From: "Tomb"

Newsgroups: warbirds.general

Date: Fri, 6 Nov 1998

Considering the attention to detail IMOL guys display in thier flight model..i wondered if they would at some future date consider the engine performance model....particulaly altitude effects

I mention this because the performance drop off as judged by WEP fall off with altitude is in possible error for some engines

Some basics

As you get higher the air gets thinner so for a normally aspirated engine the power will fall off with reducing air density...this is what WEP appears to show.

Bolt on a supercharger and the rules can change.

Merlins had superchargers and many models produced MORE power at altitude...not less

I will cite examples below and try to explain why:

Merlin III as fitted to Spit 1

take off power 880hp

12000ft 990hp

16000ft 1030hp

note the engine spluttered or cut under negative G

Merlin XX as fitted to hurri II (lancaster was a big user of this engine)

take off power 1280hp

6000ft 1460hp

21000ft 1850hp

Merlin 68(Packard) as fitted to P51B,C,D

take off power 1450hp

10300ft 1695hp

Merlin 21 as fitted to Mossie BIV

take off power 1280hp

12500ft 1480hp

Merlin 23 as fitted to Mossie BIV

take off power 1390hp

12500ft 1390hp (still)

Merlin 25 as fitted to Mossie FBVI

take off power 1640hp

9500ft 1500hp (a drop)

Lets contrast this with a quick look among some other types:

F4U-1

take off power 2000hp

15400 ft 1755hp

21000ft 1650hp

F4U-4

Strenthend "c" engine fitted and water injection fitted so in addition to above

take off power with "wep" 2450 hp

F6 hellcat

take off power 2000hp

15700 ft 1800hp

21000ft 1650hp

P40N

take off 1200hp

17300ft 1125hp

Ki84

take off power 1900hp

6000ft 1796hp

21000ft 1700hp

Tempest V

take off power 2420hp

13750 ft 2045hp

FW190a8

take off power 1700hp

18700 1440hp

Some models had GM-1 or MW-50 boost so:

take off power 2100 hp

JU88

takeoff power 1880hp

27000 ft 1420hp

with "wep" take off 2250hp

BF109g series

early such as a G2

take off power 1475hp

18700 1355hp

GM or MW boost then take off power 1800hp

late such as a G10

take off power 1900hp ( with wep ?)

24000ft 1560hp

Bf109k

take off power 2000hp (with wep ?)

16700ft 1800hp (with wep ?)

I suspect the G2 is the baseline unboosted rating and the later 109 figures are the boosted rating

The extra power in the late 109s was achieved by running on 100 octane fuel, over revving, and using GM-1/MW-50 depending on model.....engine life was a moot point if you were going to be shot down due to lack of engine power two other unusual engine performance planes are

P38

take off 1325hp

27000ft 1150hp

Notice very little power loss....it should have kept its power but severe overheating limited the availble power at altitude

P47D

take off power 2000hp

31000ft 2300hp

Water injection (wep) would boost this to 2300hp at 27000ft

Notice a gain in power with altitude. Not knowing this little gem has caused no end of difficulty for sim designers to correctly model the p47's reported supremacy at high altitude....

Note the engine would cut at sustained very high "g" that this aircraft could pull in an extended power dive...the fuel system could not maintain the fuel pressure

Some conclusions can be drawn:

Using some form of WEP gives about 20% increase in power most systems used some form of additive such as water injection or GM/MW to gain power...these have a finate limit as modelled in earlier versions of warbirds..it runs out and that's it

The Merlin used overboost to gain extra power ...with the consequences currently modelled in WB (overheat then BANG). Its major advantage was of course its reusable between cooling down periods....shortened engine life though.

The Merlin was rated in 3 versions told apart by mark numbers and catergorised as follows:

low alt (LF)

medium alt (F)

and high alt (HF)

Its a common error to think this applied to spitfire wing tips but that is incorrect......that came later...to match the engine choice (clipped,normal,extended wingtips)

Basically the particular Merlin mark was set up to give its best power in a given hieght band.

So for a given Merlin series, 60 for example it would produce say

LF 1710hp

F 1520hp

HF 1415hp

at sea level

The LF engine would drop power with alt in the usual way but have more output than normal with an oversized supercharger (engine life was short....this would be fitted to low level spits for FW190 and doodle bug chasing)

The F engine would gain power up to about 10,000 to 15000 (depending on mark) and then fall off until the second speed(supercharger) was engaged which increased "fighting altitude" to use the words of my source by another 10,000ft to 25,000ft

The HF engine had the lowest rated output of all but held its power to beyond 40,000 ft but up here propeller design is the most significant factor and the really high flyers had jablo blades

The early series had single speed,single stage merlins

Early war merlins around Spit V time had single speed two stage merlins mid war merlins around Spit ix time had two speed two stage merlins

The top of the range merlins(70 series) that i can tell were fitted to the Mossie mk BIX (and other bomber varients) which did about 420 mph at 13000ft and each engine put out 1700hp in two stage two speed (the single speed was fitted to all but a few of the FB/NF varients)

The Spit IX HF got this varient

The high level Mossie XVIII could get to over 40000ft on the HF merlins

A2000hp Merlin III was flown in a Spit I....this engine was unusable in sqn service due to its very short engine life

I have not mentioned the Spit V much because of its very wide range of merlins fitted....the most overpowerd Spit of them all was the spitV which was a light airframe with a griffin fitted with 1700hp....this had the highest power loading of any of the operertional spits and its not suprising that the test pilots (who flew them all) rated this Spit the best of them ..it was the Spit XII....only a hundred made...it was rapidly eclipsed by the typhoon in the low level role.

Packard Merlin spits/mossies were given different marks because they were not interchangable with Rolls Royce Merlins

The magic in the Merlin was because of engineering advances allowed the superchargers to run hot and fast and be small and be multi staged and multi speed....where (altitude) the power was developed was decided by supercharger ratio and speed settings and intercooler size

The other fighter that comes to mind and that is the huge P-47 whose appearance was more or less decided by the turbo supercharger layout which was huge with a vast array of ducting...contrast that to the Merlin Spit which was small by comparison

Eventually the LW produced a similar result with the FW (TA)152 and some late (a few)190d models which had two speed three stage engines

If full power was applied to a HF Merlin it would simply overboost and go bang at low level

The LF was the max that could be extracted from a particular Merlin series and used at low level in a pursuit role....over home territory usually where a blown engine was not so critical and worth the risk

The Merlin gave way in the low level role to the Typhoon/Tempest series with thier Napier engines and 400mph speeds...the Merlin was still the middle/high alt engine of choice though

The F and HF engines had a given hieght in which the second speed was used in the super charger...it was automactic and was considered the optimum height to go against a bf109 (around 10,000 ft for an F engine) and this was good up to around 20,000ft in which above this the 109 was regarded as slightly better when using WEP

Some things were noticed about the 109 and that was when WEP was engaged a characteristic black plume emitted from the exhaust and the Fw190 a/f series used to emit white condesation at high alt and high speeds...enough to obsure the view of the wings....some of those reported kills was nothing more than a 190 putting the pedal to the metal and getting outa town....puts a new perspective on those old guns films with 190s streaming white smoke

I think these engine parameters would change the charateristics of Merlin powered planes such as the Spit and Mustang....almost certainaly for the Mossie who's wep drops at 1000ft when it should really be good up to 13000....this is where the Mossie shone...its engine/airframe combo allowed it to do 380mph (Merlin 20 series) or 420mph (Merlin 70 series)..Other aircraft had to be 20000ft ot higher to even approach these speeds(if at all)....the p47 would be king at high alt as it also would be gaining horsepower while eveyone else was losing horse power....the B17 also gained more power with alt (thats why it could go so high)

It would be nice if the two kinds of WEP....fuel additive....or overboost were represented (some had both such as 109k)

Maybe a boost gauge to represent power being produced would be nice....I understand engine control likely to be of the throttle only variety..to keep it simple....well the Merlin had auto mixture and once the prop was set to combat or cruise ...not a lot of tinkering to be done...none in a 190..its automatic so one lever engine control is not a real hardship or much of a sacrifice to "realism"

Incorrect engine charecteristics however .......

Final thought

Rolls Royce named thier piston engines after flying creatures...birds usually....all except one ...which they named after the worlds most famous magician.....anyone ever wonder way

Tomb