Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Charge on August 11, 2003, 08:56:30 AM
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I have wondered what effect does engine displacement have in aircraft?
The engine of the fighter airplane usually runs in maximum or near maximum allowable RPM. So, does the the extra displacement give any bonus when compared to engine giving nearly the same power with smaller displacement? e.g. More time running at maximum engine output?
If we compare e.g. Spit1 and 109E. Spit has 1066BHP and 38ltr and 109E has 1100BHP and 42ltr. (I wonder if I got this right??)
Does the displacement difference give the 109 the ability to pull steeper climb when compared to Spit, which is a bit heavier and has smaller engine displacement (but better wingloading) because the bigger displacement gives it more torque which helps it maintain more optimal RPM in difficult situation?
-Charge+
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The Merlin's capacity was only 27 litres (I can't recall the DB's offhand, but it was certainly bigger).
My understanding is that the Merlin matched the performance of the bigger German engines because it ran at much higher supercharger boost pressures, probably due to the availability of higher-octane fuel.
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Higher displacement would equal more HP, all other factors being equal. The more cubic inches (or litres) means more ability to pump out power. This means more power to benefit performace in speed and climb.
Adding things like a supercharger to benefit high alt performance or to increase manifold pressure over a "normal" limit such as WEP would also allow the engine to put out more horses to increase plane speed and climb.
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It's kind of an apples and oranges comparison. As people have already mentioned, the Merlin developed a lot of HP out of 27 liters. To do it, it had to tolerate high RPMs, a very high boost, and it required higher octane fuel too.
The DB601 displaces something like 34 liters, but does not take the same amount of boost as does the Merlin, nor quite as high RPM. Instead, it has a higher compression ratio (the ratio of the volume at the top of the cylinder with the piston at the bottom of its stroke relative to the volume that remains with the piston at the top of its stroke). Higher compression yields more power and somewhat better fuel efficiency.
Boost vs compression is complicated. Typically supercharger efficiency declines as you design one to generate more boost. But RR developed the most efficient gear driven superchargers and intake manifolds of that time.
On the other hand, a higher compression requires a physically stronger engine, and typically implies more weight too. That is because the break-mean-effective pressure (think average pressure) of the DB601 is lower than the Merlin, but it's peak pressure is higher than the Merlin. But if the strength is there, the engine will be more durable under ordinary conditions.
I have some charts on all this I'll try to post.
Originally posted by Charge
I have wondered what effect does engine displacement have in aircraft?
The engine of the fighter airplane usually runs in maximum or near maximum allowable RPM. So, does the the extra displacement give any bonus when compared to engine giving nearly the same power with smaller displacement? e.g. More time running at maximum engine output?
If we compare e.g. Spit1 and 109E. Spit has 1066BHP and 38ltr and 109E has 1100BHP and 42ltr. (I wonder if I got this right??)
Does the displacement difference give the 109 the ability to pull steeper climb when compared to Spit, which is a bit heavier and has smaller engine displacement (but better wingloading) because the bigger displacement gives it more torque which helps it maintain more optimal RPM in difficult situation?
-Charge+
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Weight and size increase pretty much linearly with displacement. Fuel consumption goes up too.
Variation in power with RPM is not a big factor for WW2 fighters, because advanced types all had constant-speed props.
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DB601A
displacement = 33.9 L(2069ci).
max rpm (109E)
level flight = 2400, later 2700
dive = 3000
CR = 6.7:1
Merlin II
CR = 6.0:1
max rpm = 3000
DB603
44.5 L(2715ci)
2700 rpm
different CR in each cylinder bank
DB605
35.7 L(2178ci)
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You are confusing propeller RPM and engine RPM.
All that a constant speed propeller does is translate the engine RPM into the most efficient pitch of the propeller. This will affect engine RPM simply because it places a load on the engine. But that does not imply that engine RPM is irrelevant.
A piston engine is a complicated blow dryer. The more air pushed through it (and heated) the more horsepower. One way to push more air through it is to run it at higher RPM. Unfortunately forces on the engine increase dramatically with more RPM so it takes a very well designed (or small) engine to develop very high RPM.
Aces is modeled with constant speed props. Experiment with RPM and you will see it affects (1) your power and (2) your fuel consumption.
-Blogs
Originally posted by funkedup
Weight and size increase pretty much linearly with displacement. Fuel consumption goes up too.
Variation in power with RPM is not a big factor for WW2 fighters, because advanced types all had constant-speed props.
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Aces is modeled with constant speed props. Experiment with RPM and you will see it affects (1) your power and (2) your fuel consumption.
it should but it does not have an effect on fuel consumption in AH. It has been tested by numerous people already. The only use in AH of reducing prop rpm is to improve your glide ratio.
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Blogs what I meant was this:
A typical scenario for automobiles is where you have a large displacement engine and a small displacement engine (imagine 2 liter I-4 and a 5 liter V-8). They both might make the same peak horsepower if the I-4 revs high enough, but the V-8 will have a flatter power curve and still might win a drag race.
In a WW2 fighter it was not a big deal to have a peaky power curve because the CSVP prop could hold engine speed near the RPM for peak power.
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Originally posted by pugg666
it should but it does not have an effect on fuel consumption in AH. It has been tested by numerous people already. The only use in AH of reducing prop rpm is to improve your glide ratio.
It depends on the airplane. It definitely works for the Mustangs. It doesn't perfectly match the book figures for gal/hr at various rpm/mp settings, but it's reasonably close.
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I does work for the FW190 in Aces. I'm surprised it doesn't work for all planes.
-Blogs
Originally posted by funkedup
It depends on the airplane. It definitely works for the Mustangs. It doesn't perfectly match the book figures for gal/hr at various rpm/mp settings, but it's reasonably close.
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My finding was that reducing RPM always reduced fuel flow rate, but it didn't always increase specific range.
To fly at a given speed, you would get better specific range at maximum RPM than at any other RPM. Except on the Mustangs.
BUT
It's been a long time since I've tested it, so maybe it works on all of them now. I don't think it was an error by HTC, just a choice not to spend time implementing a feature that 99% of the user base won't even notice.
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DB-605A
Displacement: 2179 cu-in (35.7L)
Compression ratio: 7.3:1 port cylinders, 7.5:1 starboard cylinders
Bore, stroke: 6.1x6.3 in
P&W R-2800
Displacement: 2800 cu-in (45.9L)
Compression ratio: 6.65:1
Bore, stroke: 5.8x6 in
Packard V-1650-1
Displacement: 1649 cu-in
Compression ratio: 6:1
Bore, stroke: 5.4x6 in
Alison V-1710F
Displacement: 1710 cu-in
Compression ratio: 6.65:1
Bore, stroke: 5.5x6 in
Specs lifted from:
http://www.eaa1000.av.org/technicl/engemp/engemp4.htm
http://www.aviation-history.com/index-engine.htm
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Delta Six's Flight School (http://www.worldaccessnet.com/~delta6)
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Funked got exactly what I was wondering about. The larger engine should have it easier to reach max RPM due to its bigger torque and hold it there easier in difficult loading situations. Going up-hill would propably be a proper equivalent.
The difference is propably not so big in aircraft, however, as the power transition from engine to substance in effect (ground/air) is different. (e.g. in aircraft the engine still has power to rev but the wing has already lost the lift, stalling, causing the propeller to just cavitate through air.)
I guess that in this comparison it doesn't really affect anything whether the engine has a bigger displacement or works on higher boost as long as the output power is fairly similar? The high-boost engine is just more expensive and more difficult to manufacture and requires more maintenance.
-Charge+
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you make a good point about higher rpms and smaller engines. take for example (apples to oranges to a Merlin and a Benz) the 2 airplanes i am most familiar with...
Cessna 172
Cessna 150 (looking at one to buy to fatten up the ol logbook)
RPMs i use as a current renter (not worried about saving fuel since i rent them Wet anyway)
C-172 = Lycoming O-320, cruise rpm 2400 leaned
C-150 = Continental O-200, Cruise rpm 2550 leaned (important in these little continentals)
Just making a point that higher rpms and size of engine go hand in hand. Same with RC engines, you can have some small small small Norvel's turning 20,000 rpms whereas a larger engine might spin 12-13 grand.
That's all
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The problem there is inertia. The bigger engine has longer and heavier conrods and pistons. A well designed smaller engine will accelerate to its max power faster.
There is a similar problem with the supercharger. Suddenly changing the amount of air sucked into the engine causes all sorts of transitory effects. They are worse the larger the supercharger...
-Blogs
Originally posted by Charge
Funked got exactly what I was wondering about. The larger engine should have it easier to reach max RPM due to its bigger torque and hold it there easier in difficult loading situations. Going up-hill would propably be a proper equivalent.
The difference is propably not so big in aircraft, however, as the power transition from engine to substance in effect (ground/air) is different. (e.g. in aircraft the engine still has power to rev but the wing has already lost the lift, stalling, causing the propeller to just cavitate through air.)
I guess that in this comparison it doesn't really affect anything whether the engine has a bigger displacement or works on higher boost as long as the output power is fairly similar? The high-boost engine is just more expensive and more difficult to manufacture and requires more maintenance.
-Charge+
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As far the Merlin and the DB601/605 series went, its a perfect example for the "There`s no replacement for displacement" rule.
Merlins achieved just about the same powers as DBs, but they had to use higher RPM, higher grade (and thus more expensive) fuel,higher boosts to do that. Higher boost worked against the use of higher compression rate, which resulted in worser effiency, or in other words : Merlins consumed a lot more fuel than DBs for the same horsepower output, by about 1/3 more at high powers. The positive effects of larger displacement can be seen very well on the DBs: the larger, more powerful DB605 actually consumed less while producing more power than the DB601...
Also interesting to note, that in order to withstand the forces generated by the much higher boost rates, Merlins lost the possible edge they could have in size and displacement. Practically the weight of Merlin`s (27 liters) and the larger displacement DB`s (33.4 and 35.7 liters IIRC) was the same, in fact, later Merlins were a bit heavier. Dimensions were the same almost for milimeter. It seems the neccesary strenghtening of the engine block stole away the gains with a smaller displacement.
Rough empty dry weights were the following:
Single-Stage Merlins and DB601As : 610 kg
Two-Stage Merlins (61-66) : 741-749kg.
DB 605A: 740kg (approx.)
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Not same subject but realy near.
Some days ago someone ask about diferencial CR in DB series( SimHQ) except 601A. Looking for some info just found ,and not too much :( , tha was a matter of air intake/supercharger position. But know i'v readed most problabel is about a slightly different head volume.
Any clue?
bsitos
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So the Merling was a rice rocket, and the DB605 a good old BigBlock?
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Orka, there is a thread here, but seems to have died.
http://oldsite.simhq.com/simhq3/sims/boards/bbs/ultimatebb.php?ubb=get_topic;f=98;t=003080
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Originally posted by MiloMorai
Orka, there is a thread here, but seems to have died.
http://oldsite.simhq.com/simhq3/sims/boards/bbs/ultimatebb.php?ubb=get_topic;f=98;t=003080
I was in that thread as you can see, just looking for more oppinions here ;)
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What is the direction of rotation in DB? Maybe the higher CR side has a better angle against the roof of the engine (inverted V) than the other side thus enabling it to run at its fulla CR whereas the other side has worse angle and has its CR dropped?
Looking the engine from behind: The crankshaft turns antiCW and the right side has higher CR?
I don't think the pressure in intake manifold can vary depending of the side of the supercharger as the the downpipe from SC to IntakeMF goes down in middle of the engine when looked from behind.
-Charge+
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Clarification on the Spit I:
Merlin III
1649 cu. in.
27.02 litres
Net weight dry: 1375
H.P/RPM/Altitude: 1310/3000/9,000'
Rated Boost: 12 lbs./sq.in.
Do you guys realize that your Spit I is modeled with performance using 87 octane/6.25 lbs/sq.in?????? That's short about 260 hp. Hurri is short changed as well.
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The reason for the CR difference I would guess is probably a cooling system imbalance after the 6xx series block was bored out bigger?
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Originally posted by Charge
What is the direction of rotation in DB? Maybe the higher CR side has a better angle against the roof of the engine (inverted V) than the other side thus enabling it to run at its fulla CR whereas the other side has worse angle and has its CR dropped?
I think you should re-think this statement.;)
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C'mon Milo, help me out here. Could it be something that has to do with force vectors inside the engine?
I mean the same reason why the conrod is not attached to the middle of the piston but slightly to the other side of it.
:confused:
-Charge+
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Are you sure our Spit I is modelled with 87 octanes? Sheesh.....how nice it would be to have the 100 oct already used in th BoB......:D
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Achtung! Schpitfire lover!!! :D
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Angus, I'm certain. The full throttle height and very slow SL speed are dead give-aways. What you have modeled here is not representative of Spit I performance during the BoB.
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Why shouldn't it be modeled that way? The Merlin I and II engines were designed and rated for that fuel and the UK didn't get 100 octane until US tankers showed up during the Battle of Britain.
-Blogs
Originally posted by mw
Clarification on the Spit I:
Merlin III
1649 cu. in.
27.02 litres
Net weight dry: 1375
H.P/RPM/Altitude: 1310/3000/9,000'
Rated Boost: 12 lbs./sq.in.
Do you guys realize that your Spit I is modeled with performance using 87 octane/6.25 lbs/sq.in?????? That's short about 260 hp. Hurri is short changed as well.