's worth while saving on the fuel.

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I typically cruise at high RPMs ~5300 when I'm going from point A to point B.  If I'm just tooling around the patch then I throttle back to 5000.  Doesn't really save a lot, maybe 0.5 gph, but figure I'm really out flying for a period of time and not a distance so why not save a couple bucks (which I use to buy my coffee afterwards :)).

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The total cost to own and operate an aircraft falls into two budget areas:   Indirect and Direct costs.

 

Indirect: purchase price (or monthly payments plus interest), insurance, tie down or hangar fees, subscription fees, taxes, depreciation

Direct: oil, fuel, engine TBO, propeller, scheduled maintenance, out of warranty repairs

 

Example of Direct cost for a Cessna 172 (100 tt/yr):

 

Fuel burn: 13 gallons per hour -- fuel $6 a gallon: $78
Oil: 1/10 qt per hour, $90 per qt: $9
Oil change --  $2.50 if you change it yourself
Engine overhauls: 1500 TBO; that's $24k for the engine and another $2400 for labor: $17.60
Prop overhauls: Every 1500 hours; $2500 for the prop + $2000 for the governor = $3
Total: $110.10 per hour

 

Fuel is 70% of the direct cost to operate the plane per hour, therefore, any reduction in that cost has the largest effect on total Direct cost.

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