Holding: power settings/best econ. speed

[Mooneymite]

(I did a quick search of the forum for this, but couldn't find that it had been discussed. If it has, can someone send me the links?)

I recently flew to the Jacksonville area. I knew that the coast would be fogged in and planned my arrival for when the weather was supposed to break up. Surprise: it didn't....so I had to hold.

In 14 years, I have very rarely ever had to hold in my Mooney because I try not to fly into "hard IFR" and I don't usually fly into airports where congestion is a problem.

When I got into holding, I realized I really had never read much discussion about the best speed/power settings for holding. Seems like all we talk about is going fast.

Flying a 'C' model, I pulled the prop back to 1900 and set MAP where I would get about 80 Kts, and leaned aggressively....I had to keep moving the power because of the turns in holding, but it seemed like it was pretty close to 15" most of the time....fuel flow showed about 4.2-5.0 GPH. (I held for about 20 minutes and then successfully got in on an LPV approach to about 300')

What do you do when you just want to stay airborne at a typical holding altitude, burn the least amount of gas going nowhere? Techniques? Any Mooney guidance?

[jetdriven]

In a J, the absolute lowest fuel flow I have seen is 1950 RPM, and 15" of manifold pressure leaned to peak. That gives 4 GPH and 100 MPH indicated IAS at 1500'. However, for turning the fuel flow will need to go up, so I would set about 16" or so to where the IAS was 100 in the turn. Flying slower will burn more fuel as you have to add significant power to accelerate back to 100 or accept the increasing drag at progressively lower speeds until you have to add power anyway.

[Mooneymite]

In a J, the absolute lowest fuel flow I have seen is 1950 RPM, and 15" of manifold pressure leaned to peak. That gives 4 GPH and 100 MPH indicated IAS at 1500'. However, for turning the fuel flow will need to go up, so I would set about 16" or so to where the IAS was 100 in the turn. Flying slower will burn more fuel as you have to add significant power to accelerate back to 100 or accept the increasing drag at progressively lower speeds until you have to add power anyway.

So, would it be more economical still to set RPM even lower than 1900? What about 1500RPM X 15" of MAP....would that yield better economy?

I've never explored the world of low RPM ops.... I know that "generally speaking" lower prop speeds can be better, but is there a point where it is too low? Obviously MAP has to drop as well to prevent an over-boost situation.

Personally, I don't think that 80KTS in a 'C' is on the "backside". For economy, best speed would be closer to best L/D, right? But what is the "practical" best speed given the required maneuvering in the hold?

.

[jetdriven]

So, would it be more economical still to set RPM even lower than 1900? What about 1500RPM X 15" of MAP....would that yield better economy?

I've never explored the world of low RPM ops.... I know that "generally speaking" lower prop speeds can be better, but is there a point where it is too low? Obviously MAP has to drop as well to prevent an over-boost situation.

Personally, I don't think that 80KTS in a 'C' is on the "backside". For economy, best speed would be closer to best L/D, right? But what is the "practical" best speed given the required maneuvering in the hold?

.

Our aircraft has a 1500-1950 RPM limitation when MP is below 15". Also, puling the prop knob fully back only results in about 1950 RPM anyways. The power setting in our airplane is 16"-1950 RPM. Les than that results in a slow loss of airspeed in level flight until stall. Any less RPM (not possible anyway) will need to be accompanied by an increase in MP to compensate. Crankshaft HP is HP, it needs RPM and MP to deliver it. The Lycoming operators manual does show tables down to 1800 RPM and lower RPM is lower FF at a given power setting.

There is no "overboost" situation in a naturally-aspirated engine. The M20J POH shows up to 24.0" of manifold pressure for a 2000 RPM power setting. Not that it is limiting, but its the most I can find. Of course thats a ROP manifold setting, you can add two inches to that for LOP operations. However, the fuel scheduling cam in a RSA-5AD1 fuel injector found on the IO-360 in Mooneys schedules more fuel based on throttle position, so significant savings can be found by simply limiting manifold pressure to a low (15" or so) setting, and leaning to peak EGT.

LDmax depends on weight and airframe. Basically best glide speed for the given weight, and add 5 MPH or so for additional drag of a non-optimized airframe and drag imposed by turning maneuvers. In the M20J it appears to be around 95-100 MPH IAS or so. Getting behind the curve requires significant power addition to return to LDmax.

In normal operations, 2300 RPM and 100 knots is a good trade between airplane handling and fuel burn. You can get around 6 GPH. But in a real situation such as destination below minimums in the morning with fog, minimum fuel at destination, or an island holding situation, I would not hesitate to pull the prop back to the absolute lowest obtainable RPM, and set MP for LDmax, Mixture to peak EGT. You can get a <4 GPH fuel flow and loiter for 2 hours on 8 gallons this way.

[bd32322]

In my J, I could smoothly obtain about 5.2 gph at 2100 rpm at peak. Any lower the engine was not very smooth. However, i am only 150 from TBO. So maybe it will be better with a new engine and overhauled governor etc.

I kept my MP higher at 17 to 18