#51 Kelpiecross
Kelpiecross
Joined: October 10
Let's be clear. Feathering a (piston) engine means stopping the engine. It is an emergency procedure, when an engine is damaged or has run out of fuel. The process normally involves turning off the fuel, switching the ignition off, and feathering the prop. The last because, even though the engine is now officially dead, the airflow acting on the propellor blades will exert tremendous pressure to turn them. This leads to increased drag (the last thing you need when you have just lost an engine), and will probably lead to more damage to the engine if the reason for the failure is mechanical, rather than say running out of fuel. So feathering the propellor blades means they turn perpendicular to the airflow and the drag of the stationary blade is minimised. And it was a regular ocurrence in the piston engined world.
You do not idle an engine with a feathered airscrew. As far as I know no operational piston engine had a clutch mechanism to allow the engine and propellor to be disconnected.
This may not apply to the turboprop word though. The Double Mamba in the Gannet allowed one of the paired engines to be shut down in flight and decoupled from the transmission, and I have a feeling the Bregruet Atlantique MR aircraft was intended to patrol on only one of its two (uncoupled) engines, the other I believe being shut down (not idled) once it reached its patrol area.
I don't know, but I hypothesise that jets may idle an engine in similar circumstances, if the fuel cost of idling is less than the fuel cost of the extra thrust to overcome the drag arising from a shut down engine.
Parallel to the airflow?
PT6 engines always seem to start up with the propeller blades in the feathered position - which always seems odd to me.
Some light aircraft piston engines do actually have clutches - for starting I think.
I think you have to make a distinction between piston engines for commercial use, military and racing.
For commercial use I suspect the engines would be very much the same layout as the 1950's big radial engines. I don't know when the last radials were made (possibly in the 1990's?) but they were much the same as the older engines. In light aircraft the typical new Lycoming or Continental 4/6/8 is little changed in layout since the early 1950's.
Speaking of Lycoming - a place I worked at a few years ago had a forklift with a flathead straight 6 Lycoming engine - and I don't think the forklift was especially old.
#53 GreenMachine
ummm, yes. Thanks.
Good point, but it is a turboprop. I have noticed this too and think the answer may lie in the idea of lowering the loads on the gear train, and therefore the energy required to accelerate the gear train, and ultimately the prop, during startup. I could speculate that it might also be related to the fact that one popular small turboprop (I don't remember if it is the PT6) will destroy the gearbox if the prop is left to freely rotate with the breeze, or is carelessly handled - something about shock loads on the teeth as the prop reverses direction. I assume the loads are meant to come from the other direction (the engine).
That is news to me, but I don't doubt you are right.
#54 Tony Matthews
Tony Matthews
I have noticed this too and think the answer may lie in the idea of lowering the loads on the gear train, and therefore the energy required to accelerate the gear train, and ultimately the prop, during startup.
Surely if the prop is feathered it will need much more torque (or power - stop me!) to turn it. You have all the resistance of flat-on blades. The finest pitch must be best choice. Feathering just reduces the drag of head-on airflow and stops the prop windmilling.
In my aeromodelling days, the ultimate was props (or a single-bladed prop and counterbalance) that folded back along the fuselage.
#55 Magoo
Magoo
Yes indeed tho it would be rather rare. Continental was far more common.
You could be right - I remember it was one of the two well-known engine makers -I thought it was Lycoming but maybe not. I seem to recall that it appeared to be a fairly small capacity engine - 2 or so litres.
Sounds like a Continental I4 F124 or similar...ubiquitous in industrial applications and virtually indestructible.
The Allison does, indeed, have some features which make it potentially better than the Merlin. The head design, for one is more modern. The method for sealing barrels to heads is probably better. The rods are stronger - and many of the Reno Merlins run Allison G6 rods.
Where the Allison fell behind in WW2 was the supercharger development. Rolls-Royce offered single speed-single stage, two speed-single stage and two speed-two stage Merlins during WW2, but Allison only had single speed single stage engines for much of the war, with two stage engines coming out in quantity very late in the war. I think for Reno you would want the two stage engine, but probably want to lock the auxiliary stage to a set speed (it was hydraulically driven). Then the problem becomes length - the 2 stage Allison is longer than the Merlin.
If I had a zillion dollars I might do an Allison turbocharger program. Alas, I do not. However, I do know a guy (you know of him too) who has a zillion dollars AND a world-class automotive development company AND an abiding passion for WWII aircraft engines. We once discussed this idea at great length and he was absolutely enraptured with it right up to the point where his zillion dollars were obliquely in the discussion's frame of reference. At that point the idea seemed to lose all its appeal, there was a momentary, awkward silence, and the topic has never come up again. I figure this is why he has a zillion dollars.
#59 NTSOS
NTSOS
Joined: February 05
I'm thinking maybe a guy by the name of Jack!
#60 rory57
Surely if the prop is feathered it will need much more torque (or power - stop me!) to turn it.
The PT6 turboprop engine has a seperate turbine to drive the propellor. When the engine is started only the compressor shaft (compressor plus its turbine) is spun up by the starter motor. Many helicopter engines are like this, the rotor brake keeping the rotor stationary until the engine sounds like it's at good power. (Not a flyer, so just an observation)
If I had a zillion dollars I might do an Allison turbocharger program.
The Allison V-1710 was turbocharged for the Lockheed P38, later there was a turbo-compound variant but I don't think it was used.
Lots for the armchair engineer to admire in its design. I particularly like the way it is designed to be built running either way: the only change required is to install the crankshaft the other way round and a few small details like starter motor and an idler gear in the supercharger drive. Practical, one of several details that shows it was designed with mass production in mind. Such a different approach to the RR Merlin, which was originally built in small numbers by craftsmen.
When Soviet warbirds are re-built today to flying condition, their V12 engines are replaced with an Allison V1710: plenty of support, parts etc.
The Double Mamba in the Gannet allowed one of the paired engines to be shut down in flight and decoupled from the transmission
The Double Mamba was two engines, each driving a seperate, coaxial and counter rotating propellor so no "decoupling from the transmission" was required. Either engine could be shut down, its propellor feathered and stationary. This was done during patrols because one engine working hard is more fuel efficient than two engines at half the power.
A trick often seen at the airshows of my youth as here: http://en.wikipedia....55_edited-2.jpg
#63 JtP1
JtP1
The Yak 3 production line might have survived, but Klimovs are a bit hard to find.
The reason the prop ends up fully feathered when stationary is there is no hydraulic pressure to opertate the pitch mechanism for the constant speed prop.
#64 Bob Riebe
Hot rod ideas are quite valid. Using 60-70y/o reciprocating parts in these engines would be committing suicide. And the little I have read they do. Durability is obviously no 1 but it does not need to be durable for 100s of hours, just dozens. At low altitude. So I doubt that many people would be using original 40s rods, pistons, valves, springs etc. They are consumables and can all be made a lot better these days. Coupled with better oils and fuels far more reliable power and at least a few more RPM, which generally makes power
Go to one of the air racing sites, of which there are a few, where people who work on these engines post and they can give you a more detailed reason as to why car think does not apply in aircraft.
There is a reason the NASCAR component V-8 is in the five to six hundred hp range and not eight hundred plus.
Some of these sites list what parts are original, and why.
They are already turning at rpm far above, by aero standards, WWII speeds.
For those who keep speaking of exhaust blowers, unless you are going design, MAJOR DOLLARS, an entirely new engine cowl that deals with the heat, on the water cooled aircraft, exhaust blowers produce WAY too much heat for engine and pilot.
Aircraft cockpit temp in these aircraft as is are in triple digits.
The xp-47J had a 2,800 tuned to the then extremes but while it was considered for production that was cancelled by the xp-72 with the 4360. Big unstressed beats smaller and stressed.
The gent whose firm builds Allisons has rebuilt Centaurus engines, but it is extremely expensive.
I know there is one Fury flying with one.
Edited by Bob Riebe, 15 April 2013 - 19:53.
#65 Fondles
Fondles
Joined: July 11
Pratt & Whitney PT-6's are free turbines so the prop is driven by a small turbine section in the exhaust flow of the engine, or gas generator as they call it. You can hang onto the prop for a while when they start.
Other engines, like the Garrett TPE-331, have the prop geared to the engine and so have to start with the blades in the flat position for minimum drag as the engine accelerates when starting.
#66 Wuzak
Camshafts also have to be swapped around.
The reverse rotation for Merlins (on later versions) was achieved with an idler gear.
The Allison V-1710 was turbocharged for the Lockheed P38
Turbocharged in a slightly different way to what I think Magoo means. In P-38s (and all otehr WW2 turbo applications) the turbo was there to provide sea level air pressure to the carby deck. Boosting was by the integral supercharger.
I would think Magoo is talking about losing the supercharger and replacing it with turbos.
later there was a turbo-compound variant but I don't think it was used.
The turbocompound was tested but did not fly. It was going to require development of an air cooled turbine - the turbine used was from a C-series turbo (like that used in the P-47), and it couldn't cope with the exhaust temperatures.
#68 gruntguru
One of the Hugh McInnes "Turbocharging" books (I think the first one) had a photo of a V-1710 converted to Turbos although it was probably for hydroplane use - not air racing. That was about 40 years ago.
So, a 7.5l X-12?
The Bugatti Veyron engine is 8l and 1000hp. How much more could be freed up for aviation use (fewer emissions and noise laws)? How much would then be lost when it gets type tested (to be rated?
Makes 2500hp sound unlikely doesnt it? The difference is the Veyron is lightly supercharged and designed to deliver power and fuel efficiency over a wide range of speeds and loads.
Edited by gruntguru, 16 April 2013 - 04:48.
Basically only if flying F1C.
What Hooker added to the Merlin was the 2 stage supercharger which reduced the heat build up in the compressed air charge.
Edited by JtP1, 16 April 2013 - 01:01.
Before that and probably more significantly, he identified design flaws in the single stage engine and improved the output from 1000 to 1300 hp. This became the engine that was used extensively during the Battle of Britain.
Hooker improved the single stage supercharger, including the way the air was fed to the supercharger, reducing losses.
I believe the first Merlin to benefit substantially from Hooker's input was the Merlin XX. BoB Merlins were IIs and IIIs and did not benefit from his work.
Edited by Wuzak, 16 April 2013 - 01:37.
#73 johnny yuma
johnny yuma
The carb-supercharger layout was determined long before Hooker got involved.
I was trying to say he preferred carb to DI because it cooled the inlet charge by vaporising before the supercharger,not that he was there from conception.They could have gone to DI at some stage in the late 30s ,as did the Germans,but did not.
The RollsRoyceR,a bigger earlier relative of the Merlin, greatly loved by air racers,world water and land speed record seekers did not ever have to go into a sudden steep dive ,so perhaps they had committed to carbs a bit early and were reluctant to change.There does not seem to have been a pure power advantage with DI in those days ,but with today's compact computer power and instant data collection there must be.
Edited by johnny yuma, 16 April 2013 - 02:35.
#74 bigleagueslider
Whether it's 1945 or 2013, boosted SI recip piston engines have the same fundamental limit with regards to output. The ultimate limit is combustion detonation. Where a modern engine would have a huge advantage over a WWII-era engine would be due to the modern engine's use of digital electronic fuel and ignition systems. The boosted SI recip engines of WWII could only use CR's of around 6.5:1. While modern boosted SI recip engines easily tolerate CR's of 12:1 or more, thanks to closed-loop DI fuel systems, knock sensors, CVVT systems, and sophisticated electronic ignition systems.
Folding props are still very popular for model gliders with electric motor assist.
Hooker definitely made big improvements to the Merlin compressor design. But we should also remember that the original Merlin compressor design was horrible to begin with. The original Merlin compressor was basically a paddle wheel, and its design had absolutely zero analytical or lab work behind it. Hooker made huge immediate improvements in the compressor performance simply by employing the basic fluid mechanics and aerodynamic principles he had learned in college.
#77 Canuck
Knock sensitivity tends to increase with bore size though so I'm not sure we're comparing apples to apples. A modern small-bore sportbike engine runs that (12:1+) without DI, CVVT or knock sensors. Newer materials and better understanding of progressively finer nuances that affect performance add up to significant gains I suspect. The biggest though being smart materials able to withstand the load of higher speeds. With an increase in rpm, you could push the compression ratio correspondingly as well.
ADI takes care of any detonation, period.
This article is fairly old now but read it, please.
It will show why CR's of 12:1--(In a blown aircraft engine, really?) are either never going to happen or would serve no purpose.
http://www.supercool...tegearheads.php
Any aircraft engine that through "modern" whizz-bang gizmo construction is going to be putting out 5,000 hp with less displacement, it will have to put up with the same extremes these comp. engines do.
Here is a modern V-12 used in an aircraft.
http://www.bluethund...specifications/
Hooker definitely made big improvements to the Merlin compressor design. But we should also remember that the original Merlin compressor design was horrible to begin with.
As were most, or all, of its contemporaries.
It was far better than a paddle wheel. A (paraphrased) quote from Hooker himself.
The supercharger on the Merlin III although designed in 1934, was easily the best in the world at that time (1938).
Its peak efficiency was 65% and the main problem was a mis-match in the peak efficiency operating points of the rotor and the diffuser. Hooker needed to do little more than correct this mis-match to improve the efficiency to 75% (not bad by even today's standards).
The pent-roof chamber is equally responsible for increases in CR limits.
Cylinder (combustion chamber) size is also a major factor. A modern engine could not operate at anywhere near 12:1 with the boost, octane and cylinder size of a Merlin.
Don't recall whose that was but Ed Cooper Sr. ran turbocharged Allisons in unlimited hydroplanes successfully for many years until the turbines made everything else obsolete.
Indeed, camshaft rotation also had to be reversed.
What I should have made clear was that the single idler gear was added in the drive train to BOTH supercharger and camshaft drives. Look at gears K and H in these two drawings.
http://www.enginehis...ingengines.html
The PT6 turboprop engine has a seperate turbine to drive the propellor.
Ah, I didn't realise... However, am I right in thinking that a prop on fine pitch will need less effort to spin it than one that is feathered?
Ed Cooper ran until 2011 when apparently he got fed up with the same cluster-fuc-- system that has plagued hydros for over a decade.
His boat was designed with an aerodynamic cowling but as with every team that has run exhaust blowers, heat forces them to remove the cowling.
A few years back he won the Gold Cup I believe.
Vastly less, yes. A Garrett simply will not start with the prop feathered, you'd cook the engine.
That would be right.But how small would cylinders need to be to accept 12:1 with Merlin-like boost
of say 2 bar,or would you drop compression and have not-so-small cylinders and keep big boost,or drop
boost and have higher C/R ? What rpm would be used in flight ?
When designing a modern-day, balls-out piston aero engine, CR would not be a primary goal. Boost levels would be far higher than the Merlin and the effect of CR on thermal efficiency not as high as for a NA engine. This is because the benefit of CR is ER and a well designed turbo engine (particularly one optimised for a single operating point) recovers a lot of the expansion energy that would otherwise be lost due to low ER (CR).
RPM was 3000 for the Merlin. A modern version with same B & S might do 4000. A balls-out design with more cylinders would be much higher.
What boost and CR do Koenigsegg run on their latest cars?
1000hp from 5.0l V8 - makes you think that Tresilian's 10l X-16 could comfortably make 2500hp with today's technology.
Rather than try to use car think on aircraft which is like spitting in the wind, find info on the USAAC Hyper Engine program.
They were thinking smaller might work seventy years ago.
The USAAC Hyper engine program required separate cylinder construction, hemispherical cylinder heads (possibly where Chrysler got the idea after building the IV-2220), 2 valves per cylinder and coolant temps of 300F. It was soon discovered that the elevated coolant temperature allowed for smaller radiators but led to more heat being rejected through the oil, consequently requiring a larger oil cooler and cancelling out the aerodynamic benefit of the smaller radiator.
The closest hyper engines got to production was the Continental IV-1430. They spent more R&D time on single cylinder development than Rolls-Royce had spent getting the Merlin to production.
By the time the IV-1430 was ready for flight testing the Merlin and Allison were already approaching or exceeding the hyper goals - 1hp/cubic inch and 1hp/lb.
The Chrysler IV-2220 was the same capacity as a Griffon, a couple of hundred pounds heavier, slightly lower frontal area but one and a half times as long.
The Lycoming O-1230 was deemed to be underpowered and too small for further development. SO they doubled it up to make the H-2470. But apart from power a prototype or two (such as the XP-54) it went nowhere.
The Wiki page on the IV-1430 specifies a weight of 1615lb - not much less than a Merlin 60 series. But I think teh page is inaccurate, so I wouldn't trust the figures.
Probably the most telling quote:
(From Wiki) "In the end, all of these programs were canceled, and the surviving engines became museum pieces. One survivor, a Continental IV-1430, is privately owned, and is displayed publicly from time to time. Ironically, engines that were not considered under the program; the Allison V-1710, Pratt & Whitney R-2800 Double Wasp, Wright R-3350 Duplex-Cyclone and Pratt & Whitney R-4360 Wasp Major, all surpassed the USAAC requirements, and continue flying into the 21st century, primarily flying restored warbird aircraft."
Seems to suggest that the Hyper engine selection process was another example of what happens when the client interferes in the design process.
In the case of the IV-1430, Continental basically didn't put any of their own money into it. Instead they had to deal with contracts from the USAAC for each new development or stage of the development process.
. . . ie failure almost guaranteed.
RPM is an interesting one,older cars from 20th Century commonly cruised in 1:1 top gear at say 3000rpm.Since 1990s more powerful engines have allowed overdrive ratios to be ,by now,universal
thus our cars cruise at less RPM than 40 years ago. Putting aside NVH factors,perhaps the bean counters know lower revs always equals reliability and fuel economy.These two factors make me think that
even a war machine would always be better served by as low RPM as possible,as a major design consideration.
Although the requirement was for a fighter engine. Power trumps longevity and reliability every time (though there has to be a minimum for each).
In the '30s and during the war Ricardo and Rolls-Royce developed the sleeve valve 2 stroke - which became the Crecy. This was originally intended to be a Diesel, but at one point was changed to petrol. The thinking was that the Crecy would be a Sprint engine. It would be very powerful, but not especially economic with fuel. It was to be for fighters to climb quickly and engage the enemy, rather than droning on for hours.
The Crecy was always considered by Rolls-Royce as a "full throttle" engine - specific fuel consumption was considered worse at part throttle, so no good for cruising.
As for RPM, the designers of engines never consciously chose to limit rpm. It was the materials and technology of the day that imposed the limits.
The Allison V-1710 started out being rated at 2600rpm (maybe even 2400rpm), but by the end of the war was rated for 3400rpm. The Merlin, similarly, increased rpm through its life, up to 3000rpm, though some versions were permitted to fly at 3150/3200rpm for short periods.
A couple of examples - the Napier Dagger was good for 1000hp from 16.8l - 10l smaller than the Merlin and making nearly the same power on the same fuel. It did this by revving to 4200rpm. The Sabre started out revving to 3700rpm, but ended up capable of going over 4000rpm.
#98 MatsNorway
A couple of examples - the Napier Dagger was good for 1000hp from 16.8l - 10l smaller than the Merlin and making nearly the same power on the same fuel. It did this by revving to 4200rpm.
Never heard of it.
What was the drawbacks?
Air cooled H-24 (two opposed vertical twelves joined), pushrod OHV, inadequate cooling and a really horrendous exhuast sound. It was a follow-on from the Napier Rapier, a H-16.
The main use for the Dagger was the Handley Page Hereford
Most of which were assigned to secondary duties. Some may have been converted to Hampdens, which was the same but with Bristol engines.
The Dagger was also used for the Martin-Baker MB2
For a point defense interceptor.
For transports and bombers, (general civilian use) longevity and reliability trump power.
Current Reno Merlins can turn at least 3,800 rpm which would not, most likely, be used in a "new" aircraft engine developed, yet a lot of the talk about a new aircraft engine speaks of tuning at the level of the Reno racers.
When air racing used to include cross country events, the tricked up engines used at Cleveland were not used but versions, still hopped up, more suited for long haul flyiing.
I have a book on the R-4360 and it is interesting to see that the Bendix direct-injection system was built directly off of their carburettor.