Low Prop RPM? Maybe, Maybe Not

[Phenomflyer237]

Hi all new to Vans- sorry for any naive questions.

We have an RV6a 0-320 engine

curious what RPM we should achieve topped out?


The plane currently shows 2350 firewall at sea level.

if you pull it back till the engine relaxes, tach shows 2200 RPM/MP is 25"
this produces around 8g/hr fuel burn and planes does an honest 135TAS.


is the engine producing full power?

the prop is a 78
the build manual recommends a 77 or 79 i believe for the 6a 0320
The plane seems to climb very well 1500 at gross weight SL.
it also seems to cruise at a good speed and feels like it is making full power.
the prop feels like a nice combination to me


mag drops are clean
throttle linkage is adjusted to the carb stops.
Does anyone know what RPM we should be running?
also what good power settings you know for this combo
Thanks!

[Kyle Boatright]

Let's do some investigating first:

Which prop do you have? The number you referenced might be the pitch number from a sensenich prop, but we need to know the model number.

Are all of the fairings installed on the airplane? Wheel pants, gear leg fairings, etc?

Assuming you have the right metal prop from Sensenich and all fairings are properly installed, you should see 2600 RPM and 165+ knots wide open at that altitude.

[moosepileit]

Tach sounds off to the low.

Your climb rate says way more rpm, assuming you mean a metal Sens 70CM6 78" pitch.

I static 2300 and max out at 2680 (yeah, yeah, I know, just a limits check) at 14 gph down low, std day. 79 inch pitch of your prop. Never see 1500 fpm at best rate of climb near gross, 2350 to 2400 rpm. Solid 1000 fpm to 2000' DA. As Kyle says, with my taildragger, not a speedy one, 165 knots true.

Mags and carb. My carb is not overly rich at WOT.

What is your max fuel flow?

IF your MAP is accurate, you should be able to figure out if the TACH is off. A $10 RC strobe Tachometer can check the panel tach, works best flying into the sun.

I have to look up my chart, but 2300 rpm low altitude cruise is about 22" MAP.

8 gph and 135 TAS- knots, not MPH, IF down low? That should not need 25" MAP, I'd think that's 2400 rpm and 24 inches with your prop, if my assumptions are near correct.

Do you know that your speeds are accurate?

[Phenomflyer237]

70CM6 Series
prop data tag
T.C. P28NE P.C. NO. 1NE
All fairings and cowlings on

[Ironflight]

What kind of tach - mechanical, or electronic? I have seen mechanical tacos reading off by 150 rpm when doing prop balancing - it is a significant difference!

[rvbuilder2002]

I agree with Paul. The first thing to do would be confirm that the tach. reading is correct.
If it is correct, then all of your RPM values are well below what is typical for an RV-6 with a well matched propeller but it sounds like your performance is not, so that makes the tach. reading suspect.

[Phenomflyer237]

Tach is mechanical.
I did swap it out for a different tach and the results were the same.

It still could be the cable or the tach drive. I am going to work on an alternate means to verify RPM.
What numbers and performance should we be seeing with the above prop and o-320?
Now i am pulling it back to what sounds like 2300RPM to me "tach reads 2200" its doing about 130-135 on the outer true scale at those numbers.
Thanks!

[rvbuilder2002]

Originally Posted by Phenomflyer237

We have an RV6a 0-320 engine

curious what RPM we should achieve topped out?


The plane currently shows 2350 firewall at sea level.

if you pull it back till the engine relaxes, tach shows 2200 RPM/MP is 25"
this produces around 8g/hr fuel burn and planes does an honest 135TAS.


is the engine producing full power?

the prop is a 78
the build manual recommends a 77 or 79 i believe for the 6a 0320
The plane seems to climb very well 1500 at gross weight SL.
it also seems to cruise at a good speed and feels like it is making full power.
the prop feels like a nice combination to me


mag drops are clean
throttle linkage is adjusted to the carb stops.
Does anyone know what RPM we should be running?
also what good power settings you know for this combo
Thanks!

Originally Posted by Phenomflyer237

What numbers and performance should we be seeing with the above prop and o-320?

RV's with a fixed pitch prop that is pitched for good all around (total) performance will usually reach max continuous RPM (2700) at WOT at somewhere between 8000-10,000 ft.

Any lower than that, they typically require a reduced throttle setting to stay below 2700 RPM.

Looked at another way, cruising around at lower altitudes the MP value is usually at least 3 to 4 points below the first two digits of the RPM value (example - 22 inchs MP at 2600 RPM at 3500 ft).

Since your gross weight ROC seems pretty normal for an O-320 but your MP to RPM relationships are out of wack, I think your tach. reading is highly suspect, though if you MP reading is correct your low altitude TAS of 135 is quite a bit off the norm the norm. Even if you are talking Kts, it should be closer to 155 at least.