Van's Air Force
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Economy Cruise
- Thread starter N523RV
- Start date
N916K
Well Known Member
N523RV said:
Clay R has some good performance data on his website. I have the same engine and prop and my numbers line up well with his.
http://www.eaachapter.com/RV9A/Flights/Oct30/performance.htm
Kevin Horton
Well Known Member
There is fuel flow vs TAS data in the Cafe Foundation APR for the RV-9A. There are a bunch of graphs, and a data table. The test aircraft had on O-320 and a 3 bladed MT prop. The results would be slightly different with a different engine or prop.
If by best cruise, you mean the speed for max range, this will be a very slow speed. The Cafe Foundation tests never got slow enough to provide data for the max range condition.
If by best cruise, you mean the speed for max range, this will be a very slow speed. The Cafe Foundation tests never got slow enough to provide data for the max range condition.
Leland
Well Known Member
RV-9A mileage
Leland
My RV-9A with a new O320D1A and a 3-blade Catto prop gets 20 mpg at 2650 RPM and 180 mph, and 30 mpg at 2000 mpg and 130 mph. This was done at a density altitude of 5000 feet. RPM's below 2000 still gave 29-30 mpg but lower airspeeds. The information were taken with a calibrated Dynon D10A and an EIS 4000. Data is scattered due to fuel flow fluctuations.N523RV said:
Leland
gmcjetpilot
Well Known Member
As slow as you can go.
The slower you turn the prop the better. The less power the less fuel burn. Also the airframe is closer to best L/D, Lift over Drag. However not many pilots want to cruise at 110-130 mph either, but you can and will save fuel. (I think L/D is 100 mph (or 110 as the previous post said) at sea level, but it increase as you climb.)
Altitude is a player. Assuming zero wind, usually 7,500-12,500 feet, for most of us, is a good compromise altitude for a "going somewhere" flight, giving the good flight time to fuel economy compromise (about 65% power). An altitude that allows wide open throttle and gives 75% power is better for time while still giving good fuel econ. For most RV's with good induction, 75% is around 8,500'-9,500', with a moderate RPM. If you really are racing and there are no winds, than WOT / MAX RPM / down low is the way to go (if there are no winds). This will hurt the wallet.
Winds. This is the game to play when you can. I have climbed real high on a few occasion to get awesome tail winds. That's as close to a free lunch as you can get. I have seen consistent ground speeds of 230-240 mph in my RV-4. At work my record gnd speed, I have seen, was about 700-720 mph. It also works the other way; 170 kts on the nose is no fun.
The other "theory" is to climb well into the mid teens and run down at 50-55% power or less. Aircraft weight plays a roll in optimal cruise altitude. Also the time and fuel used to climb may be a detriment depending on leg length and winds. At some point it is diminishing returns, HP is to low to maintain best L/D. If you have a 150HP engine you will be altitude limited. If you are over powered with 180-200HP you can obviously fly higher. I have not crunched the numbers but it depends on the plane and weight. I guess FL180 is a good performance limit altitude for a light weight RV with a 180/200HP engine, about 15,000'/16,000' for a 150/160HP RV. Above that you are just getting near stall.
Practicality comes into play. How slow do you want to go. Most are happy with an econ cruise below 11,500 ft, 65-75%, making good time, wide open throttle (21.0 in-hg or less) and RPM as low as you can go. I say as low as you can go based on type and brand prop. I run 2300 rpm because it gives a smooth feel and is above the Hartzell 2250 RPM continuous restiriction. Also I don't want to go slower than that.
If you have a fixed pitch prop throttling back to get lower RPM can be counter productive because you are forcing the engine to SUCK past a restriction which cost HP (called pumping losses). The best you can do is fly to altitude, WOT and take what ever the RPM is. Depending on how you "SIZE" you fixed prop it will be optimal only at one speed and altitude.
This is the Nitty-Gritty where the C/S prop shines and is more efficient for a wider range of conditions. Many point to just better takeoff and climb for the C/S prop, but they can be more "efficient" in cruise. Also the Hartzell BA prop for example is metal which allows thinner blades, which makes it more efficient than thicker fiberglass over wood blades.
C/S props also let you run the RPM where its smoother, more quite. This does help a little with engine friction losses. For example, 75% power with 2,500 RPM and 75% at 2350 RPM (differnt MAP's), the 2350 RPM will actually have less fuel burn for the same power and approx airspeed. Constant speed props excel in these optiamal low RPM conditions. You can set the RPM independent of throttle setting.
If I did not have the 2000-2250 RPM restriction (which the BA Hartzell does not) and wanted to set some personal cruise MPG record, I would climb till my MAP was about 18.5-19" (or 12,500'), wide open throttle, set RPM to about 1900-2000 RPM. That would give me about 55% power, and of course lean the heck out of it. Its practical and requires no O2 legally. It still is going to give speeds near the 180's (RV-7, 180HP). For better MPG, if you don't mind sucking O2 to climb into the teens, climb higher, but your TAS will decrease, saving a little more fuel. As you climb into the teens you will need to add RPM, which is fine, but prop efficiency goes down and you get more engine friction losses.
At some point practical, efficiency and hey I want to get there today come together. Most of use rarely fly at optimal efficency. Look at the numbers above. By flying technique you can increases fuel econ by 50% that is pretty good, but most are happy with may be 20% better econ for a little loss in speed. I just want to go up and have fun and look at the country side when I go X-C. All this optimization is too much work.
The whole topic of what is best econ can't be discussed in a vacuum. Aircraft weight, sea level HP, prop, cruise density altitude, cruise winds, airframe drag and range (leg length) all factor into the mix.
With gas over $3.00/gal may be we will all fly at 130 mph. There is no one answer, but its fair to say the closer you fly to best L/D the better econ you will get. IN general less RPM and the higher you go the better.
George
The slower you turn the prop the better. The less power the less fuel burn. Also the airframe is closer to best L/D, Lift over Drag. However not many pilots want to cruise at 110-130 mph either, but you can and will save fuel. (I think L/D is 100 mph (or 110 as the previous post said) at sea level, but it increase as you climb.)
Altitude is a player. Assuming zero wind, usually 7,500-12,500 feet, for most of us, is a good compromise altitude for a "going somewhere" flight, giving the good flight time to fuel economy compromise (about 65% power). An altitude that allows wide open throttle and gives 75% power is better for time while still giving good fuel econ. For most RV's with good induction, 75% is around 8,500'-9,500', with a moderate RPM. If you really are racing and there are no winds, than WOT / MAX RPM / down low is the way to go (if there are no winds). This will hurt the wallet.
Winds. This is the game to play when you can. I have climbed real high on a few occasion to get awesome tail winds. That's as close to a free lunch as you can get. I have seen consistent ground speeds of 230-240 mph in my RV-4. At work my record gnd speed, I have seen, was about 700-720 mph. It also works the other way; 170 kts on the nose is no fun.
The other "theory" is to climb well into the mid teens and run down at 50-55% power or less. Aircraft weight plays a roll in optimal cruise altitude. Also the time and fuel used to climb may be a detriment depending on leg length and winds. At some point it is diminishing returns, HP is to low to maintain best L/D. If you have a 150HP engine you will be altitude limited. If you are over powered with 180-200HP you can obviously fly higher. I have not crunched the numbers but it depends on the plane and weight. I guess FL180 is a good performance limit altitude for a light weight RV with a 180/200HP engine, about 15,000'/16,000' for a 150/160HP RV. Above that you are just getting near stall.
Practicality comes into play. How slow do you want to go. Most are happy with an econ cruise below 11,500 ft, 65-75%, making good time, wide open throttle (21.0 in-hg or less) and RPM as low as you can go. I say as low as you can go based on type and brand prop. I run 2300 rpm because it gives a smooth feel and is above the Hartzell 2250 RPM continuous restiriction. Also I don't want to go slower than that.
If you have a fixed pitch prop throttling back to get lower RPM can be counter productive because you are forcing the engine to SUCK past a restriction which cost HP (called pumping losses). The best you can do is fly to altitude, WOT and take what ever the RPM is. Depending on how you "SIZE" you fixed prop it will be optimal only at one speed and altitude.
This is the Nitty-Gritty where the C/S prop shines and is more efficient for a wider range of conditions. Many point to just better takeoff and climb for the C/S prop, but they can be more "efficient" in cruise. Also the Hartzell BA prop for example is metal which allows thinner blades, which makes it more efficient than thicker fiberglass over wood blades.
C/S props also let you run the RPM where its smoother, more quite. This does help a little with engine friction losses. For example, 75% power with 2,500 RPM and 75% at 2350 RPM (differnt MAP's), the 2350 RPM will actually have less fuel burn for the same power and approx airspeed. Constant speed props excel in these optiamal low RPM conditions. You can set the RPM independent of throttle setting.
If I did not have the 2000-2250 RPM restriction (which the BA Hartzell does not) and wanted to set some personal cruise MPG record, I would climb till my MAP was about 18.5-19" (or 12,500'), wide open throttle, set RPM to about 1900-2000 RPM. That would give me about 55% power, and of course lean the heck out of it. Its practical and requires no O2 legally. It still is going to give speeds near the 180's (RV-7, 180HP). For better MPG, if you don't mind sucking O2 to climb into the teens, climb higher, but your TAS will decrease, saving a little more fuel. As you climb into the teens you will need to add RPM, which is fine, but prop efficiency goes down and you get more engine friction losses.
At some point practical, efficiency and hey I want to get there today come together. Most of use rarely fly at optimal efficency. Look at the numbers above. By flying technique you can increases fuel econ by 50% that is pretty good, but most are happy with may be 20% better econ for a little loss in speed. I just want to go up and have fun and look at the country side when I go X-C. All this optimization is too much work.
The whole topic of what is best econ can't be discussed in a vacuum. Aircraft weight, sea level HP, prop, cruise density altitude, cruise winds, airframe drag and range (leg length) all factor into the mix.
With gas over $3.00/gal may be we will all fly at 130 mph. There is no one answer, but its fair to say the closer you fly to best L/D the better econ you will get. IN general less RPM and the higher you go the better.
George
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N941WR
Legacy Member
From 6th issue of the 1999 RVator
The following numbers are from Van's O-235 powered -9A:
MPr | RPM | TIAS | GPH | MPG | Range (Statuemiles)
24.1 | 2700 | 169 | 7.1 | 23.9 | 859
23.0 | 2550 | 159 | 6.4 | 24.8 | 894
22.0 | 2480 | 155 | 6.0 | 25.75 | 927
21.0 | 2480 | 146 | 5.4 | 27.0 | 973
20.0 | 2260 | 138 | 5.0 | 27.6 | 994
19.0 | 2200 | 134 | 4.6 | 29.1 | 1049
18.0 | 2030 | 123 | 3.9 | 31.5 | 1135
16.9 | 1890 | 115 | 3.3 | 34.8 | 1288
14.3 | 1680 | 100 | 2.5 | 40.6 | 1461
13.3 | 1620 | 91 | 2.3 | 39.5 | 1424
The following numbers are from Van's O-235 powered -9A:
MPr | RPM | TIAS | GPH | MPG | Range (Statuemiles)
24.1 | 2700 | 169 | 7.1 | 23.9 | 859
23.0 | 2550 | 159 | 6.4 | 24.8 | 894
22.0 | 2480 | 155 | 6.0 | 25.75 | 927
21.0 | 2480 | 146 | 5.4 | 27.0 | 973
20.0 | 2260 | 138 | 5.0 | 27.6 | 994
19.0 | 2200 | 134 | 4.6 | 29.1 | 1049
18.0 | 2030 | 123 | 3.9 | 31.5 | 1135
16.9 | 1890 | 115 | 3.3 | 34.8 | 1288
14.3 | 1680 | 100 | 2.5 | 40.6 | 1461
13.3 | 1620 | 91 | 2.3 | 39.5 | 1424
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Kevin Horton
Well Known Member
Did the original data say what the weight was?N941WR said:
Did they explain what "TIAS" is? Is it TAS, IAS, IAS corrected for instrument error, IAS corrected for instrument error and static source position error (i.e. CAS), or something else?
The fact that the MPG value is equal to the TIAS/GPH, suggests that it is TAS, unless the testing was done at sea level on a standard day, or those are indicated miles per gallon, rather than true miles per gallon.
gmcjetpilot
Well Known Member
I guess I would like to know manifold pressure, altitude and prop used, but it is interesting. I notice MPG and RANGE increases down to 100 TIAS (mph). What is MPr? GN941WR said:
gmcjetpilot
Well Known Member
It does matter, just not a lot.
The only warning is keeping the engine temps hot enough. Here is a Lycoming article:
Low power flying and your Lycoming
As long as your CHT is 150F or higher and oil temp +165F, there is not problem with lots of continuous low power operations (?). Min RPM? I am reading between the lines but 1800 or 1900 RPM is a min.
As gas goes up, if you can fly at 3.2 gal/hr verse 8 gal/hr at $3.20/gal than more may try it. That is like making the $3.20 more like $1.28. Folks talk about turbo diesels, but this is a subject whose time has come. Let's face it 115 mph is not a lot slower than many typical Wichita fixed gear planes.
WEIGHT
I am with you, weight was not mentioned. Weight does matter; Some like to say weight does not affect top speed; well it does it just does not matter that much. However when we are talking economy, we are talking about squeezing out every 1% or 2% we can find. Weight does matter.
Empty weight is my pet peeve. Many justify fat empty weights and like to say it does not affect top speed or economy much. Well it's kind of true, but it does affect it a little. Moreover it affects: stall speed, climb rate, service ceiling, landing and takeoff distance. Not to mention a light RV's have a nicer feel on the controls. If you fly a light weight RV you will notice it immediately. George
That just shows you can save fuel by flying slow. I guess that is not a revelation, but who has done it. It still applies to a O-320, O-360 or IO-360 and IO-540. The Rocket Guys talk about this all the time. Since they have way extra power reserve they are often able to fly at or less than 50% power while making good speed. Of course their 50% power is like our 72%-86% power. I lot of folks say a Rocket can't get the same gas miliage. It can but it has to fly slow. You would think a small engine is better. It is. A IO540 will never get down to 2-3.2 gal/hr. Why? There's is a limit on how slow you can run an engine and keep it warm. Even if you fly slow you have to keep your engine withing limits (both high and low).N941WR said:
The only warning is keeping the engine temps hot enough. Here is a Lycoming article:
Low power flying and your Lycoming
As long as your CHT is 150F or higher and oil temp +165F, there is not problem with lots of continuous low power operations (?). Min RPM? I am reading between the lines but 1800 or 1900 RPM is a min.
As gas goes up, if you can fly at 3.2 gal/hr verse 8 gal/hr at $3.20/gal than more may try it. That is like making the $3.20 more like $1.28. Folks talk about turbo diesels, but this is a subject whose time has come. Let's face it 115 mph is not a lot slower than many typical Wichita fixed gear planes.
WEIGHT
I am with you, weight was not mentioned. Weight does matter; Some like to say weight does not affect top speed; well it does it just does not matter that much. However when we are talking economy, we are talking about squeezing out every 1% or 2% we can find. Weight does matter.
Empty weight is my pet peeve. Many justify fat empty weights and like to say it does not affect top speed or economy much. Well it's kind of true, but it does affect it a little. Moreover it affects: stall speed, climb rate, service ceiling, landing and takeoff distance. Not to mention a light RV's have a nicer feel on the controls. If you fly a light weight RV you will notice it immediately. George
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John Silver
I'm New Here
Theoretically, wouldn't best economy be had at Best Glide Speed? That is the point of least drag, and I would think the engine could put out the minimum power to maintain altitude. BTW, what is best glide for a 9A?
John Silver said:
It's actually a little faster than your best glide speed. When you're gliding, your prop is just causing a lot of drag. At the optimal AOA, you're going a bit slower than you will be in level flight with the prop giving you thrust instead of drag. It's all about flying at the most effecient AOA. I don't know what a typical ratio would be, but I'd guess that if the most effecient AOA at idle gives you 100mph airspeed, in level flight with the engine going you're probably somewhere about 110, or something like that (probably a more dramatic difference with CS prop vs. fixed pitch).
Maybe I'm not thinking through this right but that looks OK at the moment
Kevin Horton
Well Known Member
In the theoretical world, where prop efficiency and the engine's specific fuel consumption are constant, you'll get the maximum range by flying at the angle of attack for minimum drag. The speed for minimum drag will decrease as the weight decreases.
In the real world, if you measured your best glide speed with the engine stopped (and not windmilling), the speed you measured should be the speed for minimum drag. But, most people do their best glide speed testing with the engine at idle, so the speed they determine will be a bit different than the real best glide speed.
In the real world, the prop efficiency varies with rpm, power, and airspeed. And the specific fuel consumption is probably not the best at the low power setting required to fly at the best glide speed.
The upshot of all this is that the speed for best range is probably a little bit faster than the speed that most people think is best glide speed.
In the real world, if you measured your best glide speed with the engine stopped (and not windmilling), the speed you measured should be the speed for minimum drag. But, most people do their best glide speed testing with the engine at idle, so the speed they determine will be a bit different than the real best glide speed.
In the real world, the prop efficiency varies with rpm, power, and airspeed. And the specific fuel consumption is probably not the best at the low power setting required to fly at the best glide speed.
The upshot of all this is that the speed for best range is probably a little bit faster than the speed that most people think is best glide speed.
Bryan Wood
Well Known Member
RV-9A economy
This is old information from an article that Van wrote for the RVator. (5th issue of 2002) Since there are so many new builders that might not have seen this article here is the basic information.
Van flew to Flabob airport near Riverside, CA with the 9A and made the trip that was just short of 900sm with one fuel stop on the way down and it took him 5 hours.
On the return trip he flew at 10,500', full throttle and 2300rpm with a slight tail wind of around 10mph. He landed 12 miles short of his home airport at North Plains for fuel covering a distance of 860 statute miles in 4 hours and 38.5 minutes using 33 gallons of fuel. He had a true calibrated airspeed of 180.5mph with a fuel burn of 7gph including climb, and covered 26 miles per gallon of fuel.
To me this seems like an extreme example of the capabilities of the 9a and its economy. This is a realistic altitude for trips without the burden of oxygen, but still taking advantage of thinner air and slower propeller speeds.
Regards,
This is old information from an article that Van wrote for the RVator. (5th issue of 2002) Since there are so many new builders that might not have seen this article here is the basic information.
Van flew to Flabob airport near Riverside, CA with the 9A and made the trip that was just short of 900sm with one fuel stop on the way down and it took him 5 hours.
On the return trip he flew at 10,500', full throttle and 2300rpm with a slight tail wind of around 10mph. He landed 12 miles short of his home airport at North Plains for fuel covering a distance of 860 statute miles in 4 hours and 38.5 minutes using 33 gallons of fuel. He had a true calibrated airspeed of 180.5mph with a fuel burn of 7gph including climb, and covered 26 miles per gallon of fuel.
To me this seems like an extreme example of the capabilities of the 9a and its economy. This is a realistic altitude for trips without the burden of oxygen, but still taking advantage of thinner air and slower propeller speeds.
Regards,
