Flying inverted..
 

I have a really (stupid perhaps?) basic question regarding aero-engines and flying inverted.

I keep seeing ads and references to 'inverted systems' on aeroplanes for aerobatics, and was wondering what is involved with this. I have also heard of engines being mounted 'inverted'. So what is it all about? Can any aerobatic aeroplane be flown upside-down or do you need +ve G manouvers thoughout (ref..something I keep hearing about chipmunks?), and does it depend upon the engine itself and its installation, or the systems attached to it?

Broad question I know, and Im sure the first response will bring a multitude of further questions.... but its something I dont fully understand and would like to get to grips with.

Regards, SD..

Lycoming article mainly covering the oil system changes. It mentions the requirement for fuel injection, but not where you get the fuel from. One way of solving this is to have another tank below the (upright) engine which fills up when you are upright. When you fly inverted, this tank is now above the engine, and drains by gravity feed. As these are usually quite small you are limited in the amount of time you can fly inverted, but you generally don't stay inverted long enough for it to run out.

There are two things here - inverted systems, and engines mounted inverted.

To take the latter first, an example is the Gipsy Major fitted to the Chipmunk and Tiger Moth. It's an in-line 4, and if it was mounted 'right way up' with the crankshaft output to the prop on the correct thrust line, the cylinders would stick up in front of the cockpit and limit the pilot's view ahead. Some early Moths and other aeroplanes are like this.

To obviate this, the engine is fitted with the cylinders hanging down, beneath the crankshaft. This allows the thrust line to be up high, without the cylinders getting in the way of the view forward. This is an 'inverted engine' installation (the carburettor is, of course, mounted right way up). Apart from being aware of the possibility of oil draining into the cyclinders while the engine is off, is of no real concern to the pilot.

Inverted systems, however, refer to the ability of an engine to continue to develop power regardless of whether the aeroplane it is fitted to is right way up or upside down. To acheive this it will need either fuel injection or a special carberettor (normal carbs - like that fitted to the Chipmink - will cease to function when inverted). It will also need some means of ensuring that the oil system continues to operate, so that the engine will not be damaged through oil starvation while dveloping power when upside down.

SSD

SSD,
Interesting this, and it could explain something for me. Recently, whilst making a fairly lengthy series of very steeply banked turns over a point on the ground, the engine in my 172 "coughed" a couple of times (swiftly followed by my bottom, but that's another story ;) ). Wings level, full power climb, away we went no problem.

I know this wasn't carb-ice related, I'd been taking care of that carefully. So could this have been carb-related ? What angle would you have to bank to to cause problems with a standard carb ?

FF :ok:

C172: If you weren't careful in keeping the ball in the middle and there was a certain amount of fuel in the tanks (I've no idea how much), the prolonged sideslip might have resulted in the fuel outlet from the tanks being uncovered, interrupting the fuel flow to the carburettor/fuel injection. Returning to wings level or balanced flight would solve it.

That's why you are only supposed to use the L and R selections on the C172 tanks in balanced (level) flight. BOTH for everything else.

FF

Any angle of bank where the 'g' is still positive should not affect the carb. Only negative 'g' will cause it to malfunction. Since steep turns simply increase the 'g', they should not affect carb operation.

SSD

Skydriller

By 'normal carbs' people are refering to a standard float chamber set up where fuel is pumped into the chamber at the top via a metering valve that is closed by the float rising when the camber is full, as fuel is used by being drawn into the venturi for mixing the float lowers, opening the valve and allowing more fuel in etc etc. As the fuel is drawn from a drain in the bottom of the chamber any inverted flight or sustained negative G would mean fuel starvation.

Most aerobats will use drirect fuel injection (no carb.) With this system fuel/air mixture is fed under pressue directly to an injector rail mounted just above the cylinder head where it's fed to the inlet valves and so making most of the previous problems obsolete. Of course, like everything, there are drawbacks to fuel injection, the main ones being difficulty in starting hot engines (very difficult for some) and fuel icing problems can also be a factor.

An aircraft designed for negative/inverted flight will have a dry sump oil system. In essence dry sump is similar to fuel injection where oil is stored in a seperate tank, scavenged by a pump when used and then fed at pressure to the vital areas in need of lubrication/cooling then returned to the storage tank.

A standard wet sump engine as found in your 152, chippy and tiger moth will work just as well for inverted/negative flight no problem, you may see a greater loss rate however. Wet sumps just has its oil load slushing around at the bottom of the crankcase (sump) or, if the engine is mounted 'upside down' such as the tiger moth and chipmunk then the oil simply sits around the pistons and exposed cylinder. This is the reason you should pull the prop through before start to clear any oil via the valves that has slipped past the oil and compression rings on the piston. Failure to do this can bend cranks as liquid is incompressible!

The reason for a moths engine being 'upside down' is not to provide the pilot forward vision (sorry, shaggy sheep driver) that just happens to be a nice side effect. It is because the aircraft is a taildragger and mounting the engine in the conventional sense would mean a smaller prop would be required to prevent contact with the ground upon raising the tail on take off. This larger prop due to better clearence also means a bit more thrust and christ knows those little piston engines need it if memory serves me correctly!

Anyway happy flying chaps, it could be worse you could be stuck with turboprops on nightfreight but that's another story :ok:

"A standard wet sump engine as found in your 152, chippy and tiger moth will work just as well for inverted/negative flight"

Really?

Sounds to me like a very good way to destroy an engine.

Usually with a wet sump I thought when inverted the oil pressure will gradually fall away over about 10 seconds as the sump has nothing left to pump but air.

The ideal features for inverted flight for any length of time are therefore fuel injection and an inverted oil and fuel tank.

Stellair, sorry old chap but that stuff reagarding the Gipsy is bollox. A Chippy or Tiger's Gipsy major is dry sump with a seperate oil tank. A moments thought will tell you that an inverted engine cannot possibly be wet sump. Unless someone's invented anti-gravoty oil (or are you suggesting the entire engine's oil capacity sits in the rocker covers??). ;)

It's not an inverted system, however, since the oil is drawn from the bottom of the tank by the oil pump. So when the tank is inverted, the pump draws air, not oil. That's why it's important with a Gipsy to close the throttle if the engine dies under negative 'g', otherwise on restoration of positive 'g' it will burst into high-power life with not much oil in the bearings.


Wrong again Stellair. As I stated, the reason is to put the propellor thrust line in the right place. It's the extra ground clearance for the prop that's the nice side effect, though neither the Chippy or Tiger's props come anywhere near close to the ground with the tail up, so that isn't an issue.

Having put the thrust line where it needs to be, the cylinders would indeed block the pilot's view (look at some early Moths), so the designer made use of the space available below the thrust line and inverted the engine.

SSD

There are no stupid questions. Only those to which you do not know the answer, so well done for asking.

SSD and stellair answer things quite succinctly.

This bit seems to have been missed by most, though some of the answers have touched on it, yes you can fly any aerobatic aircraft inverted and indeed do negative g manouvers to a limited extent, though you do need to take into account that the engine will stop producing power and will have no lubrication unless it has inverted fuel and oil systems - no inverted oil will be the main consideration here and how long you can stay upside down may well be laid down in the engine limitations - with no inverted fuel the time will be mainly limited by how much height you have spare!:ooh:

Not only that, but even an aircraft with full inverted oil/fuel systems can't sustain zero 'g' for any length of time (I believe) -- after all, if there's no applied 'g' there's no way of recovering the oil...

For this reason the aircraft I fly has a limit of 3 minutes for sustained -ve 'g', but only 15 seconds (IIRC, don't have the books to hand) for knife-edge or zero 'g' flight.

There are two key pieces of equipment that have escaped the conversation so far which allow for unlimited inverted flight, even with a wet sump engine.

The first is a flop tube. This is just a piece of plastic tubing with a sieve on the end which is mounted inside the tank. It flops around inside according to the g-loading on the aircraft, thereby always staying inside the fuel, wherever it is in the tank.

The second is the Christen inverted oil system. It's mounted to wet sump engines and consists of tubes that pick up oil from both the sump at the bottom of the engine and the breather at the top. When the engine is upright, the oil is drawn from the sump. When the engine is inverted, the oil collects in the upper part of the engine and is therefore drawn from the breather. The inverted system is a seperately mounted cannister which has some tube and ball bearing contraption inside which I've never fully understood but keeps the oil coming and going properly and keeps the air coming and going properly (from the breather upright, sump inverted). Don't ask me exactly how it does this but it works.

My Pitts has both (as do most but not all Pitts, as far as I know) and can fly inverted all day long (or until said flop tube can no longer connect with any fuel because there isn't any.)

Pitts2112

Dimensional,

The aircraft that I fly can sustain inverted flight for an unlimited length of time due to having the oil system that Pitts2112 describes above. When it is inverted with constant -g applied the oil system functions quite happily, although sometimes with slightly reduced pressure.

The aircraft is, however, limited to 10 secs of zero g flight as you mention due to the oil and ball-bearing valve being neither in the 'top or bottom' of the engine.

ST

Sorry, that was my idiotry. 'Negative 'should've read 'zero' -- most engines with full inverted systems are quite happy upside down for extended periods of time -- although mine does still have the three minutes limitation.

--D :O

Managed to get the flying clubs Grob 115D up for a blast last weekend for some aero’s, all part of the class rating of course…!

Had a bash at sustained inverted flight for the first time and was generally surprise by the amount of forward elevator input required to maintain altitude. I know you had mentioned this Speed Twelve when we last spoke, but it still surprised me.

Back to the point of the thread, the 115D uses a Lycoming 320AEIO which is fuel injected and has a wet sump with pick-up lines at the top of the engine and within the sump, which do exactly as Pitts2112 explains.

Just out of curiosity and not that I would really want to try it, can the C152 aerobat achieve sustained inverted flight?

Cheers!

At risk of being a pedant, 2112's description is almost spot on - however, he has failed to mention a couple of other vital components in the Christen inverted system -

the air/oil seperator is what I think he is referring to when he saysthe oil valve is the clever bit of kit generally mounted on the firewall that works out correct direction of flow and egress for the breather

The other nifty devices are the T piece out of the top breather and the oil pick up strainers in the sump - one is very tall to help with knife edge oil pickup!

Stik

Funkie, C152 inverted - only gently downwards and quietly :p . The amount of forward stick needed in the Grob/Firefly etc. is because the aerofoil is very inefficient when inverted (much higher AoA needed to generate same lift at a given speed). Stall speed is much higher too. All much easier in a Pitts or similar with symmetrical aerofoil.
Next time you're up, try working out which way to bank in order to turn in a particular direction. Great fun :ok: .

DB6,

Getting a bit off-topic, but I suppose it demonstrates how efficient a cambered airfoil is when used correctly and with +g in effect.

Banking while inverted - now at a guess it would be greater forward authority and balanced turn to the left/right. Obviously reducing forward pressure would result in a height loss. I’ll give it a try?

Cheers.