Teaching correct use of a VP prop installation.

Multi rating books, eg Robson, have good background on VP mechanics and propeller aerodynamics.

For most aircraft, older Beechcraft aside, the principle of moving forward controls from right to left when increasing power, and bringing power back from left to right, applies – this brings in mixture into the power management motor memory as well.

I am somewhat sceptical of extensive cycling of the prop on run up – when cold two or three cycles of up to 300 rpm suffice, when warm a single cycle to ensure no surging – excessive cycling on the ground is not kind to either prop or crankshaft.

Reading up on good engine management and performance planning is also useful in more complex aircraft, as the cost of overhaul is much higher than your simple fixed pitch four bangers.

See “Those Marvellous Props” by John Deakin. Helped me a lot.

Also, don’t miss out on Barry Schiff’s demonstration of an engine out glide, where the prop in full coarse pitch yielded the best glide distance (no wait, the best glide distance was actually with the prop stopped…)

A couple of points:

Stretching the glide by pulling the prop lever back to fully coarse works well on some installations, but not all. On my Piper Dakota, it does not make any difference. It depends on the minimum governable RPM, which should be low for the trick to work. On the C177RG’s I have flown, it has a significant effect.

If the tachometer needle wavers during cruise, it could be a problem with either the governor or the tachometer. This can be checked by moving the prop lever fully forward. During cruise, this will cause the RPM to increase to max governable RPM, often 2700. Then reduce power until the RPM falls somewhat. Now the propellor is at the low pitch stop, acting as a low-speed, fixed-pitch, climb propeller. It the RPM needle still wavers, the problem is with the tachometer. If not, it is the governor.

But, these are anomalies. When introducing new pilots to the concept, I find it is important to guide them where to look when adjusting power. If not helped, most new pilots will look at the RPM when reducing power (throttle) during climb or after leveling off. If reducing power shortly after takeoff, this will often result in the pilot reducing power so much that the airplane will stop climbing and threatens to stall, because the RPM will not drop immediately. This should be trained until the pilot automatically looks at the manifold pressure gauge whenever the throttle is moved, and at the tachometer only when the prop lever is moved.

This is one more reason not to fiddle with the engine shortly after take-off. But even for experienced pilots, I am no firm believer in the 25/2500-religion. Just after take-off, power equals safety, and less fiddling equals safety. I leave everything forward until at a safe altude; if I reduce anything at low altitude, it is the prop RPM, and only for noise. All conventional non-turbo engines are happy with full throttle at 24-2500 RPM. Just look at their fixed-propeller counterparts – they do it every time.

Many pilots will ask how to choose an rpm/MP combination for cruise. POH tables usually suggest several combinations for a given % power. When so asked, I do not introduce the concept of “oversquare” (when the inches of MP exceeds hundreds of RPM). With a little luck, the pilot will live happily without ever hearing about it, but will just select a setting based on economy, noise and vibration.

Another note when introducing: Low-time pilots or pilots flying unfamiliar aircraft tend to take longer to note increasing or decreasing airspeed with a constant-speed propeller, as there is no RPM change to give it away. So more attitude awareness is needed, also because of the higher airspeeds and more slippery airframes ususally associated with these airplane types.

My Piper Dakota has no engine analyzer or fuel flow meter, and the old single-cyl EGT is not reliable. I lean by watching the RPM needle and move the mixture lever back in tiny jerks. Once I am past best-power mixture, the small, but instant power reduction caused by leaning will make the RPM drop a little bit, just for a fraction of a second; then the governor has adjusted the propeller and brought the RPM back up. Now I have established a reference for my leaning and can work from there (or leave it there).

Plenty of sources will explain the principles of the hydraulic constant-speed propellers, but I think pilots new to this should be assured that these propellers are reliable, robust and generally take care of themselves. Follow the check list, and very few things should be critical from a pilot’s point of view. Operating the prop control should not take priority very often.

Stretching the glide by pulling the prop lever back to fully coarse works well

Stretching the glide is a road to disaster and should not be encouraged in any shape or form!

Whilst gliding at a constant speed you can experiment with the rpm to see if it has any effect on ROD, in some cases its not detectable in others it is.

Perhaps it is my bad English. It is not my native language!

Tumbleweed, is it the phrase “stretching the glide” that is inseparable from the concept of trying to glide further by pitching up, bleeding off speed to below that giving best glide performance? Which is bad because then you tend to stall and die, if you do it at low altitude?

I believed the phrase could simply mean that you increase glide performance, enabling a longer glide distance. Which is what could save the day in an engine out emergency.

Huv, that was and is completely understandable and I think your write-up hits a lot of interesting points.

I have a fairly unique situation in having an electric CS prop that can be set into manual mode and adjusted to max coarse to increase glide performance dramatically. It sinks like a rock with the prop in CS mode. The only problem is that pitch actuation moves 1/6 as fast as a hydraulic prop so you will be in deep trouble if you don’t allow enough time to re-pitch for go around when practicing. Not really relevant to the original posters question but another nuance. Another is that by switching to manual mode you can lean by RPM and then turn it back to CS mode when leaned.

Try to impart a solid understanding of why you are doing certain things. Not just “this is what you do now”. For example explain why you want a finer pitch for takeoff and explain why you are reducing the rpm in the cruise. Also make sure there’s an understanding of what is happening and the effects of it. For example in what is talked about above, don’t teach “low rpm gives better glide” but make sure the reason is there.

It may just be my learning style but I am a believer in knowing why things are done makes it easier to do them.

It may just be my learning style…

That’s indeed the best learning and teaching style. Explain why things need to be done. I always tell newbies on their first encounter with VP props that it acts like the gearshift on a car (we still have more manual gearshift cars here than automatic, so the analogy works for most ). When going slowly or uphill or braking with the engine, you use a small gear (= fine pitch), for speed or coasting downwards, you use the highest possible gear that will not stall your engine. Do you shift gears under load? Certainly not. So when changing the prop setting, reduce throttle first. So simple.

And regarding the number of prop cycles required during run-up: Most VP props are operated hydraulically with the normal engine oil acting as hydraulic fluid. The piston inside the prop mechanism has quite a large displacement, because it needs to generate large forces from a small(ish) pressure, at least compared to other hydraulic systems. In order for this mechanism to work smoothly it needs to be completely filled with warm, fluid oil. When an aeroplane was parked outside on a cold day, this oil inside the propeller hub will have very high viscosity, even if your engine itself is staring to warm up. (In Goose Bay, I once saw oil crystals on the dipstick after overnight parking at -45 degrees C – oil crystals won’t move the propeller blades for sure.) So unless you cycle your prop a sufficient number of times to replace all of that cold oil with warm oil, it can not operate smoothly. When the engine is still warm from the previous flight, one time is usually enough. On a really cold engine, you might have to do it a dozen times. Always slowly and carefully with a maximum drop of 200-300 RPM until the RPM needle follows the motion of your lever as if connected directly. Working like this, I have never had the least problem with any hydraulic VP prop so far. Only an electrical one once “ran away” from me, which was not funny, because in order to keep the RPM within limits, we had to throttle back so much that we couldn’t maintain altitude. Luckiliy we were not far away from our departure airfield.

Only an electrical one once “ran away” from me, which was not funny, because in order to keep the RPM within limits, we had to throttle back so much that we couldn’t maintain altitude. Luckiliy we were not far away from our departure airfield.

Been there done that, and its a dubious distinction to say I’ve done it several times. My approach (somewhat on topic!) was to pitch up and slow down to load the prop, then throttle to 2800 rpm, 100 rpm over redline. That gave me 200-300 fpm for climb out. I’m glad it wasn’t an underpowered plane.

I have the prop working reliably now but the ‘top name brand’ certified electric constant speed prop has taken more sorting out than the rest of plane put together. The manufacturer provided good support during the 2-3 years it took to fix, I’ll give them that, and intermittent prop problems were part of the reason I got a deal on the plane. So it was a risk = reward situation and I’m OK with the result given that I didn’t kill myself

I surely agree with the why teaching style and the manual gearbox analogy: don’t lug the engine, downshift for more power instead…