Is my prop *too* flat?

[FastEddieB]

True vs. indicated airspeed is directly affected by altitude. The higher you go . . the greater the spread.

 

Here's my old Cirrus at 17,500':

 

 

(click for larger image)

 

Indicated: 121k

True: 161k

 

And only 42% power is available up there, normally aspirated.

 

Note that's on 9.3 gph. An amazingly efficient design.

[FastEddieB]

Back on point, my experience is similar to Andy and Rich's.

As delivered, my Warp Drive prop had the factory 13.5° pitch at a certain station. I was pulling not quite 5,000 rpm on takeoff and climb, which seemed to leave a lot of potential hp unused in those regimes.

I took some pitch out - I forget how but I have it in the prop log. As a result, I now take off and climb at about 5,200 rpm, for slightly better performance there.

But I think my cruise has suffered. To get the same TAS, it sure seems like I need about 200 more rpm, and hence more fuel flow and less range.

May put some pitch back in in the future. Takes a couple hours to remove the spinner, back off the clamping bolts and the prop mounting bolts, measure the change and then tighten and safety the mounting bolts and reinstall the spinner. Wish it was easier.

Really wish it could be done in flight!

[FlyingMonkey]

True vs. indicated airspeed is directly affected by altitude. The higher you go . . the greater the spread.

On a standard day, at a cruise of 7,500' MSL, the difference is about 15 knots. i.e. 110 knots indicated will produce 125 true.

 

That is why I suggested guys distinguish between the two when noting performance.

 

Bill, my original post numbers are IAS.  Here in Georgia with high density altitudes, I usually see cruise speeds of 124-127kts TAS, if that info helps at all. 

[FlyingMonkey]

Back on point, my experience is similar to Andy and Rich's.

 

As delivered, my Warp Drive prop had the factory 13.5° pitch at a certain station. I was pulling not quite 5,000 rpm on takeoff and climb, which seemed to leave a lot of potential hp unused in those regimes.

 

I took some pitch out - I forget how but I have it in the prop log. As a result, I now take off and climb at about 5,200 rpm, for slightly better performance there.

 

But I think my cruise has suffered. To get the same TAS, it sure seems like I need about 200 more rpm, and hence more fuel flow and less range.

 

May put some pitch back in in the future. Takes a couple hours to remove the spinner, back off the clamping bolts and the prop mounting bolts, measure the change and then tighten and safety the mounting bolts and reinstall the spinner. Wish it was easier.

 

Really wish it could be done in flight!

 

Given that your Sky Arrow has both short legs and relatively low speeds, assuming you could keep climb performance decent I'd probably coarsen it up, so you can get a little better cruise speeds for a given RPM and get a little better range.  I don't know what your climb numbers are like, but I bet it does just fine in that regard.

[Tom Baker]

For an owner, yes it does make a difference. Not for a renter on wet rates. But, as you said, not really a huge amount unless you do a LOT of flying.

 

For most airplanes that is likely the case, but if you are using clock time like Rotax suggest it might be different. It might be worth running the numbers to see.

[Ed Cesnalis]

Given that your Sky Arrow has both short legs and relatively low speeds, assuming you could keep climb performance decent I'd probably coarsen it up, so you can get a little better cruise speeds for a given RPM and get a little better range.  I don't know what your climb numbers are like, but I bet it does just fine in that regard.

 

'coarsen it up, so you can get a little better cruise speeds'  this only works with a retarded throttle.  If you do this when  you use WOT you will have reduced your cruise speed and risk lugging the engine.  Its best to pitch for best climb or best speed (both have similar flat pitches) and throttle back for economy.  Pitching coarse for economy/cruise limits your power settings more than other pitch settings.

[FlyingMonkey]

'coarsen it up, so you can get a little better cruise speeds'  this only works with a retarded throttle.  If you do this when  you use WOT you will have reduced your cruise speed and risk lugging the engine.  Its best to pitch for best climb or best speed (both have similar flat pitches) and throttle back for economy.  Pitching coarse for economy/cruise limits your power settings more than other pitch settings.

 

I guess I was not clear.  I meant coarse within the bounds of "can still make 5500rpm at anticipated cruise altitudes" not "so coarse as to put undue load on the engine."

 

I admit/agree that the pitch tolerances for the Rotax engine to be in the correct rpm ranges is pretty narrow, but there is a little room to play there.  Like in my case, I'm going to coarsen a bit and should see a little bump in cruise speed and economy, yet still get 5500rpm up to 9000-10000 ft. 

[Ed Cesnalis]

I guess I was not clear.  I meant coarse within the bounds of "can still make 5500rpm at anticipated cruise altitudes" not "so coarse as to put undue load on the engine."

 

I admit/agree that the pitch tolerances for the Rotax engine to be in the correct rpm ranges is pretty narrow, but there is a little room to play there.  Like in my case, I'm going to coarsen a bit and should see a little bump in cruise speed and economy, yet still get 5500rpm up to 9000-10000 ft. 

 

There is a lot of confusion, most talk of a climb vs cruise speed compromise when the compromise they are talking about is climb / speed vs economy cruise.

 

Only in the economy cruise range can you get more speed by coarsining the pitch, like using a high gear at a low power setting in a car.  So when you say coarsen for more speed it sounds as though you would not be in the 'can still make 5500' range.  In fact when you flatten to get in the 5500 range you will get less speed at a cruise/economy setting.

 

Another thing you said that I question is burning 6gph at 9,500'.  I don't have a fuel flow meter so I am just guessing but I typically fly at that altitude and 5 gph is what I think I burn.  Figure if max burn is 6.8gph and that would be determined by a full rich condition and the main jet at sea level. When we fly at 9,500' WOT we should be full rich determined by the main jet but with 70% the volume pumping through our 912s.  6.8 x 70% = 4.76 gph.   hmmmmmmm,   Am I missing something?

[FlyingMonkey]

Another thing you said that I question is burning 6gph at 9,500'.  I don't have a fuel flow meter so I am just guessing but I typically fly at that altitude and 5 gph is what I think I burn.  Figure if max burn is 6.8gph and that would be determined by a full rich condition and the main jet at sea level. When we fly at 9,500' WOT we should be full rich determined by the main jet but with 70% the volume pumping through our 912s.  6.8 x 70% = 4.76 gph.   hmmmmmmm,   Am I missing something?

 

Just noting my experience.  I took off with approximately 30 gallons, flew 4.1hrs, landed with ~6 gallons.  That's 24 gallons burned over ~4hrs = 6gph.  Now, all I have for fuel management is the sight tubes, and the line man filled the tanks, so it's possible I had less than 30 gallons, or my tubes and/or dipstick are mis-calibrated, or the airplane was not sitting perfectly level, and I landed with more than the 6 gallons I thought I had.

 

Cruise was ~5500rpm @ 9500ft for all but the descent portion of the flight, about the last 20min.

 

My understanding might be faulty, but I was under the impression that maintaining a given RPM requires a given volume of fuel and air, meaning that 5500rpm @ 3500ft and 5500rpm @ 9500ft should require roughly the same volumes or air and fuel.  I could see where the less dense air at 9500ft might have less rotational drag on the prop and allow the engine to spin more freely.  There may be subtlety here I'm not quite grasping.

[Ed Cesnalis]

 

My understanding might be faulty, but I was under the impression that maintaining a given RPM requires a given volume of fuel and air, meaning that 5500rpm @ 3500ft and 5500rpm @ 9500ft should require roughly the same volumes or air and fuel.  I could see where the less dense air at 9500ft might have less rotational drag on the prop and allow the engine to spin more freely.  There may be subtlety here I'm not quite grasping.

 

The volume is fixed, your cylinders and combustion chambers do not change size.  The mass changes with air density.

 

You pump the same volume but the number of molecules depends on DA.

 

Compare dense air at sea level, your cylinders have 1 bazillion molecules that compress 11:1 compared to 70% of 1 bazillion that compresses 11:1 and then fires.  The main jet should maintain the same mixture in both scenarios meaning a reduced amount of fuel at altitude.  Otherwise the mixture at 9,500 would be too rich.

[FlyingMonkey]

The volume is fixed, your cylinders and combustion chambers do not change size.  The mass changes with air density.

 

You pump the same volume but the number of molecules depends on DA.

 

Compare dense air at sea level, your cylinders have 1 bazillion molecules that compress 11:1 compared to 70% of 1 bazillion that compresses 11:1 and then fires.  The main jet should maintain the same mixture in both scenarios meaning a reduced amount of fuel at altitude.  Otherwise the mixture at 9,500 would be too rich.

 

That makes sense.  Still trying to wrap my head around two identical engines at identical RPM making different power.  I guess the denser (lower altitude) mixture has less combustion force against the pistons.  Which is why you need greater % of throttle to get the same RPM.

 

EDIT:  But now I just talked myself out of it.  You need greater % of throttle to be able to get the same amount of air/fuel into the cylinder to *allow* the same RPM at higher altitude.  After all, the rotational mass of the engine has not changed, and it still requires the same force to sling the engine at the same RPM...

 

Hmmm.... 

[IrishAl]

 

 

Another thing you said that I question is burning 6gph at 9,500'.  I don't have a fuel flow meter so I am just guessing but I typically fly at that altitude and 5 gph is what I think I burn.  Figure if max burn is 6.8gph and that would be determined by a full rich condition and the main jet at sea level. When we fly at 9,500' WOT we should be full rich determined by the main jet but with 70% the volume pumping through our 912s.  6.8 x 70% = 4.76 gph.   hmmmmmmm,   Am I missing something?

You took the words right out of my mouth. 

But it's not just Andy who's saying it - look at Eddie's Cirrus figures above.  I bet he would't use 9.3 gph at 42% while flying the beach.

So where does the extra fuel go, as the engine isn't running rich...?

[Ed Cesnalis]

That makes sense.  Still trying to wrap my head around two identical engines at identical RPM making different power.  I guess the denser (lower altitude) mixture has less combustion force against the pistons.  Which is why you need greater % of throttle to get the same RPM.

 

EDIT:  But now I just talked myself out of it.  You need greater % of throttle to be able to get the same amount of air/fuel into the cylinder to *allow* the same RPM at higher altitude.  After all, the rotational mass of the engine has not changed, and it still requires the same force to sling the engine at the same RPM...

 

Hmmm.... 

 

Lower altitude means more combustion force against the pistons.  All other things being equal the higher you go the less power you produce at a give RPM/pitch.

 

You need greater % of throttle to get same RPM at higher altitude because less power is available as altitude increases.

 

The rotational mass of the engine is only part of the load.  If you ran without a prop you would overspeed, the prop and air density present additional load that decreases as altitude increases. Also at higher altitudes you have less air mass to compress into the combustion chamber presenting less resistance.

 

You need more throttle as you climb due to less power available and you present a decreasing load as you climb if you RPM is constant and above 7,500' the decreasing power begins to outweigh the decreasing load and performance begins to deteriorate. 

[Ed Cesnalis]

You took the words right out of my mouth. 

But it's not just Andy who's saying it - look at Eddie's Cirrus figures above.  I bet he would't use 9.3 gph at 42% while flying the beach.

So where does the extra fuel go, as the engine isn't running rich...?

 

I'm guessing he is using only 42% because he is normally aspirated and only 42% is available at 17,500'

 

You are guessing that extra fuel is being used vs 42% at sea level.  Can you fly at sea level with 42% power?

[IrishAl]

I'm guessing he is using only 42% because he is normally aspirated and only 42% is available at 17,500'

 

You are guessing that extra fuel is being used vs 42% at sea level.  Can you fly at sea level with 42% power?

Yup, that's what I'm guessing. 

Andy definitely could fly at 75% power at sea level but wouldn't use 6gph.

[FlyingMonkey]

Thanks guys, the lightbulb is starting to illuminate.  

 

Basically, The fuel/air charge is making less power, but the RPM is still the same because the drag on the engine through the prop is decreasing at the same rate or faster than the amount of power reduction, correct?  So as we climb even higher there will be a point where maintaining that RPM (say 5500) will be impossible with the ambient air density, and thus RPM will drop with further increases in altitude at that critical point.

 

Most aviation engines lower the amount of fuel to match the decreasing air density with a manual mixture knob, but the Rotax instead uses altitude-compensating carbs that automatically adjust the mixture through a diaphram attached to the needle jet.  Thus at higher altitudes less fuel should be consumed for a given RPM because of decreasing fuel required for the lower density air charge.

 

I think I have it now. 

[Ed Cesnalis]

Most aviation engines lower the amount of fuel to match the decreasing air density with a manual mixture knob, but the Rotax instead uses altitude-compensating carbs that automatically adjust the mixture through a diaphram attached to the needle jet.  Thus at higher altitudes less fuel should be consumed for a given RPM because of decreasing fuel required for the lower density air charge.

 

 

 

You are assuming that the needle jet circuit has control and leaning is taking place.  Above 92% throttle the main jet has control and no leaning takes place.  Except for descents and approaches I do all of my flying at WOT because otherwise power is limited fro climbs and cruising above 7,500' DA.

 

Your conclusion is right.  The leaning in your argument may not be taking place if the Bing is in fact going full rich as designed at throttle settings that make sense at altitudes like 9,500'.

[Ed Cesnalis]

Reading my post above I gave you a reason you would burn more fuel at 9,500 and that would be a high throttle setting where leaning isn't supposed to happen.  Then again that is offset by the lower air mass.

[Skywalker]

I am curious does this discussion ,which seems a little dynamic, apply to the ctls or only the ctsw.I ask because I assume in my ctls that I cant exceed 5500 rpm which is in the yellow.Are you flying in the yellow above 5500.?

[Tom Baker]

Continuous operations above 5500 no

 

I am curious does this discussion ,which seems a little dynamic, apply to the ctls or only the ctsw.I ask because I assume in my ctls that I cant exceed 5500 rpm which is in the yellow.Are you flying in the yellow above 5500.?

 

It applies to the CTLS. I don't think anybody is flying continuous in the yellow, but it is OK for the RPM to get there on occasion.

[Roger Lee]

You can fly up to 5500 continuous for the life of the engine. You'll burn more fuel at 5500 continuous and not get much in return for the extra rpm over 5300. After 5500 and up to 5800 you can run up to 5 minutes. I set all my clients props to around 5600-5650 rpm WOT at their average altitude. It's the sweet spot to balance all our flight characteristics. Setting the WOT prop pitch for this rpm affects the prop efficiency throughout its rpm range and not just cruise and climb. If you need more short field capability and a better climb prop or fly over 9K-12K ft. all the time then set it for around 5700-5750. This will get you better climb and better high altitude performance, but you will loose some cruise and fuel economy. Over pitching has zero redeeming characteristics, increases engine temps and can cause crankcase damage depending on how low the WOT rpm is set.

 

 

Most of us have ground adjustable props so we don't see above 5500 rpm in climb. This is more applicable for the in flight adjustable guys so they can use up to 5800 rpm on take off.