B-25 Superchargers and Manifold Pressures

[Guest tkjake]

Hi guys.I generally fly the B-25 at or around 20,000 ft for longer hops. No matter what I do, I can't seem to get to cruise speed listed in the POH. I engage the blowers when passing 11,000 or so and by the time I get to cruise level, I am lucky to have 25in of manifold pressure. Also, the procedure for engaging the blowers is to reduce RPM to 2000 and mixture to FULL RICH, engage the superchargers, and adjust RPM and mixture as needed. Well, if I go full rich on the mixture, the engines quit, no matter how fast or slow I change it. What the heck am I doing wrong here?

[Guest tkjake]

Anyone?

[Guest allcott]

Don't think FS models superchargers. so flicking those buttons makes no difference. Are you leaning effectively?Allcott

[Guest tkjake]

Hi. I belive I am following everything right, including leaning. Is it possible that at 20000ft the engine wont produce 30in man psi? Power settings per the charts list cruise economy as 29.5-31 MAP at 2100RPM at 213kts and 208kts. Right now I am at 19000ft at 50%fuel on all tanks, 2100rpm, 26MAP, and only showing 168Kias. that is leaned out correctly, blowers engaged, NO WIND, FULL THROTTLE. I must be doing something wrong to be short at least 30kias, but I dont know what. Todd

[Guest tkjake]

Update to airspeedsCurrent alt 19100wind 0 kts27MAP, 2100RPM, Blowers high, carb air on, cowl flaps closed, flaps and bomb bay up, leaned to best operation, throttles fullspeeds 164 kts ias 190 mph ias 218 groundspeedare these numbers about right for the listed conditions? Todd

[Guest allcott]

Seems about right, given that there is no supercharger. Remember the IAS is INDICATED air speed and this may not be the same yardstick that the manual uses (do military manuals like to go in for the same hype that commercial aircraft suffer from? You may have to read between the lines to get a true picture). And unless the manual tells you whether it's IAS, TAS or something else, also at what altitude, then comparisons are meaningless. Allcott

[Guest kennethg]

I just so happen to have a complete reprint of a B-25C/D Pilot's Manual with the performance charts intact. According to it, this is exactly what a clean B-25C and D will do with R-2600-13 engines at a gross weight between 30,000 and 34,000 pounds at 20,000 feet:RPM:2100Manifold Pressure: 27.5 in. HgIndicated Airspeed: 160 mph/138 ktsFuel Flow: 155 gallons per hour.The next step down on the chart is 15,000 feet and shows:RPM:2250M.P.: 25.5 in. HgI.A.S.: 190 mph/165 kts.F.F.: 165 gphEven though this is for an earlier model B-25 than MAAM's B.T., the engines are nearly identical, and I'd say that you're getting accurate cruise figures. That's also pretty good test flying, Todd!Ken G.>Update to airspeeds>>Current alt 19100>wind 0 kts>>27MAP, 2100RPM, Blowers high, carb air on, cowl flaps closed,>flaps and bomb bay up, leaned to best operation, throttles>full>speeds 164 kts ias> 190 mph ias> 218 groundspeed>>are these numbers about right for the listed conditions? >Todd

[Guest tkjake]

Ken,Thanks for the info. That seems to settle it for me. Looks like I'm going to have to back the throttle off a little at 20000ft and sacrifice speed for fuel economy on these long flights. Todd

[Guest kennethg]

If you're interested: according to the manual, you're already at or near maximum range at 20,000 feet with this power setting. With less power you'll lose more in induced drag than you make up for with lower fuel burn. In order to go farther, you need to lose altitude. The highest altitude that's listed in the Maximum Range power settings column is 9,000 feet and the numbers are:RPM: 1850MP: 26 in. HgIAS: 170 mphFuel Burn: 110 gphCompare this to Sea Level max range settings which are:RPM:1650MP: 29 in. HgIAS: 180 mphFuel Burn: 87 gphBoth of these settings give you the same general range, which is 1,990 statute miles with 1,100 gallons on board in a no-wind condition. This is all from the performance chart, I'm not computing anything myself.Basically, piston engines like low altitudes and jet engines like it high up. And remember to keep the rubber side down when landing! Have fun!Ken