Text Box: WARBIRD NOTES # 43       20 Aug 2000  (18)    
A week or so ago, I was conducting a proficiency check in a North American B-25.  The pilot moved the throttles forward noticeably slower than I would’ve.  Certainly within a rate that would be normal – but still far slower than I do.  They reached the maximum manifold pressure around the time we'd accelerated to approximately 70 MPH or thereabouts.  When we discussed it afterwards, he mentioned the case of another pilot that also flies this airplane; his rate of throttle advancement is even slower!  In his case, the throttles might not reach the takeoff manifold pressures until liftoff – or later.  Hmmmmmm!  I asked him if he knew why the other pilot used such a slow throttle movement?  As I'd suspected, his answer was "makes it easier on the cylinders, doesn’t build up the pressure as quickly". Now, I don't like "burst" accelerations either (just think of those poor supercharger impellers having to accelerate to about ten times faster than the reading on the tachometer) – but – all aviation is a compromise.  Now let's examine this thought a little further, OK?  And keep in mind that we’re discussing airplanes without turbo-superchargers here, the needs of those airplanes may –  or may not – differ.   

A grizzled chief pilot once told me to always “wiggle” or “walk” the throttles with your wrist.  You simply cannot jam the throttles with your elbow while simultaneously wiggling them with your wrist, it appears to be impossible!  It works in either direction, both for power being advanced for takeoff or power being retarded for the landing.  Now, as to the rate of application, or how fast to do it.  Use of the phrase, ”apply it normally", could very likely have a different meaning to each person reading this.  So, you’ll have to be able to determine this for yourself – and utilizing this method, you can!  In an attempt to make this as clear as possible, let’s just say to apply the throttle(s) as rapidly as possible – but without overspeeding the propellers.  That’s more descriptive of what we’re trying to say here. If you apply them so rapidly that you hear an overspeed taking place followed by the governors returning them to the desired RPM, then obviously you’ve applied them too rapidly!  Conversely, if you “dawdle” in the application (even though it may fit your particular definition of normal), taking many seconds and noting no overspeed, then that also is indicative that you’re taking too long to get this accomplished!


Take a look at the tachometer, do you see that little-bitty lettering down there in the middle of that instrument?  Says “Revolutions Per Minute”, right?  That’s “Per MINUTE”.  Well, it’s fairly easy to figure out about how many engine cycles will be made as we advance the throttles (the tach measures the engine’s RPM, not the propeller’s RPM, right?)  And remember that every other stroke is a power stoke on a four-cycle engine.  To make it easier to visualize, we can convert that tachometer reading to cycles per second.  Let’s use the B-25 engine’s 2600 RPM here.  2600 divided by the 60 seconds in a minute equals about 44 engine cycles per second, then divide that in half to get the number of power strokes, about 22.  Now we’ve got a figure that we can work with here.  If it takes you about 5 seconds to advance the throttles to the limiting MP, then that means about 110 power strokes.   If we “dawdled” and used 10 seconds to reach the maximum power, the figure would be 220 power strokes. Even if you’d “jammed” the throttles to the redline in 3 seconds, that’d still be 66 power strokes; however in that case the impeller’s acceleration would be extremely rapid!  So, from the above analysis, it’s readily apparent that the RPM of a normal throttle application permits a plethora of power strokes, allowing a perfectly adequate combustion pressure increase.  In other words, not a good reason to utilize a more leisurely or slower throttle application.


So, move those throttles up, remember that as (or) if you “dawdle”, the runway’s rapidly disappearing behind you – and – you’ll not be accomplishing at all what you’d thought you were doing!            


R. Sohn   © 2000

write yet:

Check of instruments afterwards, Braly?

Martene Hall


 Back to the Warbird Notes Index Page