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  #1  
Old 08-07-2017, 11:42 PM
az_gila's Avatar
az_gila az_gila is offline
 
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Question Sensenich prop pitch Q's

A propeller pitch question

The PLANE - RV-6A still in Phase I testing, now with gear leg, intersection fairings and Sam James wheel pants. No other speed mods and a stock O-360-A1A with a carb and 2 magnetos.

The PROP - Sensenich PROP 72FM8S9-1 (83)

I notice Sensenich rates this as a "standard" prop for a 180 HP -6A, while Vans sells the 85 inch pitch version.

I did some high altitude flying today (cooling off a bit to 44F ) and tested full throttle speeds.

At 12,500 ft I got 2710 rpm and 168 kts TAS
At 13,500 ft I got 2670 rpm and 166 kts TAS

So it seems like my full throttle 2700 rpm number is essentially 13,000 ft. and 167 kts (191 mph)

Note that today this 13,000 ft pressure altitude was about 15,200 ft. density altitude.

Climbing the last 1,000 ft at 100 kts IAS I was at 700 fpm.

Above solo with 3/4 tanks and an estimated weight of 1470 lbs.

At this time of year I'm taking off at 5,000 ft DA early in the morning so don't know the effect of pitch at lower altitudes.


So the questions are -

Should I stay with 83 pitch or have it re-pitched to 85, the same as Vans sells?

If I change, what effects would I see?

Are the "full throttle rpm" recommended altitudes usually quoted as pressure altitude or density altitude?


PS - the TAS speeds are not quite fully calibrated yet, but are probably within 1 to 2 kts accuracy as I'm slowly adjusting the static port "bumps".

PPS - not ready to switch to Constant Speed yet, so please keep replies to Sensenich FP props.
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Last edited by az_gila : 08-07-2017 at 11:49 PM.
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  #2  
Old 08-08-2017, 04:15 AM
moosepileit moosepileit is online now
 
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What was the static RPM at runup?

How much staic rpm are you willing to lose if you increase pitch?

I would guess 2300 or more rpm, static.

Folks can chime in on if you would stay above 2200 and what your speed gain would be with one ot two more inches of pitch.
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  #3  
Old 08-08-2017, 05:57 AM
dtw_rv6 dtw_rv6 is offline
 
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I'm running the same prop but at the 85" as supplied from Van's. I can climb out at 1,500-1,700 ft/min with no passengers. I haven't collected any WOT numbers because I don't routinely fly above 10,500. I can tell you that I have considered adding a couple inches *more* to the prop so I can run WOT lower.

Take these numbers FWIW. Assume nothing here is "calibrated". I was LOP coming back from OSH with WOT, but had to drastically reduce fuel flow/mixture to stay below the 2700 RPM limit (speed in kts). Based on this run, I am looking at reducing exit ramp area for less drag.

Static RPM is 2200.

Density Altitude Indicated Airspeed True Airspeed OAT RPM1/N1 RPM2/N2 CHT1 CHT2 CHT3 CHT4 Fuel Flow
11357 143.8 170.5 54 2702 2702 345 371 385 366 8.3
11352 144 170.8 54 2704 2704 345 371 385 366 8.3
11411 144.5 171.6 55 2708 2708 345 371 385 366 8.3
11411 144.9 172 55 2710 2710 345 371 385 366 8.3
11404 145 172.1 55 2712 2712 345 371 385 366 8.3
11343 145.1 172.1 54 2712 2712 345 371 385 366 8.3
11343 145.1 172 54 2712 2712 345 371 385 366 8.4
11404 144.9 172 55 2712 2712 345 371 385 366 8.3
11404 144.9 171.9 55 2712 2712 345 371 385 366 8.3
11404 144.8 171.8 55 2712 2712 345 371 385 366 8.4
11398 144.5 171.4 55 2712 2712 345 371 385 366 8.3
11399 144.1 171 55 2710 2710 345 371 385 366 8.2
11404 143.3 170.1 55 2702 2702 345 371 385 366 8.2
11411 142.8 169.5 55 2700 2700 345 371 385 366 8.1
11417 142.4 169.1 55 2700 2700 345 371 385 366 8

Last edited by dtw_rv6 : 08-08-2017 at 05:59 AM.
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  #4  
Old 08-08-2017, 06:35 AM
rvsxer rvsxer is offline
 
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I put 1450 hours on that same prop, same engine, ignition and carb. Started out at 83" pitch and it seemed okay. My numbers were almost identical to yours. I finally had it repitched free + shipping by Sensenich, and my redline RPM altitude was about 12,000, speed at 12,000 was 172 knots. Top speed was 179 knots at 8,000.
Having said that, your comment about the relatively high density altitude takeoffs makes me wonder if you should leave it alone, if it is doing what you want it to. Tough decision...
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  #5  
Old 08-08-2017, 07:18 AM
Bill Boyd Bill Boyd is offline
 
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Default I'm probably wrong

Gil, aren't the fixed pitch Sensenich props for the -6 all redlined at 2600 rpm? Mine is.
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  #6  
Old 08-08-2017, 07:34 AM
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Sam Buchanan Sam Buchanan is offline
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Quote:
Originally Posted by Bill Boyd View Post
Gil, aren't the fixed pitch Sensenich props for the -6 all redlined at 2600 rpm? Mine is.
Only the Sensenich metal prop for the O-320, the O-360 prop has no restriction.
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  #7  
Old 08-08-2017, 07:38 AM
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To expand on Sam's post; The 70CM series (O-320) is certificated and is restricted to 2600 RPM. The 72FM series (O-360) is not certificated and does not have the restriction.
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  #8  
Old 08-08-2017, 08:30 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by az_gila View Post


So the questions are -

Should I stay with 83 pitch or have it re-pitched to 85, the same as Vans sells?

If I change, what effects would I see?

Are the "full throttle rpm" recommended altitudes usually quoted as pressure altitude or density altitude?

As I'm sure you already know.... a fixed pitch prop is a compromise. It is a bigger compromise on an airplane with a wide min. to max. speed range like RV's have, when compared to something like a C-172.

Only you can really answer your question because everyone's needs are a bit different.

My O-360 6A has the 85" prop. It works well for me and puts my WOT/2700 RPM altitude at about 11,000 (depends on the density altitude on a given day)

If you re-pitch you would likely see the same with about the same speed you have now (I get about the same speed as you... maybe a couple Kts faster), but your climb rate at any given altitude would probably drop about 100FPM and your take-off distance will get slightly longer.

For engineering comparisons density altitude is generally used, but it doesn't really matter in this case because like I said, there is no standard with a fixed pitch prop on an RV.... Everyone's needs are a little different.

One thing to keep in mind
A re-pitch to 85" would not get you the exact same prop as having started with an 85" pitch. When the shop twists the blades they are only able to change the outer half to 2/3rds of each blade. The inner portion is too thick to effect much change.
The inner portion doesn't do as much of the work because of its thickness, but it does do some. Just something else to consider.
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  #9  
Old 08-08-2017, 09:25 AM
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az_gila az_gila is offline
 
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Thanks Scott... yes, it's all a compromise.

I'm now guessing that the extra 2 inches of pitch would give me about plus 4 kts top speed assuming the "airscrew" theory is correct.

My wish is for cross-country cruising, but living at 3000 ft in the hot country with mountains around I need a reasonable climb rate. If the climb rate doesn't suffer too much I assume I could essentially get the same cruise speeds as I do now with about 100 rpm less. I would think this would be a bit more comfortable in the cockpit.

Your (Scott) 11,000 ft full rpm altitude would have translated into 8700 ft DA yesterday. Interestingly, the same 11,000ft std day altitude is also the altitude that my Grumman Tiger goes full throttle at 2700 rpm.

I haven't done a full power static run up check, but I'm running 2430 rpm in climb at 1000 ft AGL (DA = 6600) and at 115 kts. IAS

I'm preparing the data/paperwork for an OP (Observer Pilot) so I can do some more climb testing at close to gross weight and re-evaluate.


On another point - I keep seeing this comment -

One thing to keep in mind
A re-pitch to 85" would not get you the exact same prop as having started with an 85" pitch. When the shop twists the blades they are only able to change the outer half to 2/3rds of each blade. The inner portion is too thick to effect much change.
The inner portion doesn't do as much of the work because of its thickness, but it does do some. Just something else to consider.

However , if the pitch changing is done by the Sensenich factory I think all props would be the same at each pitch, regardless of later "twisting". Sensenich's web site says that all props in each series start off at an identical pitch and are then twisted appropriately beoforef shipping. This is part of a manufacturing description -

Every Sensenich propeller basic model (e.g. 74DM) comes off the CNC machine with the same median pitch. Depending on the specific aircraft the pitch can be raised or lowered.

http://www.sensenich.com/files/docum...1297368227.pdf
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EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ

Last edited by az_gila : 08-08-2017 at 09:33 AM.
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  #10  
Old 08-08-2017, 09:30 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by az_gila View Post


However , if the pitch changing is done by the Sensenich factory I think all props would be the same at each pitch, regardless of later "twisting". Sensenich's web site says that all props in each series start off at an identical pitch and are then twisted appropriately beoforef shipping. This is part of a manufacturing description -

Every Sensenich propeller basic model (e.g. 74DM) comes off the CNC machine with the same median pitch. Depending on the specific aircraft the pitch can be raised or lowered.

http://www.sensenich.com/files/docum...1297368227.pdf
I have not ever seen this, and I am only familiar with props that have be re-pitched in the field by shops (and in that case what I said seems to be true).
If the result is different when done at the factory, that would definitely be the best way to go.
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