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RPM Vs MAP

Vicflyer89

I'm New Here
I have an RV6 with a 0360-A1A and a 3 bladed Catto FP prop. I usually cruise at around 2450-2500 RPM. On a recent trip at 9500ft I noticed at 2450 RPM I was only getting 18.6 MAP and 62% power 50 degrees ROP. I believe my prop is under pitched and think the engine should be loaded up a bit more in regards to MAP abs RPM. I’m relatively new to RV ownership should I be setting my cruise power working off a MAP instead of RPM to appropriately load the engine?


Cheers

Adam
Melbourne, Australia
 
Manifold pressure is based on air pressure so full throttle at sea level usually gives you something a bit less than 30” depending on how efficient the air is driven into the engine. You can expect to lose about 1” for every thousand feet of altitude on a standard day. So on a standard day at 9500 feet you might expect to get something around 20” manifold pressure, some setups a bit more using ram air and others could be a little bit less if the air filter was partially plugged. 18.5” is certainly on the low side so I’d be checking my filter. If you were to get closer to 20” mp then with a fixed pitch prop your rpm would show an increase.
 
I have an RV6 with a 0360-A1A and a 3 bladed Catto FP prop. I usually cruise at around 2450-2500 RPM. On a recent trip at 9500ft I noticed at 2450 RPM I was only getting 18.6 MAP and 62% power 50 degrees ROP. I believe my prop is under pitched [/B]and think the engine should be loaded up a bit more in regards to MAP abs RPM. I’m relatively new to RV ownership should I be setting my cruise power working off a MAP instead of RPM to appropriately load the engine?
Cheers
Adam
Melbourne, Australia

If you were WOT at 2450 rpm, then your prop would be over pitched. Go to 8500' DA and set 2700 rpm. Your MP should about 21" +,- for 75% power.
 
We need to know pressure altitude to really understand what your engine is doing. 50 degrees ROP is also not a great place to run your engine. It’s about where the highest internal cylinder pressures occur.
 
And if I missed it, you should always be at full throttle at 9500 feet unless your trying to stay in the air longer.

Carb or FI? Makes a little difference but not worth debating.

IN GENERAL (once again, trying not to start a debate), flying at WOT and setting power with RPM and mixture will give you more the most benefit. Note I'm not using the term efficiency. There are other contributing factors and most of the time, you'll be splitting hairs. Closing a throttle plate increases the volumetric inefficiency of the cycle as the intake stroke has to work a little harder.

To the OP. Try what Mr. Sailor suggest on your next CC regardless of altitude. If you have accurate FF and AS indications, you might be pleasantly surprised

Most specific to your point, from the little bit of data provided you might be a bit over-propped.
 
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I have an RV6 with a 0360-A1A and a 3 bladed Catto FP prop. I usually cruise at around 2450-2500 RPM. On a recent trip at 9500ft I noticed at 2450 RPM I was only getting 18.6 MAP and 62% power 50 degrees ROP. I believe my prop is under pitched and think the engine should be loaded up a bit more in regards to MAP abs RPM. I’m relatively new to RV ownership should I be setting my cruise power working off a MAP instead of RPM to appropriately load the engine?


Cheers

Adam
Melbourne, Australia

With an FP prop, you will want closer to 2600 or 2700 at 10K' if the pitch is optimized for a happy medium between cruise and climb. Try going to full throttle at 8K and see how much RPM you get. If you don't get 2700 when WOT at 8K, you are over pitched. I get 2800+ at 8K, but I am purposely a bit under pitched.

Larry
 
A fixed pitch prop maker that knows what they are doing when it comes to getting the most performance with the compromise of a fixed pitch prop will sell you a propeller that will allow quite a bit of over speed at altitudes below 10,000 feet. This is part of the compromise of a fixed pitch prop. In general running wide-open throttle with a carburetor is not necessarily the best for performance either but it varies from one engine to another. My airplane as an example has very balanced CHT‘s and pretty closely balanced egts when the throttle is pulled back a little bit from wide open.
With all of this being the case, to get good performance out of an RV that has a properly matched fixed pitch propeller, you have to be willing to turn the engine faster than 2400 or 2450. My typical cruise rpm is 2600 -2650. This is on an RV 6A with 180 hp and a fixed pitch Sensenich propeller. At lower altitudes even at this RPM the manifold pressure will be about 21 inches. As I climb to higher altitudes I continue opening the throttle and still have manifold pressure at about 21 inches . The actual true airspeed keeps getting higher as I go higher because the RPM and manifold pressure are the same but drag is lower.
At higher altitudes (above 10K) my airplane trues out at about 194 mph.
With a fixed pitch prop if you don’t use a pitch that will allow this type of RPM at crews, you will greatly suffer and takeoff and climb performance.
 
With a fixed pitch prop you have to decide what you want most, good field performance, good climb rate, speed at 2000', speed at 10,000', something else? You can't have everything. Some props will be pretty good in most areas but there will always be someone who is faster, takes off shorter, climbs faster than you if you have a "generally good" compromise prop.

If you get a prop with more pitch then your take-off roll will increase and your climb rate will decrease. You may be able to tolerate that but look into all of the implications to changing the pitch of your prop before ordering. Also note that all prop manufacturers measure pitch differently, and the diameter of your prop has as much effect as the pitch. If you have an 80 inch pitch prop from Mr Catto who knows what you might need from another maker.

As you climb your engine makes less power. You must open the throttle, and turn more rpm, to make the same cruise power. It is unlikely you were getting 62% power at 2450rpm at 9500ft. Take a look at the performance chart for your engine, I would guess you were closer to 50% power.

If you have a prop that gives good take-off & climb performance you may well be able to turn it at over 2700 rpm at full throttle at altitude. It depends what your prop was built to do, and also how you want to operate your engine. There is no right answer here just a spectrum of choices. I would pick a cruise speed and open the throttle until I get it. Then adjust the mixture to minimise fuel flow - I would second the comment about reviewing how the red knob works, 50F rop is not a great place to operate. If the engine is turning too fast for your liking then you may need a new prop, or throttle back some and accept a slower cruise (burning less fuel).

Its all a compromise, just decide what you want the most. I found Sensenich metal props are the best compromise for good all-around performance, but I fly mainly below 5000'.

Pete
 
We need to know pressure altitude to really understand what your engine is doing. 50 degrees ROP is also not a great place to run your engine. It’s about where the highest internal cylinder pressures occur.

Can you tell me more about this? I thought this WAS an ok place to run the motor.

Thanks.
 
When I built my RV-8, Van's advertised the 180 horsepower RV-8 as having a 75% power cruise speed of 205 mph or 178 knots TAS. I asked Craig Catto to build me a prop that would deliver that performance. Craig nailed it exactly.

8000 feet density altitude is about where the max output of a Lycoming is 75% of its sea level performance.

So, if the original poster's performance at 8000 feet DA turning 2700 rpms is equal to the RV-6 advertised 75% power cruise speed, I would say the prop is properly pitched.

More pitch will get you a lower rpm for the same cruise speed, but it will cost you climb performance.
 
At 9500 feet altitude on a standard day at full throttle you are probably below 65% power and if Carberated you can operate at peak EGT and get your best speed and economy. There is no red box when your operating a Lycoming at 65% or less power, you can’t hurt it with leaning. Some say you can go lean of peak with a carb and a bit of carb heat but I’ve never had it work for me.
 
At 9500 feet altitude on a standard day at full throttle you are probably below 65% power and if Carberated you can operate at peak EGT and get your best speed and economy. There is no red box when your operating a Lycoming at 65% or less power, you can’t hurt it with leaning. Some say you can go lean of peak with a carb and a bit of carb heat but I’ve never had it work for me.

Pulling about a tenth of an inch of MAP helps, too. You'll still not get very far LOP tho.

Ed Holyoke
 
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