RV-9A Prop Overspeed

[Larrybia]

I tried to do an airspeed run today (three runs at 0, 180, and 90 degrees). I have finally completed the wheel pants and wanted to see what the change was. I could not fly full throttle because I kept getting prop over speed. I have an RV-9A with a 160 hp 0-320. I am using the Van?s recommended Sensenich fix pitch prop which has an over speed limit of 2600 rpm. At indicated of 162 mph and ground speed GPS of 175 mph the prop would go to 2600 rpm at 6500 ft, 80 OAT and 24 mp. I did the speed runs at 23.4 mp, 2500 rpm and got an IAS of 159 with a TAS of 173. With out the wheel pants I got IAS of 142 and TAS of 164. Same outside air temperature for with and without pants.

Has anyone seen this problem before? Is it typical that the RV-9A can not cruise at full throttle at 6500 ft with a the Van?s recommend fix pitched prop and a 160 hp engine without exceeding the max prop rpm limit?

Comments?

 

[N941WR]

Larry,

At 6,500 this would be possible as you can push it past 75% power. Get up to 8,000' density altitude and give it another try. At that altitude you should be limited to that magic 75% power and then probably can't over speed the engine.

FWIW, the prop on my O-290 is such the wrong pitch that at 10,000 DA I can spin it well past the 2800 RPM red line. (O-290-D2 has 2600 RPM for continuous power and 2800 RPM for five minutes.) On the plus side, I can climb at 1800 FPM at gross on 80+ degree days. I hope to have Catto make me a new prop later this year.

 

[tc1234c]

I have similar setup as your: O320D2A with Sensenich FP prop from Van's.
Following is my test results (at 8,500 ft, OAT 25 degree F)

I set my idle at 550 rpm. Given the limited throw of the throttle cable, I don't get full power. Unless I enter a dive, I usually don't red line (2,600 rpm) my propeller.

 

[Bob Axsom]

Yes I've Heard of that Limit

Charlie Heathco has an RV-6A in the hangar row across from mine and we entered the Texoma 100 air race. He flew his in the RV Red class because he has the O-320 and I flew in the RV Blue class with my O-360. He flew the race using that limit and his speed was held down to 186.17 mph. He could have gone faster but was held down by that limitation. I assume that it is not uncommon to have to hold back with that prop to avoid exceeding 2600 rpm. I don't think that would be much of a problem for normal cross country cruising but for racing it is not competitive. The other RV Red flown by Chris Murphy averaged 211.40 mph for the course.

Bob Axsom

 

[GrayHawk]

Another Data Point

RV-6A, O320-D1A, Sensenich 70CM7S16-0-79

Cannot even get close to 2600 rpm unless I dive. Maybe I need to repitch the prop?

 

[N916K]

I also have the 79" Sensenich with a 320. With my 9A I would just reach 2600 rpm at 8000 feet at 164 knots. At lower altitudes I had to reduce power. Now that it's a 9 I have to reduce power a bit at 8000 feet.

Now about your data. Between 2500 and 2600 rpm you saw a 3 mph change? Are you sure about that?

 

[PCHunt]

Throttle Mis-Rigged

".....Given the limited throw of the throttle cable, I don't get full power......"

All throttles can and should be rigged to be able to be fully open, and fully on the idle stop, with a "cushion" on each end. Check your throttle rigging, something's not right.

 

[tc1234c]

The limited throttle cable throw has been discussed in another thread. As for rigging, I am already using the inner most hole (of the three) on the cab throttle arm. I measured the full travel of the throttle arm that is longer than the throw of the cable. It may have a way to make it work but I don't know how. If I set the idle higher then I will be able to reach the full open throttle position.

My propeller is Sensenich 70CM7S9-0-79. It is the recommended "standard" propeller for 9. The climb propeller is 70CM()S9-0-78 and cruise propeller is 70CM()S9-0-81.

 

[N941WR]

...I am already using the inner most hole (of the three) on the cab throttle arm. I measured the full travel of the throttle arm that is longer than the throw of the cable. It may have a way to make it work but I don't know how. If I set the idle higher then I will be able to reach the full open throttle position.

Check around, it is my understanding that you can get different length throttle arms for the carb.

If you set the idle high, you just might float off the end of the runway with a long wing RV.

 

[tc1234c]

Bill,

On the carb throttle arm I will see what I can find (not the total arm length but the location of holes). For now I will assume what I have is an "over speed protection arm".

Yes, keep idle at 550 rpm is necessary for me to land without "floating to the next county".

 

[videobobk]

First, I am coming to believe the lower the rpm, the better. I am at 400 idle when stopped but only pull all the way on final. On the ground I usually use a touch of throttle. Works much better than higher rpm.

It seems to me that some electronic systems that sense MAP make a little more power at alt. Catto props take that into account and he asks what you have. You didn't specify, but that could be a factor.

I also vote that a throttle should not be cable-limited. Stops are there for a reason and not getting full power could come back and bite you. Same with idle. Get a longer throw cable of shorter carb arm.

Bob Kelly

 

[victor]

RPM accuracy

I have a mechanical tach that showed 2660 rpm at full throttle at altitude. After double-checking with a digital tach, it was actually 2590-2600 rpm. The error was throughout the entire range, being 10 rpm over at 1000 rpm, and about 40 rpm over at 2000. ( also 79" fp sensenich )