constant speed units

[luckykathryn]

i had a small problem the other weekend with an aircraft that i hired for the day.

the aircraft was a complex type and had a vp prop. prior to t/off we did all of the usual power / prop checks and everything was okay.

the take off roll was fine and we had max rpm. all of a sudden at abt 200ft we lost abt 300rpm for a split second and then the rpm returned to its original setting. it happen repeatedly again. we reduced manifold pressure and tried to control the prop surges by feathering the prop but the surges still continued albeit not as violently.

needless to say that we made a very hasty low level circuit, landed, and made our own way home!

the csu unit was later replaced and apparantly everything is now okay.

does someone with a knowledge of the csu explain to me what probably went wrong and why it only seemed to be an intermittent problem.

[182P]

sometimes you need to cycle the prop a few more times before departure to get the oil pressure up in the prop. Three cycles is normaly enough for first flight of the day, and one if the engines warm.

IMHO

[A and C]

If this was not an "aerobatic" prop this sounds like a lack of oil px from the CSU oil pump or a regulator spring failure.

[luckykathryn]

thanks 182 but it was not the first flight of the day. the outbound leg was totally uneventful and we were only on the ground for 45mins. even if the engine is still warm i always cycle the prop 3 times. it is one of those in built habbits.

[MLS-12D]

This article (http://www.avweb.com/news/columns/182082-1.html) might provide some assistance.

[luckykathryn]

thanks mls...a very interesting article.

[smarthawke]

Luckykathryn,

I presume by trying to 'feather' the prop during your quick circuit you mean coarsening the pitch (singles don't feather, thats the domain of twins and then only to prevent windmilling when the engine is shut down in flight).

Aerobatic props fail in coarse position if all pressure drops off, non-aero props go to fine. Feather is as coarse as it is possible to go!

What aircraft was it you were flying? If your prop was a 'fail fine' type its interesting that the rpm dropped!

The prop cycling pre take-off is not strictly to get the oil pressure to the prop but to check the function of the CSU and prop and get warm oil through the system and into the prop.

Glad you made it back to terra firma though!

[luckykathryn]

it was a piper saratoga.

it definitely increased the blood pressure and got the heart beating.

[smarthawke]

Seeing as Saratogas are not exactly known for their aero capability, it is interesting to note that a CSU can fail in such away that it causes a 'fail fine' prop to coarsen off uncommanded albeit intermittently.

Purged arteries for the crew then?!

[luckykathryn]

i'm going to find the engineer who replaced the csu and ask what actually failed and why the events unfolded as they did.

i'll keep you posted.

[GRP]

This all sounds like a CSU oil leak as happened to me a while back on an Arrow. The only reason I can think you would get an RPM reduction though is if the prop was generally at higher RPM than the lever position was commanding and every now and then the pressure was enough to cause the prop to move towards the commanded setting. I understand the 'unnerving' comment though. When it happened to me I was miles from anywhere somewhere over Brecon and limped back to Kemble, staying high and passing near Filton just in case something let go!

[ROB-x38]

A bit off topic but I think i've just learnt my one thing for the day - I wasn't aware any singles failed to coarse pitch. So now the questions: :D

Are all aerobatic aircraft fitted with props which fail to coarse pitch with a loss of oil pressure?

Does the prop actually feather?

Why are these aircraft fitted with 'failing to coarse' props as opposed to the traditional single engine setup?

What are some of the more common aerobatic aircraft which have this arrangment?

Cheers :ok:

[smarthawke]

ROB

I don't know if 'all' aerobatic aircraft are fitted with 'fail coarse' props, things like Bulldogs and Pitts' with Hartzell props certainly are. There are many 'composite' props out there and I presume these operate the same way.

The CSU works the opposite way round so you have to make sure you get the right one!

The easy way to tell is that at the blade root there are counterbalance weights which assist the blades to coarsen off if the oil pressure drops.

I have always presumed that this is to stop the prop from trying to unscrew itself from the front of the aircraft during manoeurvres that result in fluctuating oil supply.

The prop doesn't actually feather, it just goes very coarse (if that makes sense!).

Hope that helps!

[ROB-x38]

Thanks for the info smarthawke. Sounds logical :ok: