Ibra wrote
POH even lists 35%-40% ranges where one is probably flying best gliding L/D, the engine seems to run ok with 2000rpm but is that good to keep the engine warm and happy with 12h of flying?
This looks like a Mooney manual.
Mooney does not base their performance data on constant percentage, like Piper does for instance, but on RPM vs MP. While this makes their performance parts messy, it also gives a lot of data. The regime you mention appears in all the POH’s I know up to the 201 and appears to be a sort of “ultra long range” regime based on low prop RPM, 1800 for the C model and it appears 2000 RPM for the J model.This at least in the vintage models has to do with various RPM restrictions, usually between 2250 and 1900 RPM (propeller depending), so they give one power point below that restricted RPM. In your example, 2000 RPM appears to be the choice because there is a restricted range below 1950 RPM.I’ve had a look at this weird regime but if you ask in the scene, I know almost nobody who’s ever used it apart from some “worldrounders” so basically nobody could tell me anything about it. In other words, nobody uses them.
I’ve thrown some numbers around based on the C model. Speeds get really ridiculously low, even below Carson speed and shows an increase of some3 hours of endurance and 200 NM in rage nominal. TAS in those cases are around 100 kts or below. Now, not many people I know would fly a Mooney at these kind of speeds. The benefit is not too large either and hugely wind dependent: At 150 kts a 15 kt headwind will cost you ten% speed, at 100 kts, 15%.
At the table you quote:
you get a 2 hour increase of endurance and about 200 sm range increase (I can’t see if this manual is in kts or mph) with a speed loss of 20 kts/mph. Realistically, you’d have to calculate a 45 mins reserve resulting in about 100 miles less and approximately 8 hours of endurance at a fuel flow of 5.6 USG. For the C Model, the figures are worse: The 1800 RPM ULRC costs 40 mph of speed to gain 100 sm of range for which the plane takes 3 hours more flight time over all. And this would be somewhere around 35% power.
As these are out of the POH I guess someone must have tried these regimes and I’ve read about 2 people who have flown like this in heavily modified E models (with up to 200 USG fuel on board) to tackle oceanic flights so they do work somehow.
Properly leaned, i doubt that flying like this will do any damage to the engine, but I guess your butt might end up pretty sore after 8 hours in one of our Mooneys. For me, the 100 mile range increase vs a 40 mph speed loss simply never made sense unless I really need that range or rather the endurance. What a power setting like this does make sense for would be in a prolonged holding pattern when you are waiting for a closed destination to open and need all the fuel conservation you can get. In the holding, speed is not really very important other than it should stay above Vs (45°bank) by a margin.
45% values however I’ve seen in many manuals. Again, the main goal is to gain range or endurance or both. Yes, you can fly those, I’ve seen it done, e.g. with the Seneca and I believe I read about someone flying his Twin Comanche like that on ultra long range missions.
I guess the only way to find out if this is viable is to try it.
Peter wrote:
If you do too many long flights at low power (e.g. a TB20 at FL180) with no interspersed low level flights at normal power, you get this.
I think this is only an issue if you do not keep the oil temperature warm enough. So if the engine keeps warm it’s not going to harm…
I would not agree, because the IO540 has temperature control on oil.
It is the low cylinder pressure which, after extended periods, enables glazing of the bore.
I know almost nobody who’s ever used it apart from some “worldrounders” so basically nobody could tell me anything about it. In other words, nobody uses them.
Yes I don’t know if we have enough data from people doing it regularly?
Appart from 16h ferry flights like this one 
I flew M20J on 22LPH locally but those were max 30min flights (let’s call that microlight local flying with 4 seats)
Ibra wrote:
I flew M20J on 22LPH locally but those were max 30min flights (let’s call that microlight local flying with 4 seats)
On what RPM?
Ibra wrote:
Appart from 16h ferry flights like this one
I somehow doubt it was flown on that low RPM setting. They put large ferry tanks for this kind of thing and I believe that airplane had Monroy tanks.
I can use my Bonanza for refueling other GA aircraft in flight :-)
Vref wrote:
I can use my Bonanza for refueling other GA aircraft in flight :-)
You accept credit card or cash only? 
On what RPM?
If not going anywhere: 2000rpm LOP that gives about 100kias on 5.5gph (20LPH), most of my slow cruise is 55%: I do 2200rpm LOP that settles about 115kias on 6.0gph (24LPH)
UdoR wrote:
You accept credit card or cash only?
Any card will do, not sure though which VAT regime to apply if the refueling goes X-border
Low RPM gives better MPG especially together with LOP (lower RPM works better with lean mixture if ignition timing is fixed-angle) but you don’t get much power; on the TB20 2200rpm it is OK for say FL130 or below. So it is for good-wx flight only.
