Acheiving Best Economy Speed

[docmirror]

Bad thumb. It may work well for aircraft with similar aerodynamics to your Bo, but it is not even close to correct across the board, especially for fixed gear aircraft. The only way to find the right speed is to get in the book for your plane and do the figuring like ol' Mel says.

Well, first note that I said complex. Next if you'll go to the link you provided you'll see it's very close for both the A36 and the Mooney M20J. The Baron is higher spread, and the Cessna 172RG is a lower spread(no surprise here). But, that's what you do with the 'rule of thumb' type info, you modify it slightly to fit the conditions. 20MPH increase is a good place to start for all the complex planes, but is it prefect fit? Nope, that's why it's not an absolute.

Smaller engine, smaller plane? Reduce the rule a bit. Bigger engine bigger plane, increase the rule a bit.

 

[poadeleted20]

Well, first note that I said complex.

You did say that, but it still isn't true across the complex board. Like I said -- "bad thumb." Better to go in the books and learn your plane than to guess based on what works for a different plane.

 

[docmirror]

You did say that, but it still isn't true across the complex board. Like I said -- "bad thumb." Better to go in the books and learn your plane than to guess based on what works for a different plane.

Well, since that info isn't in the books (as noted again by the link you provided) you'll have to calculate it or work it out empirically. If you don't like the rule don't use it. Or, conversely disprove it with a real world example from the ranks of the complex singles.

 

[Henning]

By "best economy" do you mean lowest gph, or the "sweet spot' where gph and speed meet?

Lowest gph means a very low power setting, which means MCA if you want to maintain level flight.

No, MCA is typically on "the back side" of the power curve. The power to fly L/D max will give you the lowest gph in level flight. If I slow below that, I have to add power to maintain level flight.

 

[sba55]

I asked Mike Bush last year whether he thought that RPM made a difference in engine life. His answer was that RPM doesn't make a difference since there shouldn't be friction between components once the engine is warmed up.

There is lots of evidence that disuse and startup decreases engine life significantly.

Felix

 

[Henning]

I asked Mike Bush last year whether he thought that RPM made a difference in engine life. His answer was that RPM doesn't make a difference since there shouldn't be friction between components once the engine is warmed up.

There is lots of evidence that disuse and startup decreases engine life significantly.

Felix

Stress and strain increases with RPM leading to fatigue failure in reciprocating components, primarily rod bolts. I agree that disuse is more detrimental though.

 

[tonycondon]

exactly lance.

i went through the numbers of with the 421 when i first started flying pt. 135 stuff. throttling back saves a bunch of money in gas, but it costs us in TBO time, since we'd be flying more hours and are limited to the TBO. so i just fly it like the boss says and go fast.

 

[sba55]

Stress and strain increases with RPM leading to fatigue failure in reciprocating components, primarily rod bolts. I agree that disuse is more detrimental though.

I'm sure RPM makes _a_ difference, but it doesn't appear to be significant, especially considering that the components that usually break in the engines we use do so because of heat/pressure. That said, RPMs that are too low are likely to lead to problems rather quickly.

 

[RotorAndWing]

I'm sure RPM makes _a_ difference, but it doesn't appear to be significant, especially considering that the components that usually break in the engines we use do so because of heat/pressure. That said, RPMs that are too low are likely to lead to problems rather quickly.

It's not RPM. Engine health depends upon frequency of use and proper start up, warm up and most importantly cool down prior to shut down.

Also, maintenance of spark plugs, ignition system, filters and fuel injectors go a long way in maintaining engine life.

 

[sba55]

It's not RPM. Engine health depends upon frequency of use and proper start up, warm up and most importantly cool down prior to shut down.

Also, maintenance of spark plugs, ignition system, filters and fuel injectors go a long way in maintaining engine life.

Yeah, I agree RPM isn't very important.

But....cool down prior to shut down? What sort of shut down are you advocating? One data point I found to be interesting is that idling a turbo after landing actually increases some temperatures while doing rather little to decrease others.

Tony, you don't get extensions to the TBO from FSDO? It was my impression that most 135 folks do that.

-Felix

 

[gismo]

40NM/12,000' is excessive, but you generally regain fuel you used in the climb in the descent. It's pretty close to a zero sum game.

I've found that it costs me about a half gallon for every thousand feet of climb when compared to the same flight at a lower altitude assuming zero wind. That's in a B55 Baron and I've heard from other Baron owners that this is typical. IIRC in the Bonanza I used to have the number was a bit more than half that (about 3/8 gallon per 1000 ft climb). Part of this is due to the fact that I climb ROP which is less efficient than my cruising LOP, but I suspect that if I shut one engine down during the descent and kept the IAS as low as it is in cruise, there would be less wasted fuel during the climb.

 

[RotorAndWing]

Yeah, I agree RPM isn't very important.

But....cool down prior to shut down? What sort of shut down are you advocating? One data point I found to be interesting is that idling a turbo after landing actually increases some temperatures while doing rather little to decrease others.

After landing the engine should be cooled down before shutdown. In the airplane world this is usually done via the taxi to the ramp at idle power and about 5 minutes.

In the helicopter world after landing the engines go through a idle down sequence such as 3000 rpm 2 minutes, 2000 rpm 2 minutes then idle for 2 minutes.

As far as turbo'ed engines it's important to idle for several minutes before shutdown to allow the turbo time to spool down. This prevents "coking" of oil in the bearings.

Even turbine engines are required an adequate cooling time before shutdown. Quick shutdowns shorten component life.

 

[gismo]

As far as turbo'ed engines it's important to idle for several minutes before shutdown to allow the turbo time to spool down. This prevents "coking" of oil in the bearings.

According to George Braly of GAMI and Tornado Alley Turbo fame, that's an OWT. As Felix mentioned, GAMI instrumented a couple of turbo installations and found that with a normal descent to landing (e.g. a 3-4 degree glideslope) the turbo housing was coolest at touchdown and warmed up from there during rollout and taxi.

 

[RotorAndWing]

According to George Braly of GAMI and Tornado Alley Turbo fame, that's an OWT. As Felix mentioned, GAMI instrumented a couple of turbo installations and found that with a normal descent to landing (e.g. a 3-4 degree glideslope) the turbo housing was coolest at touchdown and warmed up from there during rollout and taxi.

That's actually from Garrett's recommendation. I always used that and never had turbo problems or early replacements. The coking occurs when alot of thrust is used followed by an immediate shutdown.

There area I'm referring to is the bearing housing, not the turbo housing.

 

[docmirror]

According to George Braly of GAMI and Tornado Alley Turbo fame, that's an OWT. As Felix mentioned, GAMI instrumented a couple of turbo installations and found that with a normal descent to landing (e.g. a 3-4 degree glideslope) the turbo housing was coolest at touchdown and warmed up from there during rollout and taxi.

This is one of those 'they're both right'. The cool-down of some sort is needed to prevent boiling of the oil on the turbo shaft. It can either be by letting it sit before shutdown, or it can be on descent. Either way, you need to let the turbo cool before shutting down to prevent the oil boiling on the shaft, and destroying the bearings and seal. I've seen this damage many times on auto turbos.

So, if the turbo warms due to rollout and taxi, then it needs to be cooled again.

 

[sba55]

So, if the turbo warms due to rollout and taxi, then it needs to be cooled again.

Ok, but how do you cool the turbo then? Taxi power for me is as close as I usually get to idle power. If it's heating up just from taxiing around, why would it cool down while standing still?

 

[sba55]

I've found that it costs me about a half gallon for every thousand feet of climb when compared to the same flight at a lower altitude assuming zero wind. That's in a B55 Baron and I've heard from other Baron owners that this is typical. IIRC in the Bonanza I used to have the number was a bit more than half that (about 3/8 gallon per 1000 ft climb). Part of this is due to the fact that I climb ROP which is less efficient than my cruising LOP, but I suspect that if I shut one engine down during the descent and kept the IAS as low as it is in cruise, there would be less wasted fuel during the climb.

So in other words, 12,000' for 40 NM wouldn't be a great idea if you want to save fuel. But I imagine it would be funny to do that at least once IFR. Maybe I'll file KEDU-KSAC at 12,000 next time (it's a 4 minute flight). Might need some delay vectors for the climb

 

[docmirror]

Ok, but how do you cool the turbo then? Taxi power for me is as close as I usually get to idle power. If it's heating up just from taxiing around, why would it cool down while standing still?

This gets a bit complicated, but I'll give you my 2 cents worth. Inside the engine the oil is spraying around all over. This is called windage. The faster the engine goes, the higher the oil pressure and the more oil slinging is happening. At 650RPM or so, the windage and the pressure are both much lower. Since the pressure and volume of the oil is lower, you have less air entrained in oil in the crankcase. So, we have a higher ratio(by a lot) of liquid oil to vapor running around in the oil circuit. That oil goes through the cooler, and is cooled, and run through the circuit.

At the same time, a turbo works because of adiabatic and pumping pressure of the exhaust gases on the vanes of the turbine. At low speed, this pressure is much lower. The TIT is also lower, and the speed of the shaft is lower. This all contributes to less temp on the turbine and the shaft, bearings, seals, etc. If you can let this run for a few minutes, the compound action of the cooler oil with less air in it, and the cooler shaft will keep the oil from boiling on the shaft.

However, other parts of the engine like the CHT may be higher due to the lower airflow. It's a balance, but the turbo is helped by a few minutes of idle speed, oh yeah, keeping the engine super lean will help too, because there is less post cylinder burn happening too.

 

[gismo]

That's actually from Garrett's recommendation. I always used that and never had turbo problems or early replacements. The coking occurs when alot of thrust is used followed by an immediate shutdown.

There area I'm referring to is the bearing housing, not the turbo housing.

Yes the potential for coking exists in the bearing cassette but the turbine housing (and rotor) are where the heat comes from.

 

[gismo]

Ok, but how do you cool the turbo then? Taxi power for me is as close as I usually get to idle power. If it's heating up just from taxiing around, why would it cool down while standing still?

Just sit there idling until it starts to rain. Oh wait, it never rains in southern California, right?

 

[gismo]

This is one of those 'they're both right'. The cool-down of some sort is needed to prevent boiling of the oil on the turbo shaft. It can either be by letting it sit before shutdown, or it can be on descent. Either way, you need to let the turbo cool before shutting down to prevent the oil boiling on the shaft, and destroying the bearings and seal. I've seen this damage many times on auto turbos.

Me too, on my own car. The first turbocharged engine I owned was on a 78 Saab Turbo which toasted it's turbo when the bearings were damaged by coking. These days virtually all automotive turbos have a water cooled bearing cassette making them pretty much immune to the coking problem.

But I think you're right that if you have maintained high power throughout your approach to landing, you should spend some time at idle (often the taxi to parking will be sufficient) before shutting down. I've said before that if I had turbochargers I'd add a temp probe to monitor the turbine housing so I'd know when it was or wasn't safe to turn off the oil flow.

So, if the turbo warms due to rollout and taxi, then it needs to be cooled again.

That's gonna be difficult unless the taxiing was so uphill that you needed a lot of power.

 

[sba55]

Just sit there idling until it starts to rain. Oh wait, it never rains in southern California, right?

I wouldn't know. I'm all about NorCal!!!

This discussion about turbos is really interesting. I can see how low RPMs/pwr settings would help cool it. What I don't see is that the RPM/pwr setting would be significantly lower during idle compared to taxi. I taxi almost at idle (maybe 200 RPM above) and I wouldn't expect that to make much of a difference....a turbo housing temp probe would be interesting to have.

-Felix

 

[RotorAndWing]

I wouldn't know. I'm all about NorCal!!!

This discussion about turbos is really interesting. I can see how low RPMs/pwr settings would help cool it. What I don't see is that the RPM/pwr setting would be significantly lower during idle compared to taxi. I taxi almost at idle (maybe 200 RPM above) and I wouldn't expect that to make much of a difference....a turbo housing temp probe would be interesting to have.

-Felix

As an example I've watched many times an airplane (turbo'ed) taxi up to the ramp at an idle speed, then the pilot revs the engines to make a 180 turn to only shut them down as soon as he's turned. That's where the damage comes from.

 

[sba55]

As an example I've watched many times an airplane (turbo'ed) taxi up to the ramp at an idle speed, then the pilot revs the engines to make a 180 turn to only shut them down as soon as he's turned. That's where the damage comes from.

Good point. I don't do tight turns with power anymore....if I revved my engine after taxi, it would die very quickly given the mixture

 

[Henning]

According to George Braly of GAMI and Tornado Alley Turbo fame, that's an OWT. As Felix mentioned, GAMI instrumented a couple of turbo installations and found that with a normal descent to landing (e.g. a 3-4 degree glideslope) the turbo housing was coolest at touchdown and warmed up from there during rollout and taxi.

It's an OWT that has root in old old old turbo bearing material, and it had to do with galling rather than coking. Neither issue has been applicable for decades.