Man.pressure Vs RPM

[Gaston]

I posted a comment in another thread ,about Man press and RPM and I wanted to explain How I see it...
I'm far from being an engine specialist,and this is only my opinion...
you take it or leave it ,it's up to you...
it's only thinking matter....

As per GPASC literature a 1835 pumps out 65 hp at 3600 MAX
60 at 3400 Cont.
2180 .................75 hp at 3600
70 hp at 3400 cont.
in order to get these numbers available at the prop for take off,your engine must be able to turn your prop at these RPM.
otherwise forget about these numbers...


Say your engine is a 1835 and your prop turns only 3000 on climb out ,your engine is not developping 60 HP ,but much less.
so if your climb rate is marginal don't be surprised...
If you have an engine that runs your prop at such a low rpm ,before blaming it on the engine ,have a look at your prop,it may be way too pitchy...
I once had a 1835 in my sonerai that was not performing very well until I discovered that the pitch was much more than what was stamped on the prop.
after reducing the pitch of this prop ,the engine began to perform very well, Static RPM was increased by about 200 rpm.
average performance was also increased considerably.

In my opinion the high Man.pressure and lower RPM,is much harder on the engine than high RPM and lower Man press.

as per a C-172 flight manual the work load of the engine (0 320) at 2000 feet Press Alt is 75% at 2500 RPM when it is
at 8000 feet PressAlt is 64% still at 2500 RPM
with a fuel burn reduced from 8.4 to 7.2 usg/hr ,fuel burn is reduced by 17%
so I think that reduced ambient air consuming was also by 17% to maintain a air/fuel ratio near 14:1
this gives a work load reduction of 15 %related to lower manifold pressure...
in our case using 25" Mp instead of 28" Mp is a reduction of 11% of the work load
this may be far from truth but it makes a certain amount of sense.

So I use low pitch prop that allow the engine to reach high rpm.and produce maximum output,and minimal fuel burn.
this is very interesting for take & climb,giving a climb rate over 1000ft/min with my 2276
and when level ,all I have to do if I want more speed is to add a little power.

VW are happy at higher rpm.in their original use HP output were measured between 4000 & 4800 rpm depending on their size...

Personally my 2276 can reach and maintain around 3350 rpm on climb out depending on actual conditions,
and I feel very comfortable using higher RPM.

Gaston

 

[pancho]

yep that is pretty well the sum of it.
I like "mp" in this kind of airplane more for reference so that I can tell where I am in relation to the last prop setting (Ivoprop on a friends avid)
I prefer a flatter pitch propeller as they tend to be a bit more climb and fuel efficient. Mainly because you power back a little farther to bring the RPM down at cruise. Best economy speed is usually just over best glide speed, but who wants to go THAT slow? Just check any flight manual for any light fixed pitch airplane and you will see that a flat (climb) prop outperforms in all areas except flat out speed. and even there the difference aint all that great. A "MP" gauge gives you a constant from which to judge prop performance and overall power setting.
You don't normally see "MP" gauges in spam cans because the airplanes are certified with only a couple propeller choices and the perimeters are already explored for you, and then printed up in the flight manual. But you can be assured that the prototype certainly had one during testing. (most Tripacers did have them )

 

[Schmleff]

I am with you guys. One of the other factors to consider that is not discussed very often is the velocity of the air that you are moving. Yes a large prop moves a larger disk of air, but a small one (turned up faster) with more pitch can move the same amount with a higher velocity.

That means that a smaller diameter high pitch prop can equal the climb of a large diameter prop while providing a higher cruise/top speed.

 

[lgiroux]

It sounds like the big difference between VW and certified engines is the RPM they produce power at. Is it fair to say that there is a lot of throttle movement at the top end that doesn't necessarily produce a difference in rpm, but does change manifold pressure? I think it was Fred who said his throttle does not produce linear change in rpm--is that the nature of the beast with these high-revving engines? Is there a favorite prop out there that is a good average for climb and cruise performance on a 2180 or 2276?
Luke

 

[Schmleff]

The "favorite" prop will be different from plane to plane. Static RPMs might be the same between planes, but it flight its hard to compare.

I have been reading about Sonex airplanes that can only get 3050rpms wide open in flight. Aircraft propped like this are giving up both climb and cruise performance. At that rpm, its probably only making 50 hp.

Like Gaston said, the vw does not mind the rpms. It needs them to make decent power.

I have a 49.5x50 Sterba if anyone wants to try it in the air on a SII. Pay shipping both ways and let me know what it does.

 

[Schmleff]

For what its worth, here is a desktop dyno chart that Chad did when he was helping make choices for my motor.

You want the engine to be able to spin up to at least the rpm that the torque peak starts. On this one, it starts at about 3500rpms. Spinning any less than that is just wasting power, unless you are purposely derating the engine. Peak torque hangs in there till just over 4500rpms.

From the Formula V days till now, the Sonerai's that are nearing the 200mph mark are able to spin the engine up. John M (and others) tell me that they used to turn nearly 4500rpms on the V's.

There is an entire book of lore and theory that goes along with VW engines in airplanes. It does not apply to a continental or rotax. Its based on the fact that a VW is a small displacement engine with no gear reduction that needs rpms to make power.

 

[Gaston]

very interesting chart Jeff (Thanks Chad)

would like to know more about this book Jeff...

You talked a few times about this air disc velocity Vs Diameter theory,and I adhere to it.So the last mod I did on the prop you sold me ,was to reduce the diameter,as per your recommendation, instead of reducing the pitch once more, and I'm very happy with that .
I almost got what I was looking for...I could make it a little shorter ,but My wallet can't afford to spoil a prop,so I have to live with that, and it's fine...

maybe one day I will make it shorter and tell myself I should have done it long before...

Gaston

 

[Schmleff]

I should have said "oral tradition" instead of book...

what is the diameter at these days?

 

[splischke]

I'm starting to see the light.

Now for a question. Do I have the best prop for my Sonerai IIL based on the following information?

1. Engine GPASC 2180cc at 8.0:1 compression ratio.
2. Prop Sterba 54x48
3. Ground static RPM 3280
4. Field elevation 3640 feet MSL.
5. M.P. on ground engine not running around 26".
6. Initial climb-out 3200-3300 RPM at 85 mph.
7. Cruise RPM 3200-3300 at 21" M.P at 4600 feet MSL.
8. Avgas 100LL.

Also, my throttle response is very linear. I'm not sure if this has anything to do with the Ellison TBI I'm running.

-Scott

 

[Schmleff]

Scott,

What are these numbers?

ROC at 85mph
ROC at 100mph:
RPM at 24":
Max RPM level flight:
Fuel burn in 21" cruise flight:

 

[splischke]

Scott,

What are these numbers?

ROC at 85mph
ROC at 100mph:
RPM at 24":
Max RPM level flight:
Fuel burn in 21" cruise flight:

ROC at 85 mph at 4000 feet MSL with 10 gallons and 172 lb pilot is about 850-900 ft/min. Empty weight of A/C 595 lbs.

Since bumping up the compression ratio last winter I have not updated/recorded my new performance numbers. I'll have to get back with you on 100 mph ROC and max RPM in level flight. Same for the RPM at 24" (which is close to max RPM).

I don't have a fuel flow gauge but fuel burn at 21" is very close to 5.0 gal/hour -- perhaps a tad less. I'll get an accurate number on the next flight.

BTW, all airspeed numbers are indicated AOB 4000 feet.

-Scott

 

[Schmleff]

Scott,

I would be interested to know your new numbers. The only thing that jumps out at me right away is your fuel burn. 5 gph for 21" seems like quite a lot to me. Seems that you might be a tad over propped or cooling with fuel. What is your egt in cruise? I am seeing 5.4gph at 24" at 3000' but turning 3600rpms.

Your plane would be the perfect test bed for my 45.9x50. It worked really well on my 1850cc engine, but it was a little too much for it. Again, not trying to sell it, just want to get some more numbers for the "oral tradition".

 

[splischke]

Testing your prop may be for not. I'm seriously considering bumping the displacement to 2276cc with 94mm nikasil cylinders this winter. I'm digressing from topic. Let's keep this topic on topic.

-Scott

 

[privatepilot03]

In my experience with the VW engines in aircraft, you seem to almost make exponentially more power with RPM. I realize that's kind of a basic statement, so let me clarify that a little better.

Lugging my 2332 around at 3100-3200rpm or so "seemed" ok until I cut 2" of diameter out of the prop. I subscribe to the same school of thought as Jeff regarding diameter of a prop (we both fly R/C airplanes and have done similar experiments) versus pitch. My Sonerai had a T4 engine on it when I bought it. The Sterba prop started out life as a 54x48. It was sent back to Sterba before I bought the plane and was re-pitched to get more RPM out of the engine due to lack luster performance. The engine was hard pressed to swing that prop to 2900rpm static on the ground and really didn't do any better in the air. I cut 2" out of the prop and picked 200 rpm across the board. ALL of my performance numbers increased. The engine still didn't make the power that I thought it should so I went down the T1 engine path and built the 2332. I talked to Sterba on the phone a couple times about what I was trying to do and he cut me a 54x54 straight blade prop. The engine would turn about 3100 static and would WOT at 3400 or so. I could "feel" that the engine was up against some kind of barrier and that once on the other side of whatever was holding me back, the plane would go faster and the engine would be happier. I cut 2.25" out of the diameter of the prop and everything was better from there. In the end, I swung a 51.75x54 prop at 3600-3700 wot in the air and the engine loved it. Excellent climb rate and cruise speeds.

The VW is happiest in the 4000 rpm range. We can't swing much prop diameter in that RPM range, so we compromise. My humble opinion is that most guys fly the VW like they would a Lycoming or Continental engine lugging them along at 2900rpm or so in cruise. Some guys will go to 3200 I guess. For anyone that isn't getting at least 3500rpm WOT straight and level, cut a 1/2" off each tip of your prop and see what happens. Once you see the improvement, go ahead and cut another 1/2" off each tip. Don't send the prop in for re-pitching, it won't help near as much as you think.

Chad

 

[JuergenA]

I agree with Chad about the rpms what the VW want to see, also about repitching the prop...
The prop must be right for the plane and the engine. For the S1, the cruise speed is realistic for 170-180mph, for the S2 about ?130-150mph? (less weight, 2180 or bigger engine,cruise rpm at 3600-3800). When you look in the Prop-Pitch Calculation file inside the download area, you can pick up the right prop for the plane/engine combination.
Instead of cutting the tips, you can form better wing tips, to degrease the aerodynamic drag (and noise) and so become 200-300 more rpms without cutting the tips and without reducing the static (and climb) power. About the form of the wingtips, I will send some informations next week, because I have it in my office and I am now at home...
with best regards
Juergen

 

[Gaston]

waiting to hear from you Juergen on the wing tips...
maybe my WOT could go beyond the actual 3720 rpm or so...

week end will be long...
and season ending soon...

Gaston

 

[splischke]

RPM numbers being mentioned on this thread of 3600 plus (with some references of 4000-4500) are at or above the Great Plains Aircraft published redline of 3600. Great Plains recommends a cruise of 3200 +/- 200 RPM. Are they just being conservative and perhaps more concerned with engine life than getting every last bit of horsepower? Are the guys here that are running 3600 to 4000 RPM in a speed and racing mindset? What RPM is good for "Joe the Sunday Sonerai Pilot" who is looking for a good balance between performance and engine longevity?

I'm playing devil's advocate and not making any judgment on those that are exceeding the GPASC published redline.

Anyone know what is the RPM Redline on the new AeroVee? What about the Revmaster and even the Limbach?

-Scott

 

[privatepilot03]

RPM numbers being mentioned on this thread of 3600 plus (with some references of 4000-4500) are at or above the Great Plains Aircraft published redline of 3600. Great Plains recommends a cruise of 3200 +/- 200 RPM. Are they just being conservative and perhaps more concerned with engine life than getting every last bit of horsepower? Are the guys here that are running 3600 to 4000 RPM in a speed and racing mindset? What RPM is good for "Joe the Sunday Sonerai Pilot" who is looking for a good balance between performance and engine longevity?

I'm playing devil's advocate and not making any judgment on those that are exceeding the GPASC published redline.

Anyone know what is the RPM Redline on the new AeroVee? What about the Revmaster and even the Limbach?

-Scott

Aerovee lists redline on their engine @ 4000rpm. They list their 80hp number @ 3400 rpm. Peak horsepower numbers are VERY sensitive around their stated RPM. If you have an aerovee that doesn't produce 3400rpm in flight condition, you're not making published horsepower numbers. Fuel consumption is a dead accurate way to calculate your HP as well. Brake specific fuel consumption math is pretty easy to do. You'll find that you need to be burning about 6 gallons an hour in cruse config to be making much more than 70hp. So by that same formula the 4-4.5 gallon an hour crowd is making 50-55 hp in cruise. Increase that RPM and you'll not only increase your fuel consumption, but you'll also drastically increase your HP output.

I believe the redline numbers listed by VW manufacturers take into account expected prop diameters. If you do the math on RPM figures north of 3600 rpm you'll see that you can't swing the 54"-58" diameter props that these manufacturers recommend without the prop tips going outside the 80% MACH speed that is considered the speed limit on wood props for a VW.

I'm not in a racing mindset at all. I have recommended my prop mod to everyone I've ever consulted in the last 5+ years that was flying behind a VW. So I believe even Joe the Sunday Sonerai Pilot can benefit from trying to turn a few more RPM.

Chad

 

[EricM]

My Sonerai is powered by a Revmaster which is listed as 65hp at 3200rpm. I could not find a redline in the manual supplied with the engine. There is a power curve chart which indicates 62hp at 2800rpm and 65hp between 3050 and 3300rpm. The chart stops at 3300 rpm.
I have a Sterba 54X48 prop which is overpitched for my plane/engine. I get 3100rpm at full throttle static runup and 3000 rpm in climb at 90 to 100mph indicated. Cruise figures are:

RPM MP IAS MPH
2800 21 115
2900 22 120
3000 23 125
3100 27 130
3200 28 140

All speed runs were done at 2000ft msl and OAT around 80f. The airspeed indicator agrees with the GPS with calm winds. At WOT, the rpm will build up to 3200 and the plane seems to “get on the step” and airspeed will increase to 140. The 140mph at 3200rpm mystifies me because that is 96% efficiency and I didn’t think a wood prop would perform that well. Another interesting note is the engine smoothes out at 3200rpm and feels like it is not lugging like it does at lower rpms. I usually cruise at 21 to 22” MP which is 2800-2900rpms. At this power setting my fuel burn is 3.5 to 4 gph so I don’t think the engine is putting out 62hp.
I am planning on having my prop repitched and or cut down or even getting a new prop. Being the frugal guy I am, I will probably do whatever is cheapest.

Eric

 

[privatepilot03]

My Sonerai is powered by a Revmaster which is listed as 65hp at 3200rpm. I could not find a redline in the manual supplied with the engine. There is a power curve chart which indicates 62hp at 2800rpm and 65hp between 3050 and 3300rpm. The chart stops at 3300 rpm.
I have a Sterba 54X48 prop which is overpitched for my plane/engine. I get 3100rpm at full throttle static runup and 3000 rpm in climb at 90 to 100mph indicated. Cruise figures are:

RPM MP IAS MPH
2800 21 115
2900 22 120
3000 23 125
3100 27 130
3200 28 140

All speed runs were done at 2000ft msl and OAT around 80f. The airspeed indicator agrees with the GPS with calm winds. At WOT, the rpm will build up to 3200 and the plane seems to “get on the step” and airspeed will increase to 140. The 140mph at 3200rpm mystifies me because that is 96% efficiency and I didn’t think a wood prop would perform that well. Another interesting note is the engine smoothes out at 3200rpm and feels like it is not lugging like it does at lower rpms. I usually cruise at 21 to 22” MP which is 2800-2900rpms. At this power setting my fuel burn is 3.5 to 4 gph so I don’t think the engine is putting out 62hp.
I am planning on having my prop repitched and or cut down or even getting a new prop. Being the frugal guy I am, I will probably do whatever is cheapest.

Eric

Coping saw from your local Home Depot is $5. 10 minutes of your time to cut the prop down is basically free. Can't imagine it getting much cheaper than that.

chad