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-   -   Yet again, another prop/speed question (http://www.vansairforce.com/community/showthread.php?t=152102)

Dreamin9 08-06-2017 03:48 PM

Yet again, another prop/speed question
 
Here's some numbers from a recent test flight, I'd like to find more speed in the 6.5 to 12.5 altitude range. Considerations are re-pitch or new prop or burn gas.

RV-9A, O-320 160hp, 405hrs
Prop Sensenich metal 70/79 (actually 69.5 after repair from nose collapse/flip at 13 hrs)
Power set to 75% after initial climb for CHT cooling, leaned to peak, AS in knots, static +/- 2200
This engine runs smoothly LOP; FF/RPM/speed/CHT's drop off quickly LOP.

ALT DALT RPM MAP %pwr FF TAS

Rotation 750 2140 2275 29.4 93 12.5 72

Level@ 1800 3100 2380 25.1 78 12 138

Level@ 6500 7800 2470 23.4 76 8.4 144

Level@ 8500 9815 2510 22.5 70 7.5 147

Level@ 10500 11840 2480 20.9 67 7.1 148

Thanks,



Carl

lr172 08-07-2017 12:00 PM

I would suggest a prop that will let you turn 2700. I have a 6A /160 HP with an older Catto prop. At 8000' I get around 2740 in the neighborhood of peak EGT (maybe 10-20 LOP) and burn 8.2 GPH for 161-162 KTAS. The additional RPM equates to greater HP production. At 10,000' the fuel burn is under 8 for 161 and 2720 RPM.

My prop is a bit under-pitched, but I would only rarely want to be ROP at cruise anyways and this prop gives me good climb and the ability to turn 2700 at 12,000 (14,000 DA).

The Catto is very smooth and I have no complaints about vibration at those RPMs.

Larry

Raymo 08-07-2017 03:09 PM

Quote:

Originally Posted by lr172 (Post 1194009)
I would suggest a prop that will let you turn 2700. I have a 6A /160 HP with an older Catto prop. At 8000' I get around 2740 in the neighborhood of peak EGT (maybe 10-20 LOP) and burn 8.2 GPH for 161-162 KTAS. The additional RPM equates to greater HP production. At 10,000' the fuel burn is under 8 for 161 and 2720 RPM.

My prop is a bit under-pitched, but I would only rarely want to be ROP at cruise anyways and this prop gives me good climb and the ability to turn 2700 at 12,000 (14,000 DA).

The Catto is very smooth and I have no complaints about vibration at those RPMs.

Larry

I don't think he is at WOT given the fuel flows but maybe I've mis-read. Would need to know if he can get to 2700 at WOT at 8000 ft DALT. If not, reduce drag (cheap), increase HP or reduce pitch. (my 2 cents)

Sam Buchanan 08-07-2017 03:45 PM

His prop is restricted to 2600 rpm by Sensenich. Burning more gas will yield higher cruise speed, the pitch is correct for a cruise prop on the 9A.

But I have to wonder if the prop lost some efficiency due to the repair.

lr172 08-07-2017 06:13 PM

I am not a prop expert, but I believe he is currently over pitched for his goal. The only way for him to increase speed is to move from peak to ROP, which is inefficient. A lot of gas for little improvement. By backing off the pitch, he can increase his RPM from 2500 to 2700, while still at peak EGT. He will pick up 5-10 additional HP with the added RPM and with a more moderate increase in fuel flow over his current flow. With the right pitch, that 5+ HP will translate to increased speed. The only thing you leave on the table is the ability to pour on the coals (increase speed further) via best power as you are already at redline. The counter to that is improved climb rates due to the lower pitch.

According to the lycoming SL chart, 2500 @ 9 GPH / Best power = 105 HP and 2700 @ 9 GPH / Best economy (peak) = 122 HP. This, of course, is seal level power, but the comparison should be close to linear at 8000'

I understand the OP can't spin the Sensi at 2700 and is why I suggested getting a prop that could. For the fixed pitch crowd, increased RPM is the answer to increased speed. I am sure that is why the racers spin their engines at 3000 or more.

Larry

Dbro172 08-07-2017 09:39 PM

Same engine, same plane, at 75% power and 6500' I'd probably be 10 knots faster and similar rpm. I've ran both a catto 2 blade and currently a whirlwind GA, both perform similar.

Dreamin9 08-15-2017 02:26 PM

Thanks for the above responses.

Jesse Saint suggested we do 4 cardinal heading speed checks to compare with the Dynon calculated TAS. As you can see, at the two altitudes tested, True based to 4 point GS reading is 7-9kts faster than TAS shown on EFIS.
Here's the spreadsheet, WOT at both altitudes:
P Alt 7500 HDG 360 90 180 270 AVERAGE
D Alt 8457 GS 140 152 170 159 155.25
MAP 23.4 IND 129 130 129 130 129.5
RPM 2520 DYNON TRUE 147 148 148 148 147.75
FF 8.6
%Hp 78%


P Alt 9500 HDG 360 90 180 270
D Alt 10450 GS 134 141 164 159 149.5
MAP 21.6 IND 120 121 120 121 120.5
RPM 2450 DYNON TRUE 140 141 141 140 140.5
FF 7.4
%Hp 65%

Static at 1748 Dalt 2180


That's a comfort for sure!
On the other hand, it seems I can't get the Sensenish FP to the prop limit of 2600.
Does that mean that ideally, the pitch should be a bit finer?

Thanks again;

Carl

Raymo 08-15-2017 02:54 PM

Quote:

Originally Posted by Dreamin9 (Post 1196523)
On the other hand, it seems I can't get the Sensenish FP to the prop limit of 2600.
Does that mean that ideally, the pitch should be a bit finer?

Carl

I would look for ways to reduce drag first, if possible. I thought I had performance issues with my -7A until I installed the gear fairings and picked up about 20 knots and 140 RPM (currently getting to 2690). I still need to fill in a bunch of gaps between control surfaces (e.g. elevator fairings) and hope to find another 5-10 knots.

TXFlyGuy 08-19-2017 08:15 AM

Here is my question on props, and rpm...according to the Warp Drive calculator, the most efficient rpm for the prop in cruise is 475 mph tip speed. Above that, the prop loses aerodynamic efficiency, makes lots of noise, and you lose IAS.

True? False? Old wives tale?

Dreamin9 08-26-2017 10:55 AM

Hi, OP here, I don't mind the hijack.

From what little I know, those issues listed become more a concern as the tip approaches the speed of sound.


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