Tried the two-blade prop for the first time...

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lambourne
Airman Basic
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Joined: 07 Mar 2021, 15:41

Tried the two-blade prop for the first time...

Post by lambourne »

Evening all! Mine's a pint of mild and yes, thank you, I'll certainly have a packet of dry roasted with that...

In a sort of reverse type conversion earlier I went from the Mk.I with the three-bladed Rotol airscrew to the two-bladed prop. It was such an experience I simply had to tell somebody about it!

Duxford, K9795 outside the tower. Clamber in, strap in, shut the door, cockpit preparation checklist... last two items, radiator lever fully forward for open and flying controls have full, free and correct movement in all axes. Bit of a neckache trying to see the rudder but all was normal! Top tank wasn't quite full but we've still got 40 gallons and all I'm intending to do is some basic general handling before returning to land, 20 minutes tops.

Pre-start checklist, brakes set, pressure's a bit below 300 on the gauge but we've still got 80 on each wheel so I shan't worry about that. Mags and starter mag on, fuel cocks both on, throttle cracked to immediately in front of the gear horn button. Five strokes on the primer. Call "clear prop!", wait a second in case someone shouts something back, then press the button. Takes a little while and 3 or 4 primer strokes to encourage her but the Merlin coughs her way to life at last. Throttle set for a steady idle, oil pressure leaping around safely below 120psi. Starter mag off, thumbs up to the groundcrew who take away the GPU, then wave away the chocks and get her rolling with a gentle jab of power. Brakes and right foot, brakes and right foot, and we're trundling down the line to 06 grass.

Keeping a weather eye on the rad temp as we roll out and she's warming up a bit fast for my liking so I pull the throttle back as far as I dare while we approach the turnoff for 06. There's 4 or 5kts of wind almost straight down the runway so maybe it's that blowing hot air back into the rad... Drat, the runup in a Spit needs groundcrew, this isn't my C152! Left foot and brake, on the radio to the tower but I'm already being trailed by a Landrover of amused groundcrew wondering when I'd realise. Two of them drape themselves over the tail, brakes set, throttle up to the regulation 2,000rpm... bloody hell, this is vibrating a LOT and we're barely nudging 1,800rpm.

Pull the power back just in case there's a problem and the vibration eases. Maybe it was my imagination? I try it again, this time with a bit of spirit. Up to 2,000rpm and we're being rattled around like a stone caught in Lewis Hamilton's tyre. Flick both mags in turn, drop seen - never mind the amount, I can barely read this - and chop the power. Idle reached, groundcrew back in the Rover to the hangar... aaaaaand 110 C on the rad temp.

Decision? Easy. I'm practically on the threshold already, I know how the Mk.II cools down once airborne, let's get her up there before I have to shut down. Full right rudder trim, brakes off, bit of left foot and a blip of throttle and we're lined up. Brakes one last time, almost stopped, check the elevator trim's 1 division nose down and I open her up smoothly and without hesitation.

Everything vibrates like mad. RPM is well down on the Mk.II, she's barely making 2,200. Coarse pitch? I fumble for the familiar t-handle on the left, eyes darting between rev counter, airspeed and horizon. Nose is rising, I think that's 80mph on the clock, ease the stick back and she drunkenly lurches into the air, engine sounding decidedly quieter than normal. Even as I'm pulling the gear selector handle downwards a stupid sound, a silly whistling of a kettle on a stove, becomes apparent as a steady stream of white steam begins shooting out of the right-hand side of the cowling.

Oh bugger. Quick glance at the rad temp and she's hit 120 and then some. Is the gear up, maybe the right leg's still blocking the intake? Eyes dart across to the gear indicator on the panel but both lamps are out. A further glance left and there's the red tell-tale poking out of the wing. Recycle handle, up then down again. Eyes back to the rad via the horizon and the rpm (2,100ish and vibrating). 130. OK, something is very wrong here. Scrub the sortie, fly a shortened circuit and get her down again. Immediate priority is getting the gear up so the rad has airflow. Glance back at the gear indicator but both lights are still out. Gear failure? Hydraulic system lock?

No, of course! This is a Mk.I! Right hand snaps forward to the handle and I start pumping. Out of the periphery of both eyes I see the red tell-tale rods start to jerkily retract. Phew. OK, where's the temps at? Oil 75ish, rad 130. By now we're just turning downwind and I've been so distracted we're at about 1,200ft on the QFE. No wonder she's running hot. I level out immediately as the gear legs clunk home. The steam stops - hell, have I boiled her dry? If so we're in forced-landing territory. Glance at the rad temp, 120ish.

Mindset shift. The engine might die. If it dies we're at about 1,000ft which is enough for an abbreviated circuit and a fast wheels-up landing on the grass. The prop's a goner, that's £25,000 for the insurance company to haggle over. If the engine keeps giving power let's risk a wheels-down approach. Still fumbling for the T-handle on the left to get her out of coarse pitch but no time to look now, we're downwind. RPM is still 2,100... OK, if the engine's boiled dry and the prop's a goner anyway, let's get as much power as we can before it seizes. Balls to the wall. RPM barely moves. What? OK whatever, I can't diagnose that and I'm nearing capacity so let's ignore it.

Still got power for now. Roll left, glance at airspeed, oh bloody hell 110? Nose down quick before we lose any more, unlock the gear and pump the handle as we meander leftwards onto finals. Airspeed's rising, why did I panic and stuff the nose down back on base, we're really very close too. Sod it, snap the flaps down, nose drops, catch her with some stick, glance at the wing to check the flap tell-tale is sticking out, OK good. Runway sight picture's looking OK as the threshold passes under the wings... oh, ASI's reading below 70mph but we're still flying, adopt the landing attitude, engine still going? Thump and we're on terra firma somewhere after the numbers. Muscle memory kicks in, throttle chopped, stick fully back and brakes are slightly on to slow her down, mind catches up, throttle back up to running idle, we're level with the tower, glance at the rad and we've 100 C. When did that happen?

Everything feels a bit more normal again. Taxi back in to parking, everything seems normal, engine has crept up to 110 again. Really don't get this... but hey, I'm alive and the aircraft's undamaged!

Lessons:

1. The Mk.I with a two-bladed prop is a fixed pitch unit even though the pitch handle is still modelled in the cockpit!
2. Don't take off over 100C. Even if you were lucky before on a different mark of Spit. Especially with a different mark.
3. RTFM!

Questions:

Should the two-blade prop vibrate significantly more than the three-blade unit?
What's max RPM from the two-blade prop?
What's the aborted takeoff drill?

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Killratio
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Re: Tried the two-blade prop for the first time...

Post by Killratio »

Nice report! The Watts/Weybridge is a beast of a thing to handle. Everything must be done "just right" and in good time to keep her from boiling. I have to admit, though, that I get more satisfaction out of flying that airscrew than any of the others. Perhaps it is that the majority of my aerobatics time was in fixed pitch, or perhaps it is just that good temps with that prop show a really “refined” handling of the aircraft. I don’t know.

Your questions in order...

Vibration…..Remember that the airscrew was designed for a “best performance” at high cruise speed. So running at a slow speed and highish rpm, you are essentially “pulling out in 4th gear”. Vibration/bunny hop/shake rattle and roll.

Max RPM. The max is 3600 (momentary in dive) 3000 (short term/emergency)
2600 (flat out) . Now I am not at all sure that MY teeth are firmly enough attached these days to try 3600rpm!

Aborted takeoff. Same as any other tailwheel aircraft really:

If not past “point of no return” close throttle, brakes as hard as appropriate, as soon as you are below rotation speed, column hard aft. (In all probability, if you are above rotation speed, you're not stopping the aircraft unless on a VERY long runway).

If past point at which you can stop the aircraft, you ARE going flying. You now need to know WHY you are aborting..if due to “changing your mind” well, um, be ready to explain the crash OR go ahead and take off, do a circuit and get back on ground. If because of “failure” you need to prepare for an off runway landing within 30 degrees either side of runway heading. With the Spitfire I, it takes significant effort to retract the gear, so you won’t probably have that option (tighten your straps, shut the engine switches and fuel and use your choice of expletive or prayer) In any case I would tend to move the U/C selector valve to “Raise” so that there is a good chance the gear will at least collapse.
<Sent from my 1988 Sony Walkman with Dolby Noise Reduction and 24" earphone cord extension>


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