CSU on a Cirrus?

[Gugnunc]

Saw a Cirrus (SR20 I think) in the engineering hangar with its cowl off and wandered over to have a look. All very impressive but I couldn't see anything that looked like a CSU. Friendly engineer nearby also wandered over and he couldn't identify a CSU either and pronounced it a fixed pitch prop.

Now I'm sure all Cirrus' have this clever single lever AND a constant speed prop, so is it an electric unit or is it a hidden hydraulic unit, or is it really fixed pitch? I'm curious as I may need to do some training on our (single lever DA40 TDi) for someone who will upgrade to the Cirrus.

[moonym20]

Hi, the cirrus aircraft is indeed fitted with a conventional CSU using oil for the hydraulic movement on the prop.

to my knowledge the VP prop control is purely mechanical, there is a complex linkage attached to the throttle on the centre console which sets the pitch according to the throttle position, also at 1,700 RPM (if i remember) there is another mechanism to exercise the CSU which happens automatically once the throttle is held at 1,700 RPM

I think the wobbly prop on the DA 40 (Tdi models) is electronic and you push a button which exercises the CSU. (hope im right there...)

[IO540]

The Cirrus has a CSU, with the RPM control linked to the throttle such that above a certain throttle position the engine runs at max rated revs all the time.

It sacrifices a bit of fuel on the alter of marketing "simplicity" but hey as they as in the USA (or used to, until recently) fuel is the cheapest thing you can put in your engine.

[Gugnunc]

So the CSU is very compact and virtually hidden then?


From a training point of view, if the Cirrus is always running at Max rpm the throttle response must be pretty instant - is there much yaw on the go around ? (DA40 TDI has a bit of lag and the yaw is easy to counterract)

How do you cycle the prop during the engine tests? (DA40 is a doddle, I love watching the faces of the people I do the diesel differences training for when they press the test button for the first time!)

[BackPacker]

I think the wobbly prop on the DA 40 (Tdi models) is electronic and you push a button which exercises the CSU. (hope im right there...)

Not quite correct, but close. The prop is hydraulically controlled variable pitch, but the hydraulics are controlled by the ECU (also known as FADEC) and the DA-40 TDI has two of those.

It is not a constant speed in the traditional sense where the RPMs are kept constant regardless of the throttle position. Instead, it has a load lever (0-100%) which controls both the throttle (actually manifold pressure and injected fuel) and the RPMs. It works generally fine and hassle-free, but at 50% load (typical downwind setting) the prop "hunts" a bit.

[Gugnunc]

"It works generally fine and hassle-free, but at 50% load (typical downwind setting) the prop "hunts" a bit."

Not experienced that in ours - I've been general handling and doing student nav at 50% (105kts) and it is lovely and smooth (and just under 4 USgph). The only time I've really noticed the prop adjusting that wasn't smooth was when a student did a 360 deg steep turn and we went through our own wake - there was a bit of a thump from the prop then.

I didn't start the topic to talk about the DA40 - but as it is still all shiny and new I can resist saying that it is gorgeous and I love every single second in it (and so do the students!).

Would still appreciate feedback on the yaw characteristics on go around of the Cirrus though.

Thanks

Gugnunc

[BackPacker]

Gugnunc, does yours have the newer 2.0 engine by any chance? Ours is the 1.7, where 50% is a mere 90 knots at 3.3 USG/hr. We need 60% (4.0 USG/h) for 106 knots but generally cruise at 70% (115 kts, 4.8 USG/hr).

Thread creep, I know, sorry. But the DA-40 is indeed gorgeous. Can't stop talking about it.

[Gugnunc]

Our is a 1.7 but it has only limited VFR avionics and standard tanks, 789 kg BEM. I polished it when I got it, and we get:

50% 105 kts 3.8/3.9 usg ph

70% 120 kts 6 usg p h

100% 135 kts 8 usg

All with pilot & student. 3 people and MTOW seems to take 5 kts off even though the cofg is in a more efficient position. Looking at the fuel consumption figures I suspect our % load measurments are not the same - ie my 50% is your 60%.

I'm at Perth, Scotland. We have Thielert and Diamond factory certified engineers on site so come over if you want a trip or need some maintnenance done! pm me for details

[BackPacker]

Okay, I see. The figures I have here are the POH figures at MTOW. But I'm still a bit puzzled. As far as I know, the load figures vs. fuel consumption should always be equal, since load% is defined as a fuel consumption issue, or not? Here's what it should be, according to the POH:

100% - 7.4 USG
90% - 6.6
80% - 5.7
70% - 4.8 (but at 70% ours indicates 5.2 USG/hr...???)
60% - 4.0
50% - 3.3

Ours is airways equipped, 815.3 kg empty. And I don't think it's been polished recently - or ever...

Our club is large (700+ members), owns 20 planes or so and owns its own maintenance outfit which is licensed for Diamond/Thielert. I don't think they'll appreciate me flying to Scotland for a bit of tweaking... :=

[moonym20]

Not quite correct, but close. The prop is hydraulically controlled variable pitch, but the hydraulics are controlled by the ECU (also known as FADEC) and the DA-40 TDI has two of those.

Sorry, i meant to say with the DA40Tdi the govenor was electronically controlled but the output was the standard hydraulics.

with the Cirrus there is a 'gate' on the throttle, when you are retarding the throttle and you hit the gate the CSU places the prop towards the fully fine pitch, a change noticible when your making a deadside decent.

Same thing again when you open the throttle once the throttle passes the gate the CSU alters the pitch of the prop slightly.

I also think it runs 50-100 rpm below the red line, you can also lean the engine off (much more helpful with EMAX) however if you get it wrong it fouls the plugs, most people i know of just leave the mixture fully rich (unless EMAX assisted) as the fuel pump is altitude compensating anyway. I think it was around 11/12GPH @ 25/25 3,000FT