engine rated power...

[[Sg]ShotGun]

is manifold pressure the ruler to measure engine power output for our genre of AC?

if MF pres is constant with alt, will eng ouput be constant w/alt?

will speed of climb and rate of climb be constant with a constant MF pres?

[Nashwan]

I think it depends on how you maintain that constant manifold pressure.
A geared supercharger should take more power at lower alts, if in the same gear, because it is compressing a denser amount of air than at higher alts.
A good example is the Spit IX tests reports at MW's site http://www.fourthfightergroup.com/eagles/spittest.html
The test on Spit IX BF274 shows a constant boost of 15lbs from sea level up to just over 12,000 ft, and a climb rate going up from 3680ft/min to 3860ft/min. It's as expected, because the supercharger is using more power at lower alts.
However, if you look at the test of Spit XIV JF319, climb does remain constant with constant boost. Why, I don't know.

[[Sg]ShotGun]

so rate of climb and speed of climb shood stay constant or at least improve if MP is constant as alt increases?

[Maverick]

When figuring power gained from a super charger please keep in mind the wastegate. Many engines will selfdestruct if too much pressure is used. A wastegate "bleeds" off the excess air to keep an overboost situation from happening. The engine should maintain the same, or rated, HP at altitude as long as the turbo or supercharger is able to maintain the manifold pressure at the same level. The big difference is the thinner air gives the propellor lass to work with and it becomes a limiting factor in performance as well as the loss of lift due to lack of air density.  The engine and HP (horse power) aren't the only factor to consider in high alt flying.

 

[HoHun]

Hi Sg,

>if MF pres is constant with alt, will eng ouput be constant w/alt?

With the Engine Calibration Curve for the R-1820-56 with an engine-driven supercharger in front of me, I'd say: Constant manifold pressure doesn't mean constant power.

The power gained from a certain manifold pressure/RPM setting is lowest at sea level and then increases with altitude to the critical altitude of the first supercharger gear. For the R-1820-56 at 43" Hg/2600 rpm, power output is 1120 HP at sea level and 1200 HP at 5800 ft. (Above this altitude, manifold pressure drops, reaching a minimum of 34" Hg at 13000 ft, yielding 960 HP.)

At 13000 ft, the pilot switches (manually) to the 2nd supercharger gear, restoring 43" Hg manifold pressure, yielding the same 960 HP. Power will slightly increase to 17800 ft, then manifold pressure will drop of again, reaching something like 27" Hg at 30000 ft where the curve ends. The engine will be down to 620 HP there.

>will speed of climb and rate of climb be constant with a constant MF pres?

First, let me modify the question to "... at constant power" to make things a little easier :-)

With constant power over altitude, the rate of climb will drop. If you'd climb at constant true air speed, this drop would be pretty bad, since the thinner air would require you to increase the angle of attack quickly, making the wing more and more inefficient, until you'd stall out.

To climb more efficiently, you have to increase the true air speed to keep the angle of attack shallow (thereby avoiding the stall).

Now you could try and keep the angle of attack at the optimum for climb at sea level, which would mean keeping the indicated air speed constant. You'd still lose climb rate over altitude since the higher resulting true air speed in the thinner air at altitude would mean that the thrust produced by the propeller would decrease, leaving you with a decreased climb rate.

The solution is to find the optimum speed where the angle of attack is still pretty shallow, and yet the thrust is still pretty good. This solution involves true air speed increasing with altitude, but indicated air speed actually decreasing.

In other words, the climbing aircraft would fly with a greater angle of attack, but nevertheless climb on a shallower flight path for the optimum rate of climb.

However, even flying at the optimum speed, the climb rate would still drop with altitude. There's no way around that :-)

Regards,

Henning (HoHun)

[[Sg]ShotGun]

-56???
u flying an FM2??

but why wood engine output decrease with alt if MF is constant? (lets disregard climb performance for now

if MF is constant than does that not mean the waste gate hasent fully closed and the supercharger (SC) is still providing SL atmospheric conditions for the eng?

the SC is supposed to maintain some semblance of SL conditions..correct? so if takeoff (TO) MFpres is maitained till 12k than shoodnt HP be constant till 12k?

prolly no..right?

[gripen]

Well, there is no simple and universal answer to those questions but anyway here we go...

Single speed superchargers (or two or three speed or anything with constant gear ratio) give their best efficiency for given MAP only in the altitude where they operate at full throttle (in other words at rated altitude, rammed or unrammed depending on the case). In practice this means that below rated altitude throttle is not fully open because it is used to adjust MAP to the given level. So we can see that at below rated altitude supercharger and throttle system actually wastes some energy and that is why the constant speed system reaches it's max power at rated altitude (generally). The waste gates are used in the turbo systems to adjust how much exhaust gases are directed to the turbo so there is no such devices in the single stage mechanically supercharged systems.

gripen

[ 12-25-2001: Message edited by: gripen ]

[[Sg]ShotGun]

i kno that the 38s n 47s use GE turbos, so they r the ones with waste gates.

i can understand why AC lose climb performance with alt, some faster than others of corse. but dont understand why power output from the eng wood decrease if the MF can be maintained.

so if u take a WC 3350 from SL to pikes peak, the HP wood be less at the top of pikes peak even if u hauled it up there in a truck, and even if the SC cood keep MFP constant?

[HoHun]

Hi Sg,

>so if u take a WC 3350 from SL to pikes peak, the HP wood be less at the top of pikes peak even if u hauled it up there in a truck, and even if the SC cood keep MFP constant?

With engine-driven superchargers, it's different from what you describe: They usually develop their maximum power for the selected supercharger stage at critical altitude. Below this altitude, they actually develop less power - as Gripen explained, the throttle is not fully open there to prevent overboosting, and the throttle plate is a restriction for the airflow into the engine, forcing the supercharger to pump harder.

Using the high supercharger gear at high gear critical altitude, the engine also will provide less power than at low gear critical altitude. The reason is that the supercharger has to spin a lot faster to maintain the same manifold pressure, eating up much more power in the process.

The power the pistons feed into the crankshaft is indeed equal in all three cases. However, in the case of the R-1820-56, the supercharger eats up about 80 HP more at sea level than at critical altitude due to the partially closed throttle at sea level. At high gear critical altitude, the throttle is wide open, but the supercharger has to be spun much faster, eating up 200 extra HP.

The combustion process in all three cases delivers the same power, but the supercharging system subtracts varying amounts from that, leaving a smaller amount of usuable power at the propeller shaft.

Regards,

Henning (HoHun)

[gripen]

Well,as noted before there is no universal answer. Anyway, the case of the single speed single stage system is described above. In the case of the two speed system (or three, four...) higher gear ratio can maintain given MAP at higher altitude but higher rotation speed of the supechargeralso requires more power. So in the case of the multispeed fixed ratio systems there is two or more efficient peaks and lower gear ratio results higher peak than higher ratio.

Then there is variable speed systems which have a hydraulic coupling and in that case speed of the supercharger is continously variable at given range. Then there is several combination of two stage systems and in some cases these can be used as single stage systems at low altitude. And then there were some other systems but I'm too lazy to explain them all unless somebody pays...

gripen

[Seeker]

<throws a buck into the ring for Gripen....>

With this talk of critical altitudes, I've never understood why RR kept with geared blowers when they already had pre-war knowledge of the German variable rate blowers, as had been convincingly displayed in the prewar Le Mans battles between the blown Mercedes and the blown Bentleys. (The constant drive Bentleys drove the Merc's so hard an unblown Bently went on to win, all the blower cars blew up, go figure.....)

What would have been the engineering reasons to have perservered with what seems, in theory at least, to have been an inferior product?

Also, we've been talking centrifugal type blowers up to now, no? These by their nature rely on the mass of the intake charge to work for them, and a lower density at alt implies a lower per unit volume mass. Were Roots type blowers more efficient at alt? I seem to remember reading of an high alt Ju 88 (or maybe a 188) having a third engine mounted in the aft fuselage specificaly to power a rootes type blower to supply the other two engines. Why would a Rootes type be used for this, when ordinarily the current practice would have been a centrifugal type?

[gripen]

Well, I see two main reason why manufacturers like RR and Junkers stayed on fixed ratio gearings in their supercharger systems:

1) Hydraulic coupling wastes always a bit of energy (around 2-4%) so while it gives optimal rotation speed at given range for the supercharger, it also wastes some energy all the time.

2) Hydraulic coupling is a quite complicated system and there were a lot of problems with it (oil cooling and so on...).

Generally hydraulic coupling systems were used in the fighter engines (there is couple exceptions) where smooth power curve is an advantage and actually Packard developed hydraulic coupling for the Merlin too (V-1650-11). Fixed gear ratio systems were preferred in the bombers for efficiency and reliability reasons.

And yes, we have been talking about centrifugal blower systems.

gripen

[ 12-27-2001: Message edited by: gripen ]

[gripen]

Originally posted by gripen:
Packard developed hydraulic coupling for the Merlin too (V-1650-11).

My mistake, the Packard Merlin with variable speed supercharger was the V-1650-19 with Sunstrand supercharger drive (ratio was variable between 5-8,3). This engine was developed from the V-1650-11.

gripen