High or Low?

C

Clipper5895h

New Member
Recently I had a chance to ride right seat in a K model Bonanza. When we were told by ATC to decent, the pilot proceeded to lower the RPM and increase the Manifold pressure, a technique I considered to be very odd when descending. I even noted a increase in airspeed because, in my opinion he was placing the propeller to a lesser drag, "feathered" condition. When we had put the aircraft away, I brought up his peculiar technique. He explained to me that Bonanzas (especially the V-tail) where notorious for picking up speed in a decent (the frontal drag and profile of a the aircraft are incredibly minimal, I believe it is equivalent to a 3 foot square board going through the air). He responded by saying that he had read an article in the Bonanza association magazine that claimed that this technique would actually prevent the aircraft from accelerating in a descent (essentially, you are in a downshift condition). With regards to propeller drag, there is two relative winds affecting the prop. First, the relative wind generated by the aircrafts forward momentum through the air, causing drag on the prop blade. Then, there's the relative wind that is working parallel and opposite to the actual propeller arc. What it boils down to is which one causes the most drag in a decent: 1) The forward relative wind striking essentially the wall of drag the propeller makes in a high RPM (low pitch)...Or 2) the relative wind that is striking the propeller parallel and opposite (in that casue, the most drag condition would be a lower RPM and Higher pitch). So, High or low?
 
It's hard to say exactly what was happening on your particular flight because you didn't specify how much he decreased RPM or increased MP.

I'm skeptical that this technique would create any drag. Decreasing the RPM (increasing blade angle) will make an engine more fuel efficient and produce less power in cruise flight, but it won't actually add drag to the aircraft.

Why wouldn't the pilot just reduce MP during the descent? Or accept an increase in airspeed? I've got a couple hours in a V-tail Bonanza...it's a slick plane, but not THAT slick.
 
jrh said:
It's hard to say exactly what was happening on your particular flight because you didn't specify how much he decreased RPM or increased MP.

I'm skeptical that this technique would create any drag. Decreasing the RPM (increasing blade angle) will make an engine more fuel efficient and produce less power in cruise flight, but it won't actually add drag to the aircraft.

Why wouldn't the pilot just reduce MP during the descent? Or accept an increase in airspeed? I've got a couple hours in a V-tail Bonanza...it's a slick plane, but not THAT slick.
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I recall the RPM was at 2000 and the MAP was at 19. He also said that this technique aided in preventing shock cooling during descents. I wish I could find that article, he was sincerely convinced that this was the proper technique.
 
Clipper5895h said:
I recall the RPM was at 2000 and the MAP was at 19. He also said that this technique aided in preventing shock cooling during descents. I wish I could find that article, he was sincerely convinced that this was the proper technique.
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Well, that's fine. It won't hurt anything to run that power setting. But it's not really necessary, either.

What I suspect was happening was a reduction in power as a result of the lower RPM. It was this reduction in power that allowed the plane to slow down, not an increase in drag.

I just looked at a Cessna 206 POH to get an idea for the relationship between MP/RPM/%BHP.

Let's suppose we're cruising at 8,000 feet in a normally aspirated 206. Running 19" MP and 2500 RPM will produce 57% power.

Now suppose we set power to 20" MP and 2100 RPM. This will produce 50% power.

Voila! An increase in MP, yet a reduction in total power. This 7% power reduction would be enough to accomplish a shallow descent without an increase in airspeed.

And as I said above, drag has nothing to do with it. Leaving the RPM at 2500 and setting 17" MP would accomplish the same 50% power output in this scenario.

Sometimes I just shake my head and let pilots believe whatever they want to believe. If this pilot's technique makes him happy, he can keep using it as long as he wants.
 
Lower RPMs=quiet=stylish too! I'll operate at the lower end of my RPM range whenever possible. However I don't do it under the guise of lowering the power 7%.
 
Codyjp said:
Lower RPMs=quiet=stylish too! I'll operate at the lower end of my RPM range whenever possible. However I don't do it under the guise of lowering the power 7%.
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It's not a guise. It's reality.

And I'm not saying one way is better than another. I was just explaining why the pilot in question might have thought he was increasing drag, even though he wasn't.
 
Was he on plane? (sarcasm fo'ks.
I'm not buying the "won't accelerate" argument. I will buy the aid in not shock cooling, but even thats a stretch.
 
ppragman said:
Why would you pull the power back in the descent? I wanna go as fast as possible.
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Save gas and/or stay below the yellow arc in rough air and/or get maximum descent performance, such as when flying jumpers. Lots of reasons.
 
jrh said:
Save gas and/or stay below the yellow arc in rough air and/or get maximum descent performance, such as when flying jumpers. Lots of reasons.
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I missed the sarcasm tag, but seriously, I don't think you actually save any gas by pulling the power back in the descent the speed you pick up (usually 10-20kts) a 12kt increase in groundspeed (or 10% in a 206/207/PA32) in the descent would probably be better than any fuel savings you'd get by a 7% reduction in power - I could be wrong though. As for the "yellow arc" I can see doing that, same with jumpers, but as for fuel savings I think the jury is still out, when I get home, I'll pull out my 207 POH from the dusty bookshelf and start being scientific.
 
jrh said:
Save gas and/or stay below the yellow arc in rough air and/or get maximum descent performance, such as when flying jumpers. Lots of reasons.
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pshhhhhhhhhhhhhhh

if you're not descending at VNE you're doing it wrong!

I kid I kid
 
ppragman said:
I missed the sarcasm tag, but seriously, I don't think you actually save any gas by pulling the power back in the descent the speed you pick up (usually 10-20kts) a 12kt increase in groundspeed (or 10% in a 206/207/PA32) in the descent would probably be better than any fuel savings you'd get by a 7% reduction in power - I could be wrong though. As for the "yellow arc" I can see doing that, same with jumpers, but as for fuel savings I think the jury is still out, when I get home, I'll pull out my 207 POH from the dusty bookshelf and start being scientific.
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Oops, my bad for not picking up on the sarcasm.

As for the fuel savings, I'm willing to bet money that fuel is saved by reducing power in a descent. It's no different than reducing power during straight and level cruise flight. You'll get better "mileage" out of an aircraft at a lower power setting regardless of the phase of flight, assuming your airspeed remains higher than best range speed.
 
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