Why are F1 engine clearances so small?

[langwadt]

'Gyro' (hmm) backlash does occur .......
It is why Ken Wallis always uses a direct drive prop on the Wallis autogyro designs. (no problem with electric drive of course)
A high reving engine that needs a reduction gearbox to slow the prop always gives excess wear due to backlash vibration, it can be very bad and even lead to breakage.
Weslake designed a one off prototype engine for Ken that suffered from this problem as did the Norton Wankel prototype they gave him but to a lesser extent. The engine designed and built for Ken by Hewland was not as bad and had a damper fitted.
On light aircraft it is useful to use 'boxer' and long stroke engines to keep rpm to 4000 or lower with a direct drive.
It may be why there have been almost no real development in light aviation engines since the 1930s.
However, F1 engines dont usualy run at very low rpm.
It might also be a reason for 'blipping' the throttle at idle to keep the revs up and the gears driving.

I can see why it might be problem for an engine driving a propeller where the propeller might have bigger inertia
than the engine itself

backlash is often seen as the cause of the problem when it is just the symptom (removing backlash doesn't cure inadequate design)
eg propellor inertia depresses the natural frequencies of the whole system
major and ill-defined torque vibration (aerodynamic ripple and plane of rotation blade flexing) is always a design problem

BTW the 'real developments' in light aviation engines have been numerous and have all failed to be good enough
(Detroit V8s from Buick/Rover to bigblock, Renault, Porsche, Honda etc and diesels)
part of the long life (2800 hr TBO) of the 'old-fashioned' engines is due to their pushrod seperate-cylinder air-cooled design
(only one ever cylinder fails a compression check and is easily sorted in situ and without dismantling the whole engine)

reduction gearing is both pointless and worse than useless below 300 hp
(variable propellor pitch though is usual above 120 hp, and is very compact, very light and very reliable)

real diesels are always found in big ships and have never had reduction gears (that's a big selling point)
they are the most efficient piston engines (cruise BTE 50-55%)

don't many aircraft and F1 engines have resonance rpm (to be avoided) below their normal running rpm ?
eg Peugeot F1 engines had ring flutter resonance around 11000 rpm and blew up after a few seconds at that (ask Mr Brundle !)
isn't that type of thing part of the throttle blipping culture ?

I have seen several big ships with reduction, not all of the big diesel/HFO are (that) slow speed

apart from the complexity wouldn't modern tdi etc. be a good fit for aircrafts? power at lower rpms and they could probably be made to run on jet-a

Blipping makes sense, in neutral all the pressures and forces are "wrong" and there is very little load to dissipate any resonances

[Tommy Cookers]

I must answer one point though.
Motorcycle engines with gearboxes look ideal at first glance for light aviation use and they have a place in certain applications.
However one problem is nearly always overlooked.
The thrust on the airscrew drive shaft.
This can easily damage a light and fairly weak gear casing and usualy needs major expensive modification to become reliable.
Thrust bearings and new housings have to be made.
On a pusher design the thrust is reversed of course.
It is far easier and more reliable to choose a low reving engine to match a direct drive airscrew.
The extra thrust bearing can easily be fitted on the engine block.

yes, this is right, I was wrong, and now see why

[Tommy Cookers]

@langwadt
real diesels are what is often called slow speed diesels, the things that Dr Diesel invented

[Tommy Cookers]

The main problem (from experience with at least twenty different engines) is the lack of decently designed airscrews for light aircraft.
Wooden airscrews are still the best and that needs a skilled craftsman.
Ken Wallis still carves his own and builds his own steel/wood composite rotor blades.
They both have an unlimited fatigue life and far superior performance to other types.
Metal blades dont work nearly as well.

I know little about airplanes, just idly curious to know why this is so?

last I heard carbon fibre etc advanced composites were the big thing for small/amateur light aircraft
because they can easily have 3 blades and are designed to flex so as to have variable pitch without moving or wearing parts
(a really good use for the advanced tailored structural properties of such materials)

wood is nature's composite, it has naturally oriented structural properties (ie grain) good for (2 blade) props
but the raw material availability is a problem, it needs a lot of work, poor durability (damp etc), and can't be thin for high speed
it's valuably lighter than metal and was used a lot in WW2 (mechanically improved/densified wood in UK)

all props get stone chip impact damage (even accident damage), this affect life more than other fatigue

[langwadt]

@langwadt
real diesels are what is often called slow speed diesels, the things that Dr Diesel invented

but I'd still call a big 2-stroke HFO burning engine with manholes in the crankcase "real" even if it runs at a few hundred rpm and has a reduction

[autogyro]

many/most motorcycles have helical gear primary transmission (incl shaft drive machines that have a 'transfer gear' drive)
the thrust capacity is already there (bidirectional) and the thrust will be the same whether driving a motorcycle or aeroplane
the primary shaft is made handle some high internal reaction moments, the prop equivalent should be ok
you did look at geared primary drive (motorcycles), not chain ?

I am not talking about the side thrust produced by the geartrain design Tommy.
I am refering to the thrust on the airscrew hub created by the aerodynamic lift from the airscrew.
This thrust is larger than the aerodynamic drag of the complete aircraft and in some designs can exceed aircraft weight.
There is no side thrust like this in a motorcycle or car powertrain.

[autogyro]

autogyro wrote:
The main problem (from experience with at least twenty different engines) is the lack of decently designed airscrews for light aircraft.
Wooden airscrews are still the best and that needs a skilled craftsman.
Ken Wallis still carves his own and builds his own steel/wood composite rotor blades.
They both have an unlimited fatigue life and far superior performance to other types.
Metal blades dont work nearly as well.

I know little about airplanes, just idly curious to know why this is so?

It is way of thread so I will try to be brief.
Actualy I am not sure why the wooden props and steel/wood laminar rotor blades on the W116 are so superior to others.
Wooden two bladed props certainly absorb vibration better than alloy which are too rigid.
The two and three bladed wooden props used give more thrust than any other types tested up to 90hp.
When 120hp RR engines (given to W by RR) are used in the W117 (a stretched airframe) a four bladed alloy prop is used.
There is also a cross form supercharged air colled two stroke Italian engine that uses the four bladed alloy prop in an enclosed cockpit speed and height record aircraft the W122.
No four bladed wooden props were made to compare these with.
The standard W116 uses a wooden two blader on 60hp. (for almost VTOL flight, beat that).
At this scale of airscrew and rotor, there is a huge potential for future development.
I am looking forwards to the first all electric, practical rotor wing aircraft when a budget becomes available.
However, the lessons learn by Wallis over 50 plus years would be ignored at a high cost.

The laminar wood/rolled steel rotor blades give nearly twice the climb rate of any other available material rotor blades.
The design is also exactly correct for the cetrifugal tensioning of the blades in all parts of the flight envelope.
Again alloy is too rigid (dangerous) and it would take a lot of development to exceed the current blades performance in modern ceramics.
I am sure it could be done, again it is available budget.

[PlatinumZealot]

The clearances between the bore and the piston is actually larger when the engine is cold. This is because the operating temperature of the piston is higher than the aluminum block or liner. The piston itself is conical and oval. The reason for the conical shape is the higher operating temperature near the piston crown, while the oval shape is due to compensate for the change in shape caused by the cylinder pressure when the engine is operating.

The clearances between the crankshaft and crankcase, and camshaft and cylinder head is smaller when the engine is cold. This is because the steel crankshaft or camshaft will expand less than the aluminum crankcase or head by the heat. This issue is generally smaller with iron block engines, and many aluminum blocks use cast in iron inserts for this reason.

Another reason for small clearances is the use of low viscosity oils. To reduce the frictional losses in the bearings a thinner motor oil (typically similar to 0W-20) is used in combination with a higher surface finish. Unless the bearing clearances are reduced the resulting oil flow through the engine would be very high, which is undesirable.

This is a pretty good explanation. What about the Reynolds number of the low viscosity oil in the passages? Do you think a smaller clearance allows the oil to keep in laminar regime?

[Tommy Cookers]

@autogyro
about the thrust bearing issue, in haste to air an idea I had 30+ years ago
I was wrong (and had never thought it through in all that time !)

about the props .... IMO
at low powers and speeds (ie low Reynolds nos) the prop will be efficient if aerodynamically (%) quite thick
due to the low density of wood, it does this job better than metal
for lightness, solid metal props are too thin to be as efficient for this job
the reverse applies at high powers and speeds
similarly F1 wing thickness and camber is not efficient for Boeing/Airbus stuff

BTW Dr Porsche did an electric helicopter in 1917 at Lohner (it served in military observation)
it was tethered and powered from the ground

[autogyro]

@autogyro
about the thrust bearing issue, in haste to air an idea I had 30+ years ago
I was wrong (and had never thought it through in all that time !)

about the props .... IMO
at low powers and speeds (ie low Reynolds nos) the prop will be efficient if aerodynamically (%) quite thick
due to the low density of wood, it does this job better than metal
for lightness, solid metal props are too thin to be as efficient for this job
the reverse applies at high powers and speeds
similarly F1 wing thickness and camber is not efficient for Boeing/Airbus stuff

BTW Dr Porsche did an electric helicopter in 1917 at Lohner (it served in military observation)
it was tethered and powered from the ground

Thanks Tommy, you are not alone, it is surprising how often such issues are overlooked by the best engineers.
I think further discussion on aircraft engineering risks completely derailing this thread.
So I will simply say, it is the tip speed at a maximum of 4000rpm (90hp) and the airframe clearance dictates on prop diameter that fix the W116 prop type in wood up to date.

Dr Porsche was indeed one of the timeless engineering geniuses.
His tethered electric helicopter is the forunner of my current push to establish race tracks with under surface induction charging to create the future FI, using electric traction and NO onboard energy storage.
However the W116 is capable of electric flight of one hour plus on batteries.

If the moderator does not move this to another thread I will be surprised.

Mod edit - The aviation posts are an interesting discussion. I'll split them into their own thread in the next few days

[flynfrog]

Even though it is completely off topic. Its one of the better threads here in a while. Auto why would you think that wood would out preform a modern composite prop? You could build it exactly to the stiffness you want and it would be lighter. Metal erosion protection on the leading edge.

[riff_raff]

Even though it is completely off topic. Its one of the better threads here in a while. Auto why would you think that wood would out preform a modern composite prop? You could build it exactly to the stiffness you want and it would be lighter. Metal erosion protection on the leading edge.

At further risk of going OT, a wood prop would not give better efficiency or weight performance than an epoxy/graphite prop, if both were properly designed. However, Rolls-Royce continues to use hollow, diffusion-bonded titanium fan blades instead of composite fan blades on their large commercial turbofan engines, due to the fact that the titanium blades can be made with a thinner airfoil cross-section which gives better performance.

[autogyro]

I agree that the aerodynamic performance of a wooden prop could be equalled by a composite prop.
I do not believe that this has yet been achieved and development is needed.

However, with all such applications it is not just one thing that needs to be addressed.

I have nosed over in light aircraft on two occasions. The first experience shattered the wooden prop, on the second I reacted quick enough to turn off the engine without serious damage.
The shattered wooden prop still required an engine strip down to check the internals.
All the internals were found to be serviceable.
A composite prop in this circumstance could easily have bent the crankshaft or other components.

There is no comparison between the high rpm compressor blades in turbofan engines and the low rpm airscrews used in light piston engined aircraft. The W116 is a pusher design with a maximum prop rpm of 4000.

Wood also absorbs vibration better than composite at thicker aerofoil sections.
Slight damage to a wooden prop is easily repaired, any damage to a composite requires replacement.

[350matt]

the engines are pre-heated because the bores are honed hot - ie at running coolant temperatures and the piston clearances are matched to this hot bore size

so they stil turn over by hand as the pistons are cold and have a lot of clearance at room temperatures but if you were to fire them up without pre-heating the piston will scuff and seize at high rpm

these aren't like a road engine bear in mind

[autogyro]

the engines are pre-heated because the bores are honed hot - ie at running coolant temperatures and the piston clearances are matched to this hot bore size

so they stil turn over by hand as the pistons are cold and have a lot of clearance at room temperatures but if you were to fire them up without pre-heating the piston will scuff and seize at high rpm

these aren't like a road engine bear in mind

I think it is the cold clearances of the crankshaft and camshaft journals that result in a tight cold engine that needs to be pre-heated.
The piston (crown) will be far hotter than running coolant temp when the engine is running and has warmed up.
The piston rings will match the bore dimensions when the cylinder block is at running coolant temperature prior to start.

As explained by Edis, the piston when cold is of a completely different shape to when it is at running temp.
I would expect the initial start up with initial running temp in all parts, to be a delicate condition for the pistons untill their much higher eventual running temp is reached.
Maybe two minutes in time?