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  #61  
Old 02-06-2009, 03:14 PM
Finley Atherton Finley Atherton is offline
 
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Quote:
Originally Posted by Steve Brown View Post
Its true my Catto requires closer attention to speed management in the pattern. On the other hand, glide ratio is much better than with a CS prop. So, if glide ratio is included in the performance equation, it is not at all clear that CS has better performance.
Steve,
I agree that a FP prop would give a better glide ratio. However, I would guess that in the majority of engine failure situations, the engine would be windmilling and producing oil pressure. In this case, a C/S prop can most likely be put in full coarse pitch and that will greatly increase the glide ratio. It would be interesting to compare glide ratios of FP V C/S with the C/S in full coarse. As a point of reference, my 0-320 9A at 9,000 ft DA has a sink rate of 857 f/min at 80 kts IAS with the engine dead (mixture at idle cutoff) and the prop windmilling at full coarse pitch.

Fin
9A
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  #62  
Old 02-06-2009, 03:54 PM
whifof100ll whifof100ll is offline
 
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Quote:
Originally Posted by N941WR View Post
Dale,

I have to disagree with your reason 1, up/down drafts may cause fluctuations in RPM but not power. Just like when coasting downhill in your car, you may accelerate even with your foot off the gas pedal doesn't mean you are putting out more power. Back when I was taking instrument training in a Archer, we just set the power and didn't worry about it.

As for reason 2, I don't understand. The airplane may be a bit more stable but that also has to do with passenger and baggage compartment loading. As long as you are w/in CG and W&B this should be the same between similar airplanes.
of the same plane may seem stable.
An FP Warrior is more stable than an RV, so it is not as noticable, reason 1 does apply.

To disprove your car analagy, when I point my RV downhill without reducing the throttle, the RPM's pick up. The MP stays the same. The fuel burn goes up. This is easy to see with my AFS. Fuel burn is an indication of power, so there goes your car dowhill argument. When you go down hill in your car, you slightly decrease throttle (manifold pressure) to maintain speed. If you did not, the speed would pick up and the power would pick up. Look at the power charts for your lycoming or most internal combustion engines and you will see that unless you are way up on the power curve, power output increases with RPM for any given MP. When you point a CS RV downhill, the speed goes up, but as long as the MP does not appreciably change, the RPM, power, and fuel burn will be the same.


Reason 2, Let me reword this one a bit. Airplanes with down flying tails have less dynamic stabiltity as CG moves aft. Dynamic stability is defined as the ability to restore pitch attitude after a pitch upset. As you go past the aft CG limit, you can pull the nose up, and as the speed decreases the nose will rise instead of fall (pitch instability). Neutral dynamic stability means that if you pull the nose up, it will stay there as speed falls of. Draw a free body diagram (showing the forces at work) of your RV in flight and you can figure out why.

As a plane gets near neutral dynamic stabilty, it is more of a handfull to land as relaxing back pressure does not have the same affect has with more dynamic stability.
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  #63  
Old 02-07-2009, 07:42 AM
Steve Brown Steve Brown is offline
 
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Default IMO: FP verses CS negligeable factor in fuel efficiency

Quote:
Originally Posted by lsu-rv View Post
......
I don't want to start an argument, just some of you who have flow both please enlighten me on fuel burn at same speed and LOP with each?
jeff h
I have run LOP in a couple of different Mooneys, but they are so different than and RV I don't think its a good comparison.

The M20E (injected) that I had in the 90s seemed to burn about 0.5-1.0 GPH less than an M20C (carburetor) I rented occasionally at roughly the same airspeed (or more). Not a very scientific comparison, but I'm convinced injection is the way to go.

Obviously having CS prop gives RPM options and these engines do burn less fuel at lower RPMs, even for the same power setting. On the other hand, they weigh more, which decreases efficiency because you have to drag them up to altitude. Depending on specifics, it may be a wash, or go either way.

I run my O320 / FP LOP all the time. It can be quite economical. Check this link to a non stop tip I did last year:
http://www.vansairforce.com/communit...ad.php?t=31715

I don't know if I'll do it again unless I've got a stiff tail wind, because staying on top of the fuel management was just too stressful, but the point is that it can be done.

I doubt that having CS prop would make that trip less stressful, but fuel injection might tip the balance.
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KRHV - Reid Hillview airport, San Jose, CA
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  #64  
Old 02-08-2009, 06:48 AM
penguin penguin is offline
 
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Quote:
Originally Posted by Steve Brown View Post
Obviously having CS prop gives RPM options and these engines do burn less fuel at lower RPMs, even for the same power setting. On the other hand, they weigh more, which decreases efficiency because you have to drag them up to altitude. Depending on specifics, it may be a wash, or go either way.
Everyone's entitled to an opinion, but I'm not sure this is accurate. With a C/S prop you can run at an rpm that will burn less fuel for a given power setting/speed. 2200/24" burns less fuel than 2300/23, and both will likely burn less fuel than the size engine with a FP prop. If these engines are O-360s they will again burn less fuel (up to 1.5 gph less) than an FP O-320 going the same speed (I realize that the 360s will be producing less % power). Injecting the 360s will reduce the fuel burn if the pilots are prepared to work at it (run LOP, etc). Using a 200hp IO-360 will further reduce the fuel burn (by up to a further 1 gph) at the same speed (again less % power than the O-360s). This is fuel consumption at the pump - all start off with full tanks, all climb together to cruise alt (8000' for sake or arguement), although C/S airplanes will get there much quicker, and all fly at same speed. An FP 150hp O-320 will burn around 9 gph, chock to chock flying WOT when cruising. The (I)O-360s will use less over (say) a 2 hr flight. I believe the C/S props used sensibly - max MP and control speed on rpm - are responsible for most of the fuel savings.

If the larger engined airplanes were to fly at the same power settings (% power) I have no idea what the result would be.

Pete
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  #65  
Old 02-08-2009, 07:20 AM
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n5lp n5lp is offline
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The theoretical reasons that a constant speed prop airplane should be more efficient are certainly valid but advantages like that are often overpowered by operation techniques.

Way back before the advent of 200 HP IO-360s on RVs, I took a trip with three other airplanes. The group had a good cross-section of the recommended powerplant propeller combinations of the time and all the airplanes were RV-6s or RV-6As.

Fuel burn results (from my website):

The two tankfuls where everyone flew together gave a little opportunity to compare the fuel efficiency of the four RVs. The fuel burn order was consistent, but three of the airplanes were very close on fuel usage. The order was, from least burned to most:

180 HP O-360, fixed pitch Sensenich

160 HP O-320, constant speed

160 HP O-320, wood (lightest airplane)

180 HP O-360, constant speed (heaviest airplane by quite a bit)


My airplane (O-360 F/P, carb, mags) burned the least fuel. That is just because of leaning technique. I lean according to the Lycoming guidelines which results in one cylinder lean of peak, one quite rich of peak and a couple of cylinders near peak. The whole business is a theoretical mess.

For the entire trip, covering large parts of the western US, my fuel burn was 7.4 GPH.
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Last edited by n5lp : 02-08-2009 at 07:26 AM. Reason: more information
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  #66  
Old 02-08-2009, 08:18 AM
Steve Brown Steve Brown is offline
 
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Location: Alviso, CA
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Default I don't think so

Quote:
Originally Posted by penguin View Post
......With a C/S prop you can run at an rpm that will burn less fuel for a given power setting/speed. 2200/24" burns less fuel than 2300/23, and both will likely burn less fuel than the size engine with a FP prop.
What I wrote is that this is probably true, but (expanding) depends on the specifics in terms of FP prop pitch and aerodynamic / propulsive efficiency of each prop.

Quote:
Originally Posted by penguin View Post
If these engines are O-360s they will again burn less fuel (up to 1.5 gph less) than an FP O-320 going the same speed (I realize that the 360s will be producing less % power).
Injecting the 360s will reduce the fuel burn if the pilots are prepared to work at it (run LOP, etc). Using a 200hp IO-360 will further reduce the fuel burn (by up to a further 1 gph) at the same speed (again less % power than the O-360s).
Why would a larger engine burn less fuel at same absolute power setting?

This does not make sense to me. First assume either both FP or both CS, and also assume 360s and 320s are leaned the same way. In that case they should be roughly equivalent, except that the larger engine would be expected to have more internal friction resulting in slightly less efficiency at low power settings at the same RPM. The injected engine burns less because because of better mixture control/distribution, not because it is running at lower percentage of its maximum HP.
If you reduce RPM on the larger engine to get the equivalent power, you might be able to offset the increased friction of its larger size. The slower turning prop is probably a bigger contributor due to decreased drag. Could the balance be tipped the other way? I don't know, but I'll guess the difference is relatively small either way between 320 & 360.
Anyway, fuel efficiency verses engine size is an interesting topic, but I think a different topic than FP verses CS.

Quote:
Originally Posted by penguin View Post
This is fuel consumption at the pump - all start off with full tanks, all climb together to cruise alt (8000' for sake or argument), although C/S airplanes will get there much quicker, and all fly at same speed. An FP 150hp O-320 will burn around 9 gph, chock to chock flying WOT when cruising. The (I)O-360s will use less over (say) a 2 hr flight. I believe the C/S props used sensibly - max MP and control speed on rpm - are responsible for most of the fuel savings.................

Pete
Raising the extra weight of a heavier engine and prop to the same altitude takes more energy.

In this case the tables have turned on CS verses FP because in climb the FP will typically be turning lower RPM than CS, reducing friction & prop drag. The CS pilot could decrease the RPMs even further, but then would definitely NOT out climb the FP pilot since he will be using less power to raise more weight.

Assuming:
  • Equivalent aerodynamic & propulsive efficiency
  • CS somewhat more efficient at cruise power settings because of running at lower RPM
  • CS somewhat less efficient at cruise power settings due to extra weight resulting in greater angle of attack, resulting in more drag.
  • CS takes more energy in climb due to greater weight
  • FP somewhat more efficient in climb due to turning lower RPMs
Those more knowledgeable than me could probably put numbers on those relationships. Which would win overall would depend on the time in climb, verses time in cruise, specific weight differences, specific prop efficiency differences, and possibly the phase of the moon.

I would not want to bet on the outcome, but if I had to bet, I would go with the FP prop.
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  #67  
Old 02-08-2009, 12:05 PM
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L.Adamson L.Adamson is offline
 
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Quote:
Originally Posted by Steve Brown View Post
What I wrote is that this is probably true, but (expanding) depends on the specifics in terms of FP prop pitch and aerodynamic / propulsive efficiency of each prop.



Raising the extra weight of a heavier engine and prop to the same altitude takes more energy.

In this case the tables have turned on CS verses FP because in climb the FP will typically be turning lower RPM than CS, reducing friction & prop drag. The CS pilot could decrease the RPMs even further, but then would definitely NOT out climb the FP pilot since he will be using less power to raise more weight.

Assuming:
  • Equivalent aerodynamic & propulsive efficiency
  • CS somewhat more efficient at cruise power settings because of running at lower RPM
  • CS somewhat less efficient at cruise power settings due to extra weight resulting in greater angle of attack, resulting in more drag.
  • CS takes more energy in climb due to greater weight
  • FP somewhat more efficient in climb due to turning lower RPMs
Those more knowledgeable than me could probably put numbers on those relationships. Which would win overall would depend on the time in climb, verses time in cruise, specific weight differences, specific prop efficiency differences, and possibly the phase of the moon.

I would not want to bet on the outcome, but if I had to bet, I would go with the FP prop.
I do believe that you'll find..............

That a C/S prop will beat a F/P prop to altitude nearly every time. Unless of course, the F/P prop is pitched for best climb, in which cruise would suffer. With a C/S, we can generate full RPM on takeoff. RPM translates to available horsepower. Fly two similar RV's with C/S & F/P, and you'll easily notice the difference between the two.

L.Adamson --- RV6A Hartzell C/S
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  #68  
Old 02-08-2009, 12:29 PM
Pallet Pallet is offline
 
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Location: OH
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Maybe this has already been dicussed, but with a fixed prop set up for cruise, I'm wondering how much longer ground roll this would take.
Anyone with experience that has kept track?
I'm thinking possible grass strip.
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  #69  
Old 02-08-2009, 08:54 PM
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RV6_flyer RV6_flyer is offline
 
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I would like to see some FACTs on what is being discussed. Can anyone point me to ACTUAL independent DATA showing the performance deltas?

I will post a link to the FIRST REAL HARD DATA on this thread.

You need to be an EAA member and login to their members only web site. You are looking for "Sport Aviation" November 1990 issue article on "The First Annual CAFE Propeller Competition". If the link I inserted works, it will take you directly to the PDF version of the article.

If the hyperlink does not work, the link is:
http://members.eaa.org/home/saarchiv...les/013474.pdf
The article was written by Dick VanGrunsven of Van's Aircraft.

I have been flying my RV-6 160 HP with Constant Speed prop for almost 11.5 years. I would like to know what EXPENSE there is operating a Constant Speed prop that everyone keeps talking about. The only operating expense has been grease every year. If you go to RVproject.com and look up the weight and balance issue, you will see that without the constant speed prop, my useful load would be reached at a lower weight becasue I would hit the AFT CG limit much sooner. As for the weight of a CS prop vs a fixed pitch, go to Van's web site and compare the difference in performance of any of the RVs at solo weight and gross weight. IIRC, the rate of climb on an RV-6 goes down 1 FPM for every pound increase in weight at the same HP. (Assuming a straight line function.)

I hope this is the FIRST of MANY posts with LINKS to FACTS that can be verified and not OPINION. I have Lycoming Copywrited material on 320 and 360 that are power charts. These charts show that the 360 burns less pounds of fuel per hour per HP produced. Yes the 360 will burn more fuel at full power but it is making more horsepower so that should be expected. If someone has a link to charts can be shared on the web that shows this, please post them.

Lets see some posts to articles on LIKE aircraft with Fixed and CS props showing RACE results or economy runs?

MY OPINION:
"I would rather walk than own an RV that does not have a Constant Speed prop." I have over 2,000 PIC hours in RVs when I make that statement. How many RV hours do you have to backup your opinion?
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To those who love aviation, the sky is home.

Last edited by RV6_flyer : 02-08-2009 at 08:57 PM. Reason: spelling
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  #70  
Old 02-08-2009, 10:00 PM
David-aviator David-aviator is offline
 
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Quote:
Originally Posted by RV6_flyer View Post
I would like to see some FACTs on what is being discussed. Can anyone point me to ACTUAL independent DATA showing the performance deltas?

I will post a link to the FIRST REAL HARD DATA on this thread.

You need to be an EAA member and login to their members only web site. You are looking for "Sport Aviation" November 1990 issue article on "The First Annual CAFE Propeller Competition". If the link I inserted works, it will take you directly to the PDF version of the article.


.....Lets see some posts to articles on LIKE aircraft with Fixed and CS props showing RACE results or economy runs?

MY OPINION:
"I would rather walk than own an RV that does not have a Constant Speed prop." I have over 2,000 PIC hours in RVs when I make that statement. How many RV hours do you have to backup your opinion?
Gary,

Gary Hertzler won the CAFE 400 in 1990 with his VEZ and a FP prop. Klaus Savier has recorded 100 mpg with his VEZ, also with a FP prop. The efficiency gurus can not afford the weight of the CS system. For all out speed, the CS wins. For efficiency it loses.

If you'd rather walk than fly behind a fixed pitch prop, have at it. I'd rather fly anything than walk.
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