Anyone have trouble keeping the tail down during runup?

Kooshball · #1 10-16-2020, 06:47 AM

I’m not sure if something could have changed with prop angle when my prop & hub were overhauled and the governor replaced but I experienced something for the first time last night when I did my run up.

I have a 200hp IO360 with a 3-blade MT prop, Mt hub and MT gov. Yesterday as I was approaching 2000 rpm on my way to 2100 to cycle the prop the tail started to lift even with full aft stick (stick and elevator have full travel and were at the stop). I throttled back, moved the stick, verified visually I was getting full aft stick / elevator and tried it again...same thing happened. I ended up cycling the prop at 1950rpm and as soon as the pitch changed, the tail jumped a little.

I was at 1/2 fuel, solo and I don’t weigh much but the CG was near the middle of the envelope. The air was near standard conditions so my no means exceptionally dense (I’ve flown this plane many times in much denser air without this happening). I normally top off the tanks after every flight so being at 1/2 fuel was something different and of course a new governor and overhauled prop / hub are different.

I have a call in to the prop shop but was curious if this is common with other RV4s given the geometry of the plane and the chosen power plant.

Thx!

plehrke · #2 10-16-2020, 06:51 AM

Did you have a particularly high head wind? It has been very windy here lately. Tail flies on airspeed. Additional 10 knots of wind would make it fly a lot earlier.

Kooshball · #3 10-16-2020, 07:03 AM

Quote:

Originally Posted by plehrke

Did you have a particularly high head wind? It has been very windy here lately. Tail flies on airspeed. Additional 10 knots of wind would make it fly a lot earlier.

There wasn’t really any wind yesterday during the event.

Skysailor · #4 10-16-2020, 07:22 AM

I use 1800 for run up without problems. 2100 RPM on the ground is a lot of power and dramatically increases the chances of damage to prop and/or tailfeathers due to the increased pebble and debris pick up. I set my "Canopy Open" warning at 1900 RPM as I should never see that RPM on the ground during taxi even considering hills. 1800 RPM seems to provide enough power to detect ignition issues during test and prop governing pressure to move the blades. Higher RPMs have not been needed to detect ignition issues and add a great deal of stress to the blades when cycling the propellor.

SmilingJack · #5 10-16-2020, 07:40 AM

What was your flaps setting?

In an 8 with no rear pax and flaps down the tail will lift.

Kooshball · #6 10-16-2020, 07:42 AM

Quote:

Originally Posted by SmilingJack

What was your flaps setting?

In an 8 with no rear pax and flaps down the tail will lift.

~20 deg in this case

Mel · #7 10-16-2020, 07:54 AM

Quote:

Originally Posted by Kooshball

~20 deg in this case

And, I believe, therein lies the problem. Flaps should be full up during run-up.

ANY flap deployment can cause the tail to lift.

And, 1700-1800 rpm should be sufficient for run-up.

Bob Martin · #8 10-16-2020, 07:57 AM

Quote:

Originally Posted by Skysailor

I use 1800 for run up without problems. 2100 RPM on the ground is a lot of power and dramatically increases the chances of damage to prop and/or tailfeathers due to the increased pebble and debris pick up. I set my "Canopy Open" warning at 1900 RPM as I should never see that RPM on the ground during taxi even considering hills. 1800 RPM seems to provide enough power to detect ignition issues during test and prop governing pressure to move the blades. Higher RPMs have not been needed to detect ignition issues and add a great deal of stress to the blades when cycling the propellor.

1700 for me! for all the above reasons.

Ironflight · #9 10-16-2020, 10:00 AM

Agree 100% that the flaps need to be up, stick back, and anything over about 1900 rpm is excessive for run-up on these plans. You will have to figure out for your airplane what the minimum RPM is for the prop to cycle - our four airplanes with C/S props all have different numbers between 1700 and 1900 rpm - and use that. No point in using more!

Paul

Kooshball · #10 10-16-2020, 11:52 AM

Thx for the replies folks! Interesting that with the old hub / gov we actually needed over 2000 rpm to get the prop to cycle...now it seems like it will cycle lower which I will implement immediately. Ignition check run up has always been at 1800...maybe I can get the prop to cycle there as well.

Thx