6A Performance/Prop Evaluation

Anyone who has followed the build thread for 94CR will know that I am very new to flying this RV, and will likely be full of questions in the near future. The first of these relates to some testing I did yesterday at altitude.
I climbed to 8k msl (DA was lower due to the cold, but I forgot to look and see how much) and performed four runs (N, S, E & W) at the same indicated airspeed to compare with groundspeed, check TAS, 75% performance, etc.

(Almost) Full throttle running just ROP, I snapped the following photos:






I have all of the fairings on the plane except the one that transitions between nose leg faring and bottom of the cowl. Running the numbers through an online calculator, I'm showing that 154 indicated in those conditions = about 171kts TAS. I saw "almost" full throttle because all-in, the Sensenich FP on this carbureted o-360 wants to spin 2900rpm, so I pulled it back slightly at first, and then even more which is why two of the runs show an indicated of about 150kts.

My questions are these: Is this typical performance from a 6A thusly equipped? Am I looking at the correct performance markers (MP/IAS/TAS/%power, etc)?
Perhaps more importantly, is the Sensenich prone to overspeeding in the RV, and what would be the pro's and cons of having it (or putting on something else which is) pitched to turn right at 2700 rpm max?

I hope these aren't total NOOB ?'s, but, as they say, you don't know what you don't know... Not to mention, I've got a ton of learning to do just to figure out how to set up this GRT! I'm mostly just flying the plane at this point, but I did enough digging around the AP to set it to fly a 500fpm descent. Kind of cool to see it nail the VS and keep it there, and to be able to steer using the heading bug! What someone needs to do is to put together some instructional video's on the GRT Sport and upload them to youtube! It's one thing to read the manual, but another entirely to really understand what's going on...

>My questions are these: Is this typical performance from a 6A thusly equipped?

170 KTAS is about the max my 6A with an O-320 will do.

>Am I looking at the correct performance markers (MP/IAS/TAS/%power, etc)?

Using the 4-way runs ensure your IAS is accurate.

>Perhaps more importantly, is the Sensenich prone to overspeeding in the RV,

No, not if you have the correct prop. See this page

>and what would be the pro's and cons of having it (or putting on something else which is) pitched to turn right at 2700 rpm max?

Your propeller is very much more likely to remain with the airplane - metal props don't like large overspeeds... Your take-off performance and climb rate will decrease, your max cruise may increase very slightly. The liklihood of your engine reaching TBO will increase (if that is important to you) and your fuel consumption will decrease. The airplane will feel more comfortable as the engine will not be turning so quickly. A 200rpm over speed at full throttle says you are 4 to 5" under pitched - that's a lot!

Are you sure you have anything like the correct Sensenich prop? The page linked above shows only one prop for any 180hp RV, with 85" pitch. Bear in mind that a Sensenich prop can only be re-pitched by 7" throughout its whole life. Also, if you have a prop intended for an O-320 (check the part no) then that is limited to 2600rpm - you are taking a significant risk revving to 2900.

Good luck, I've had my 6A for 14 years ...

Thanks for the input. I've got the prescribed prop for the 0-360, and the log doesn't indicate that it has ever been back to Sensenich for any adjustments. My tach is accurate, so i'm not sure what's going on here, but I completely agree with your assessment that something needs to be done to keep the RPM's down.

What speed are getting at 2700 rpm? What is your static rpm?

What's up with the TAS and fuel flow numbers on the display? Maybe you really are doing 2820kts TAS and if so, you've got the fastest RV I've ever seen

Ha! More GRT learning curve to figure out. Both of those figures are accurate when displayed in their dedicated locations (TAS can be selected instead of GS for the top left corner of the PFD page, and FF is shown on the engine page correctly...) I went into the settings and selected those labels in the boxes, but apparently I have to dig around and try to find out where its pulling that data from and change it separately.

I have to go back and check the static numbers. The speed at 2700 is about 140-145kias

I appreciate you posting the screen shots. I am working on my -320, -6 to get the cruise speed up. I am turning a 3 blade catto, 2650 rpms at 8000ft for 158.7 ktas. By your numbers, if i can get it to 2700-2750 rpms, i should see about 165ktas.

I run an IO-320 with a Catto 2 blade prop in my 6A. At 8000' I turn my prop around 2750 for 190 MPH (~165 Kts) TAS. At that altitude/RPM I burn 8.1-8.2 GPH. My prop will turn a bit over 2800 if I go ROP at that altitude. I am somewhat under-pitched, but that lets me achieve higher speeds at higher altitudes and gives me a little better climb. I would try running LOP and getting the RPM down to 2700. If that nets a speed lower than you want, you should re-pitch. I don't know much about the sensy, but I don't believe they rate it above 2700 and you need to know if it is safe there. My Catto is rated above 3000.

Hope that helps in your research.

Larry

j-red said:

Thanks for the input. I've got the prescribed prop for the 0-360, and the log doesn't indicate that it has ever been back to Sensenich for any adjustments. My tach is accurate, so i'm not sure what's going on here, but I completely agree with your assessment that something needs to be done to keep the RPM's down.

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If you are sure you have the correct prop then I would check your tach again. I believe static rpm for a 180hp engine should be around 2100rpm. If you are showing higher something is wrong somewhere. Get an optical tach, check it against a neon light, then check the prop speed. Also use the 3 (or 4 ) run GPS technique to calibrate your ASI.

Pete

penguin said:

If you are sure you have the correct prop then I would check your tach again. I believe static rpm for a 180hp engine should be around 2100rpm. If you are showing higher something is wrong somewhere. Get an optical tach, check it against a neon light, then check the prop speed. Also use the 3 (or 4 ) run GPS technique to calibrate your ASI.

Pete

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An incandescent light works just fine.... it also flickers 60 times a second and would show 3600.

gasman said:

An incandescent light works just fine.... it also flickers 60 times a second and would show 3600.

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I always thought that incandescent lights did NOT flicker due to the relatively heavy tungsten element staying hot through the cyclic voltage. OTOH, fluorescent bulbs do flicker. As do Neon bulbs.

I stand ready to be corrected!

Fluorescent tubes do flicker.
That's why the lights in an engineering shop should be of the correct type to stop lathes and mills looking stationary when they are really going full chat!

The only thing left to point out on this very international forum is that in Europe the mains frequency is 50Hz, not 60Hz.

Was able to fly again this morning and check out a couple of things, albeit pretty gusty conditions and limited flying time kept it short, slow, and at lower altitude.

First, I checked static RPM and it is just about exactly 2100rpm, as was suggested it should be. A possible conclusion, then, is that the tach could perhaps be off at the higher rpm's. I'll have to get a light and test it, but that, to me, seems unlikely since the CPI ignition box can display a tach readout that is driven direcly off of the flywheel magnets and that agrees, at least at the lower rpm's I flew today, with the grt which uses both the electronic ignition and slick mag for tach display.

It was really cold today, so the density altitude at 2500' was practically sea level. What I saw was about 135kts indicated at 2200rpm and leaned to 8gph fuel burn. That's all the faster I wanted to go given the turbulent conditions (it was actually worse up higher). From what I can tell, that's pretty comparable performance to others with the same setup. I still don't know how this engine/prop combo is capable of spinning up so high at altitude, but for the time being i'm just going to throttle back when needed to keep it below 2700rpm indicated.

One more quick question before putting this thing to rest for now: I double checked the temperature during Monday's flight and recalculated density altitude. DA was actually 6500' while doing those 4 runs pictured above where the prop wanted to turn higher than 2800. Should I expect the prop to be able to spin faster at a lower DA, or a higher one? In other words, if I was to fly at a true 8k DA, full throttle, would I expect the prop to turn closer to the desired 2700 rpm, or would it actually want to go even faster than the 2900 it was creaping toward the other day?

Max rpm will decrease at higher density alt

Maximum RPM

This was mentioned earlier, but a lot of Sensenich propellers have a 2600 RPM limit.

flytoboat said:

This was mentioned earlier, but a lot of Sensenich propellers have a 2600 RPM limit.

Click to expand...

The ones for the O-320 have the 2600 RPM restriction. The ones for the O-360 don't.

Flicker at 2f?

Doesn't an incandescent bulb flicker at 120 Hz (100 Hz in Europe, parts of Japan, etc.)? Two zero crossings per cycle in an AC waveform....

Your data boxes on the lower right are mislabelled. Looks like your TAS data field is showing rpm and fuel flow data field may be showing density altitude data. Go to the setup page to change the data field to match the same as the label field for each box.

Maybe change the "Flow" box to "% power" along with the data field and put "flow" on a bar chart added to the lower left.

The attitude indicator is showing the plane about 5 degrees nose high even though you're not climbing. That doesn't seem right at that speed and altitude. The AHARS may need a pitch calibration adjustment as may your magnetometer unless the winds were really that variable and up to 57kts. Poor mag calibration can have a large effect on calculated wind velocity and direction.

Bevan