Gauge Cockpit

Hating The Fixed-Pitch Prop……Especially It Its Attached To The Front Of A Pitts Special From The Cockpit By Brian Lecomber

The time is some 35 years ago.I am in the spring of my aerobatic life. By dint of saving and scrimping I have actually scared up the funds to buy my dream aeroplane, a pitts S1-S Special. I’ve worked hard for it…………

Now all I have to do is learn to fly it. This is a process fraught with both ecstasy and fright. Ecstasy because the S1S will do any aerobatic man oeuvre I’ve ever thought of with what seemed to me back then, as lightning rapidity.

Fright because it has already proved that it will also do things I haven’t thought of with equal alacrity….

Well, well this is only my third hour in it.Iam just beginning to lo lie with my love.And Ihave just managed a double up-vertical roll, something I could never have considered in my wildest dreams before the Dawn of the Pitts.

And the stall-turn at the top went well-or well-ish, anyway- and so I threw in a down-vertical roll and now I’m pulling out. I glance at the engine gauges…..

Hell! The rev-counter is passing 3400 rpm! How can that be?

I slam the throttle shut and pull hard, turning for home. I’ve wrecked the engine –my engine .But this is woolly thinking because I must not now be considering my almost-extinct bank account but far more profitably the fields passing below, into any one of which I may abruptly have to place this missile of an aero plane if the engine suddenly decides that so much abuse is an abuse too far…..

The engine runs serenely. I land my little loved one and taxi to the maintenance base. Twisting my flying helmet to the Chief Engineer.”I’ve over-revved it. Redlines 2700, and I took it to 3400″.

“How much? Ye Gods! How did you manage that?”

“I decline to discuss the matter. Just call it stupidity”.

I wander home, facing the second-worst life-threatening prospect which confronts any aviator foolish enough to aspire to become a professional aerobatic pilot. Not impact-which must always, of course, be the first factor- but starvation. I could no more afford a major re-build of a six-litre Lycoming 360 aero engine than I could fly to the moon……

In fact, the engine survived. Oil filters check, oil analysis, and compression check-all hunky –dory. It took two days to get all the results, and in those two days I jumped like a cat every time the phone rang.

Some-well, most-of the ashes in my mouth were self-mortification. I should never have been caught out like that. I knew perfectly well the pitfalls of aerobatics with a fixed-pitch propeller…

I am often asked what were the biggest innovations in aerobatics in my lifetime.Carbon-fibre wings-yes,certainly.The Christen inverted oil system-well, yes again-farmyard technology ,but it rendered the equally farmyard  Lycoming engine capable of inverted flight. Reflex ailerons-once more yes…..

But the most important of all? The almost universal adoption of the constant-speed propeller in aerobatic machinery. No question.

Let us consider the basics.

A fixed –pitch prop has one advantage and one advantage only-it is cheaper than a constant-speed prop. That’s it. There is no other.

Take a typical aero-engine with a fixed-pitch prop and an engine red-line 2700 rpm. Tie the aero plane down to something immovable such as the UK national debt, and run it up to full power. Depending on the prop-pitch you will get a full-blast rpm of somewhere between 2200 and 2400 rpm.

Now increase the airspeed top, say, 70 knots-and lo! You’re seeing maybe 2500 rpm, because now the blade-drag has reduced with forward speed. And so it goes on. The higher the airspeed the less the prop-blade angle of attack, and therefore the less the induced drag and the higher the rpm.Not exactly rocket science.

Except, of course, there comes a moment when the airspeed has increased enough that the engine over-revs. Normally if you climb to height, throttle back to cruise rpm, then sit there fat dumb and happy at a cruise speed of maybe 100/140 knots, this is not an issue. But start doing aerobatics, where the speed is constantly watching the rev-counter like a hawk…..

Well, , in fact –no, you don’t .Any you shouldn’t be.

The first thing to do is find the aero planes full-throttle speed, which is also not rocket science .Simply open up to full throttle at a fairly low airspeed and then ease into a gentle dive keeping an interested eye on both rpm and airspeed. When the rpm reaches red-line (2700, or whatever) ,note the airspeed. And that’s it. That’s full-throttle speed. Any time you’re below that airspeed you can use full throttle without thinking about it.Simple, yes?

Well, yes-so far. The other side of the coin, of course, is that any time you’re above full-throttle speed you need to be throttling back.

The question is-by how much?

This is not easy. Not so easy at all.

On the face of it you just glance at the rev-counter-but how good an ideal is that? Mostly when the airspeed is rising rapidly in aerobatics you’re either pointing down at the deck or leveling out from so doing. So is the rev-counter really the thing you should have your eyes on at this time….?

Ideally –no, of course .Your eyes should be on the greenery in front, the horizon, you’re positioning….

So how do you solve that?

Well, the first thing is to find out the extent of the problem-which means discovering the median aerobatic speed-range of the aeroplane.Not medium-median. Let me try to explain that. I’ll use airspeeds related to a Stampe SV4B simply because its median range is tattooed on the inside of my skull just where my tiny brain could see it easily. But the principle applies to any and all aerobatic aero planes.

When I first dived in to start an aerobatic sequence in the Stampe I would aim for Velocity Never Exceed-140 knots. This was just common sense-start as fast as you can for the maximum kinetic energy-but it was also what I named VNSA. Velocity Never Seen Again. Because I would exit that first man oeuvre doing no more than 120m knots or a tad over-the drag would see to that.

As the sequence went on it comprised the usual triplets of man oeuvres –energy-burner, energy-gainer, and energy-neutral. I needed 100 knots for a loop starting and finishing on the deck, and I could gain 10 knots in the process by v-e-r-y gently unloading around the top. I needed 110 knots for a neat half-Cuban, 120 for a reverse half-Cuban.

So, in the dearly-beloved Stampe, the median aerobatic speed-range was 100/120 knots.

So how did the Stampes full-throttle airspeed relate to the median aerobatic speed-range?

In this case, pretty well. Full-throttle speed was in fact 109 knots, so I only had to throttle back eight times in a 20-manouevre sequence when the speed exceeded 109 knots.

Which, of course, still left the question of how much to throttle back…?

The initial dive-in to V ne was no big issue –you could glance in at the rev-counter for that, ‘cos you never going to see 140 knots again anyway. It was the 109 to 125 knot speed-range that was critical-‘cos that you were going to see. So just how much do you throttle back….?

I’ll tell you a trick I’ve used on several types. Dive gently to the top end of your median aerobatic speed range, easing the throttle to contain the rpm to red-line. Then pull out, leaving the power where it is, take out one of the indelible marker-pens, and quite literally mark the physical position of the throttle lever.

Then land, and sit there fidding with the throttle for along time. See if you can find a cue you can feel with your hand. The Stampe, for example ,had a little rail over the throttle quadrant and if I pressed my thumb over the back of the rail I could pull the throttle back with my  fingers  and when the lever touched my thumb it was about right. I didn’t have to look, didn’t have to think.

Pulling out of a fact moan oeuvre I’d ease the throttle back to my thumb and then as I started  pulling or pushing up into the next figure I’d open up to full power again. The throttle became a sort of two-position switch –full chat or backed off enough to cope.

Crude? Yes, of course it was crude. Did it work? Well, yeah, within the meaning of the act. Will it work for aero planes other than the Stampe?

Well… that depends on your cockpit layout. It will if you can find a good tactile cue. In one aircraft I tried putting a strip of Velcro behind the throttle lever so I would feel it with my left knuckle as I pulled the throttle back. That didn’t work because I never felt the Velcro in the heat of the moment.

And then, of course, a Pitts S1-S comes along, with a fine –pitch prop to give it a real hang-on capability at the top of verticals. But bringing with it a full-throttle speed of around 115 mph along with a Vne of 201 mph and a median aerobatic range of 130-160 mph.

Big difference. Big, BIG difference .Which is why fixed-pittses still shriek when their prop-tips move into the transonic airflow region as the engines over-speed.

I must have done that a thousand times. And because Lycoming engines, however crude, are particularly tough, and when un-loaded –which they are always going to be on the down-strokes of aerobatics, which is when the over-revs occur-no person, to the best of my knowledge, has yet been killed by an over-speeding Lycoming blowing up.

But still think the greatest advance is the constant-speed propeller.

About the Author

Anthony Juma is the Editor and Senior Aviation Director at Wings Over Africa Aviation. 
This is an Air Charter Company that specializes on Scenic Charter Flights| Aerial  Survey  Charter Flights| Filming  Charter  flights  Kenya,Uganda,Tanzania,Rwanda,Burundi ,Eastern  & Central Africa. The website has guided thousands of travelers to achieve their dream holiday. For more information and guidance, visit the site at http:// / www.wingsoverafrica-aviation.com/index.php/services/charter-flights/aerial-survey-a-photography.html

 

 

 

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