Possibly needing prop adjustment

[FlyingMonkey]

Is 7500 a generally-recognized best compromise for all fp aircraft, or does it vary significantly for different models and 7500 is just the CT's best-speed altitude?

 

My understanding is that it's a function of an air breathing (non-turbo) piston engine.  That's just the density altitude where such an engine makes the best power-to-drag ratio.  As CT said, higher up the power drops off to quickly relative to drag, and lower the drag rises sharply more than negating the increased power.

 

Personally, I rarely climb that high since my field elevation is only 950ft, and you burn a lot of fuel climbing that high to go short distances.  But for cross country flights of significant distance, I try to cruise at 6500-8500 if the winds aloft make it feasible.

[Anticept]

TEXT WAS HERE.

 

Have a look at my response above, it explains some other stuff.

[FastEddieB]

Is 7500 a generally-recognized best compromise for all fp aircraft, or does it vary significantly for different models and 7500 is just the CT's best-speed altitude?

This may help:

 

 

Follow the 75% line up to where it starts to drop off.

 

Most NA planes will be similar, due to the air density at 7,500' which will drop their SL HP to about 75%.

 

Note also that lower power settings have different (higher) optimum altitudes.

[gbigs]

There is no such thing as a 'what you fly the most' altitude. Unless all you do is fly in the pattern and at the same temperature all the time.

 

If 7500 DA is the magic number, then FD must use it right?  If not, why not?  If so, then why change?

 

When Morden says he gained 300 fpm on takeoff out of context (no mention of DA, wind, pitch angle) it has no meaning.  A 300fpm increase in climb performance if actually achieved would be a MASSIVE increase.  He should contact FD and give them his magic secret, they can probably sell more planes if suddenly they can add 300fpm to their sales lit. 

 

Ground speed is the number that matters anyway, not an extra 100rpm and 5KIAS - and thats more a function of head versus tail wind.  And climb performance is dictated far more by DA than by prop pitch.  In other words, summer flying versus winter flying.

[Tom Baker]

There is no such thing as a 'what you fly the most' altitude. Unless all you do is fly in the pattern and at the same temperature all the time.

 

If 7500 DA is the magic number, then FD must use it right?  If not, why not?  If so, then why change?

 

When Morden says he gained 300 fpm on takeoff out of context (no mention of DA, wind, pitch angle) it has no meaning.  A 300fpm increase in climb performance if actually achieved would be a MASSIVE increase.  He should contact FD and give them his magic secret, they can probably sell more planes if suddenly they can add 300fpm to their sales lit. 

 

Ground speed is the number that matters anyway, not an extra 100rpm and 5KIAS - and thats more a function of head versus tail wind.  And climb performance is dictated far more by DA than by prop pitch.  In other words, summer flying versus winter flying.

Maybe you don't have an altitude you fly the most, but there has to an average altitude. That is the altitude to set your prop pitch for.

 

If ground speed is all that matters to you, only fly when you have a tailwind. Most evertone else here knows it is true airspeed that really matters.

 

The increase in climb that he reported is just the end result. Adjusting the pitch of the prop is about getting the best performance from you engine. With the early airplanes adjusting the pitch allowed people to get more horsepower from their engines. The CTSW's when they were shipped the propellers installed at the factory, and the pitch was already set. When they started shipping the CTLS they were shipped with the propellers removed, and pitch had to be set here. By this time people had started to realize that they were not getting the full performance from their airplanes because the engine would not turn up to develop full horsepower, so they started to adjust the propellers accordingly. In addition Rotax issued a service bulletin regarding prop pitch and the RPM's the engines should be turning. By the time you got your CTLSi this had pretty much been figured out, and Lone Mountain set your propeller based on where you live, and the altitudes in your area.

 

Edited to add the last paragraph.

[Anticept]

>Graph was here<

 

Thanks for the help with the graph . Did you see my explanation above?

[IrishAl]

Thanks for the great explanation Corey, and for the visual, Eddie, that crystallizes the explanation perfectly.

 

Unfortunately, there's not too many days in the year here when we can fly that high - we're usually limited to a 2-5k ceiling.

[FlyingMonkey]

I gave the "magic secret"...it's the same one Roger and CT have given.

 

And yes, 300fpm. My airplane was a dog in climb initially, climbing around 700fpm solo on a cool day. Changing the prop gave me 1000fpm in similar conditions. Yesterday solo on a pretty cold day gave me 1150fpm. If I said climb was increased on one day, it would be meaningless. If I say it is consistently higher than it was under a wide variety of conditions, that is meaningful.

 

It is possible the CT props going to the USA are intentionally "dumbed down" to ensure LSA compliance. But since there is no prohibition on changing prop pitch, if there is more performance to be had it's kind of weird to ignore it. It's kind of like finding out your throttle cables only allow 90% open throttle, but refusing to consider adjusting them because "the factory is always right"...

[FastEddieB]

Thanks for the help with the graph . Did you see my explanation above?

Yes I did. Good job.

 

But I think the graph helps cement the concept.

 

Always has for me, anyway!

[Roger Lee]

The factory hasn't always been right on many things, but they do make corrections after time for important items. No MFG has the time nor money to just sit there for thousands of hours just looking for little ways to make adjustments and European ideas can differ a bit from other regions in the world as to what works best in a specific area and or specific aircraft owners needs. There are many LOA's and manual revisions to prove it. 

[Runtoeat]

Surprising that the factory didn't use the full potential of the Rotax by going with a lower pitch early on.  They surely must have known about the improved speeds resulting from flatter pitch. Guess we'll never know why FD originally set the planes up for 5300 rpm WOT but I'm glad that the FD factory recognized their setting wasn't optimal and allows custom pitch resetting to be an option for CT owners.

[Roger Lee]

The pitch originally was set because of fear about top speed breakout. The big problem with setting an engine up to only get 5200 rpm back then was the weaker crankcase. Running 5200 rpm for a WOT setting over stressed the case and caused cracks. That's why engine prior to approx. May 2006 don't qualify for the 2000 extended TBO.

 

This is just one of many several examples that demonstrate LSA MFG's don't always get it right.

[Anticept]

The pitch originally was set because of fear about top speed breakout. The big problem with setting an engine up to only get 5200 rpm back then was the weaker crankcase. Running 5200 rpm for a WOT setting over stressed the case and caused cracks. That's why engine prior to approx. May 2006 don't qualify for the 2000 extended TBO.

 

This is just one of many several examples that demonstrate LSA MFG's don't always get it right.

 

No aircraft manufacturer has gotten things right the first time. In fact, it has been the mechanics more than anyone else that have found problems in aircraft designs, and one of the marks of a good manufacturer are ones that play nice with us

[Marco01]

 

• 4800 RPM - 103 KIAS - 108 KTAS
• 5000 RPM - 106 KIAS - 112 KTAS
• 5200 RPM - 112 KIAS - 119 KTAS
• 5700 RPM (WOT) - 122 KIAS - 130 KTAS

 

These numbers have a lot more meaning if a density altitude is included.

ok, done the test again today with Density Altitude:

 

 

At 2570' DA and OAT 0°C.

4800 RPM - 103 KIAS - 108 KTAS

5200 RPM - 108KIAS - 112 KTAS

5700 RPM (WOT) - 122 KIAS - 127 KTAS

Hope this will help.

 

Got a power loss during Take off due to 1 defective carb floater: will share this under the right post...

[WmInce]

ok, done the test again today with Density Altitude:

 

 

At 2570' DA and OAT 0°C.

4800 RPM - 103 KIAS - 108 KTAS

5200 RPM - 108KIAS - 112 KTAS

5700 RPM (WOT) - 122 KIAS - 127 KTAS

Hope this will help.

 

Got a power loss during Take off due to 1 defective carb floater: will share this under the right post...

 

Please specify, do you have standard landing gear or Tundra wheels/fairings?

[Marco01]

Please specify, do you have standard landing gear or Tundra wheels/fairings?

Standard landing gear with fairings. Flap set at -12°C. Using Mogas SP 95. No passenger. 90 liters in fuel tanks. OAT probe recently moved to the right place (under left wing on the inspection plexi disk: a real challenge to get the wires going along the wing without dismounting it).

[IrishAl]

Surprising that the factory didn't use the full potential of the Rotax by going with a lower pitch early on.  

 

The pitch originally was set because of fear about top speed breakout. 

 

 

 

 I'm pretty sure that there's another reason for it besides this one because it happens in Europe too, and with other manufacturers, and it has not changed for many years.  

 

The C42 has outsold the CT in Europe by a huge margin, so it's a well tried and tested airframe with about 20 years of improvements behind it and yet these have been supplied from the factory way over-propped for many years. 

 

Only today I tested out an 800 hour club aircraft (100hp, 450kg, 2000' and 1024mb) and it gave me 5000rpm/108kts WOT straight and level. 

 

I don't question the forum's correctness on the issue, but like Dick I'm surprised, especially as they have been supplied for so many years - and by multiple manufacturers - propped for 5000rpm WOT. 

 

There must be a reason for it, as I can't see how they could all be wrong for so long about such an obvious issue.

[gbigs]

I am flying with the factory set pitch and get the performance indicated in the POH.  And I am not flying at sea level.  Chasing prop pitch to gain a little more torque and RPM is a dog chasing it's tail.  It's a fixed pitch prop and you will just be redoing it for winter versus summer, and if you fly at altitude versus sea level.

 

Roger is a mechanic with a LOT of expertise and the tools and time to play the game.  The rest of us should not try to fix what isn't broken.

[Runtoeat]

IrishAl, do you have a link to the C42?  I'm curious what it looks like?  Wonder what the results might be if the prop was adjusted to allow 5600 rpm WOT with the club airplane you fly?  If the C42 is a slippery craft like our CT's, it might respond to the added horse power by providing more speed?

[Mike Koerner]

Dick,

I Googled "C42" and got Wikipedia, the manufacturer and lots of photos!

I'm surprised they get 100 knots with multi-braced wings, multi-braced main gear and even braced horizontals.

Mike Koerner

[IrishAl]

It's a most unusual design - the entire aircraft is built on a single 6" aluminium tube. 

 

The passenger cell and wings are just bolted on to the tube, as is the undercarriage.  The 6" tube runs full length, with the engine and nose wheel bolted on the front, and the tail feathers bolted on the back.  The fuselage is nothing more than a fibreglass skin with no significant structural purpose. 

It has around 35% of the German market alone, outselling every other competitor in Europe by a huge margin.  It's the C150 of Europe.

 

I think it is successful because of a combination of factors: the engine/nose wheel arrangement is immensely strong (no firewall mount to bend) and so it's great for training.  The main gear has built in shock absorbers, also making it very hard to bend.  It's easy to repair - bits are bolt off/bolt on.   A lot of people love it's flying manners - it's easy to land, and great with short/rough field landings.  Cabin is about 49" wide, so there's  lots of room.  

 

Sorry to keep going back to the same issue, but I'd love to find out why they still come out of the factory over-propped.  Can the manufacturers really be so far behind the curve on this?  

 

Dick - it's not slippery with all those struts on it, so using the extra power might not add too much more speed, but I too would love to see what it could do with 5600rpm!   

 

Come to think of it, Vne is only 121kts - could that have anything to do with it, i.e. maintaining a bit of margin between top speed and Vne? 

[FastEddieB]

Could noise be a factor?

 

I know motorcycles often were delivered "overgeared" to pass drive-by noise regulations in top gear. Dropping one tooth (or more) on the countershaft sprocket was/is analogous to flattening the pitch on a ground-adjustable prop.

[IrishAl]

I can't see noise being the issue as they are not that noisy to begin with.

 

The only logical explanation is that Rotax themselves have something to do with it.

 

Surprising that the factory didn't use the full potential of the Rotax by going with a lower pitch early on.  They surely must have known.... Guess we'll never know why FD originally set the planes up for 5300 rpm WOT.

 

It wasn't just FD that set up their aircraft over-pitched - Ikarus have been doing it for years.  

 

The only common element is Rotax - did this over-pitching instruction come from Rotax themselves?

 

It annoys me when simple things don't add up, and it doesn't add up when experienced users all say 'pitch for 5600 WOT' while the American and European suppliers sell new aircraft pitched for 5000-5300 WOT.  

 

Can anyone shed light on this?

[Ed Cesnalis]

When I first got my CTSW most pilots / owners were confused about the compromise that exists when selecting a prop pitch.  The common misunderstanding was that we needed to choose between cruise speed and climb rate.  Looking at the performance chart I could see that power for both good climb and best speed would require a flatter pitch.

 

The compromise isn't between climb and cruise but between speed (including climb) and economy.  A course prop has its place and that is at a low power setting.

 

I don't think this is true of Rotax only, both my 180hp Skyhawk and Cherokee 180 couldn't realize best performance, they were pitched more for a low altitude low power setting too.

[Roger Lee]

Rotax is absolutely vemonant that rpms at WOT be at least 5500+ and especially adamant to stay away from WOT settings down around 5100-5300. This is taught in all Rotax schoolls, it's in bulletins. Just because an aircraft MFG does things like this doesn't make them right. In the mid 2006 and earlier Rotax crankcase's it caused cracks.

 

The problem with too many aircraft MFG's is they don't go to Rotax schools, read all the manuals cover to cover or ask questions from the factory or more experienced engine people.

 

Everyone wants to reinvent the wheel, repeat history or do their own thing whether it's right or not.