@LondonMike
That’s, more or less, what airliners do many times.
With piston aircraft the shock cooling idea speaks against doing that (although not everybody believes in it, so it makes no sense to discuss that here). I don’t even pull the power in the descent, I will only do that if the air is too unstable and I get too close to the “yellow” on the ASI. In the SR22 I see a TAS of 220 many times in the descent.
In theory wouldn’t the “most efficient” way be whatever rate of decent allows you to maintain best glide speed with the engine at idle? I guess there is an argument that even at idle the engine is consuming some fuel (I’m doubt they do ‘fuel off on overrun’ like most modern cars) and there may be some speed faster than best glide which produces a lessor fuel consumption due to taking less time.
Yes; exactly. But, in piston GA, real Vbg is way too slow for most of us 
For the TB20, and most IFR SEPs (which are de facto certification-driven to have a Vs of 59kt) it is about 95kt. But the engine efficiency is not constant (it is poor at low power outputs, due to friction and pumping losses) so the best economy glide would be something a bit faster. Probably about 110kt. But this is still too slow… so I descend at 150-160kt max.
Also with no power you go down too fast. Most SEPs go down at about -1000fpm, which is usually undesirable for all the reasons posted. Plus rapid engine cooling…
The problem with these things is that (most of them at least – dont’t know about this Garmin unit) do not have an atmospheric model and know nothing about winds. All they can do is divide your distance to go (again it does not know about any shortcuts or detours which are about to come) by your momentary ground speed and compute a rate of descent from that. Which will be way to high in most cases when you come from high altitude.
Yes, totally true, but maybe this is worse for you in the Citation
The GPS calculated VS is at least continually recomputed so if you follow it you will arrive at the bottom of the descent correctly.
Peter wrote:
The GPS calculated VS is at least continually recomputed so if you follow it you will arrive at the bottom of the descent correctly.
That’s true but then you will be descending with a constantly changing (decreasing) rate. And it will be the wrong way round, because you will start with a high ROD at high altitude and end with a low ROD at low altitude. Jets like it the opposite way: Stay in thin air (high altitude) as long as possible, then cut the power and convert altitude into distance without adding power – this means a low ROD in the beginning and a high one at low level so that you spend the shortest possible time in thick and draggy air… Which in real life is seldom possible to to traffic and airspace.
Peter wrote:
Yes, totally true, but maybe this is worse for you in the Citation
Depending on the FMS installed. There are real dumb ones which will give you figures that are off by 50% and more, and smart ones which know about air density and TAS and which let you enter winds for different altitudes. Of the three aircraft I fly only one is equipped with a “smart” unit, so I do my descents on intuition. Makes one real proud on the (few) occasions when one gets it perfectly right. But it’s always better to descend a little steeper in the end than to fly at low level for extended periods of time with high fuel flow and low ground speed. This can ruin your day if you are tight on fuel…
It is totally unnecessary to descend at 1500 fpm. In order to avoid this, I plan the descent ahead of time and request to start down to avoid a kamikaze descent. If ATC needs to give you a steep descent to be below a certain altitude by a certain point, the alternative to a high rate of descent is to reduce speed to produce the same descent angle. When ATC ask you to descend at a certain rate, I am pretty sure they assume you will maintain roughly the same speed in order to achieve a certain angle.
The VNAV feature of the G1000 and the VNAV calculator on the GNS430 work well enough. I am pretty sure they both use GS and do not do any fancy density altitude and TAS computations. I take a conservative descent rate which gives me some room for increasing the descent rate if need be, and again if you need to steepen the angle you can also reduce speed.
Aviathor wrote:
The VNAV feature of the G1000 and the VNAV calculator on the GNS430 work well enough. I am pretty sure they both use GS and do not do any fancy density altitude and TAS computations.
VNAV on the G1000 keeps you on a 3 degree (or whatever you change it to) line in space. Your ROD is entirely determined then by your GS.
Even if fuel optimizing is the goal, it never makes sense to climb as slow as Vy, and it never makes sense to descend at an IAS speed slower than your cruise speed.
A speed a little faster than Vy in the climb will always give you a benefit for a minimal rate-of-climb penalty. Under certain conditions (e.g. climbing lean-of-peak, with engine efficiency being as high as in cruise) the optimum climb speed would actually be very close to cruise IAS.
A descend speed slower that cruise IAS will never be more economical than if you had just reduced your cruise speed to achieve the same average trip speed.
Of course, in real life it is a matter of ATC, winds, terrain, comfort, engine temps etc. Also a high cruising altitude would complicate the considerations somewhat, but I do not see it really changing the equation.
Only if ultimate fuel economy is the goal, which it never is in real life, because everybody tend to fly more or less for speed, then a Vy climb and a Vbg descend would be the solution, but for that to make sense, the cruise speed should not be more than Vy, which would appear ridiculously slow for most people, and also difficult to fly, because the airplane would not be speed stable, i.e. tend to drop below the power curve very easily. And the engine might very well be inefficient at such a low power setting.
A descend speed slower that cruise IAS will never be more economical than if you had just reduced your cruise speed to achieve the same average trip speed.
Agreed.
Only if ultimate fuel economy is the goal, which it never is in real life, because everybody tend to fly more or less for speed, then a Vy climb and a Vbg descend would be the solution, but for that to make sense, the cruise speed should not be more than Vy, which would appear ridiculously slow for most people, and also difficult to fly, because the airplane would not be speed stable, i.e. tend to drop below the power curve very easily
Surely, assuming constant engine efficiency, the best MPG will always be obtained at Vbg. Vx and Vy are a different issue entirely (except that they are related to Vbg).
And the engine might very well be inefficient at such a low power setting.
That’s why in reality best MPG is not obtained at Vbg but at some higher speed.
Also if you have a headwind, the best MPG is obtained at a faster still speed, and the opposite with tailwind.
As for the Vy cruise speed…
Many ferry pilots from diamond fly at FL140 with 45-55% power setting in order to fly at /around Vy. They do that because it gives a range of more than 9hrs (da40 tundra) and saves technical stops in CIS countries which charge you up to several thousand euros per landing. (Not mentioning the paper works to even get a landing permit)
And also to mention…that’s decision making in the office and not the cockpit. Any pilot obviously want to avoid a 6-7hrs leg…
But the question was fuel economy ..and drag increases v^2… So as slow as possible increases reach… Every g1000 drives sees it while watching the maximum range circle while playing with the throttle.
If time is your prime concern. Yes…100% throttle with centurion engines and 92% with AE300 (for Da aircrafts at least) and then you burn the fuel in less the 4,5hrs and reached around 75-65% of the maximum range if flown like mentioned above.
What counts for the cruise part is same valid for the climb and descend part…and again … The turbo charged and water cooled Diesel engines have a nice performance benefit against their avgas friends. no problems with idle at any stage of the flight and no time limitations. Doing this with a mooney or a sr22t would kill the engine very quickly (idle at TOD)
So again…the wanted result sets the operational setting for the flight execution.
In my opinion, going to the limits of reach (low fuel operation) is not required in GA planes. If you fuel up to,the top you burn the same and have only a marginal less climb and cruise performance. Eg. If I fly alone and with minimum fuel required a 2,5 hrs leg I reach FL160 in less then 20 minutes. If I do it with MTOW plus ;-) it takes maybe 5-10 min more… And for the decend you have the reversed effect…the more heavy you are the better you (faster) you can glide / descend efficient. So in real terms it completely equalizes this point.
Anyway to finish the entry. The ferry pilot I talked with said, that the whole hassle of IFR procedures for arrival and departure, with turnaround and refueling takes so much time, that it’s faster to fly 5-6 hrs slow than two times 2hrs really fast and loose 1-2 hrs with Procedures and ground time…
Again…sometimes a matter of taste…for me a matter of property
If you pay per flight hrs…4 hrs is cheaper than 6 hrs…if the landing is less esoenssive than 2hrs rental costs ;-)
If you pay per year for owning a plane…. You might go long leg and spare yourself a technical stop…safes your frame another cycle and your engine a full climb workout ;-)
Walter wrote:
Anyway to finish the entry. The ferry pilot I talked with said, that the whole hassle of IFR procedures for arrival and departure, with turnaround and refueling takes so much time, that it’s faster to fly 5-6 hrs slow than two times 2hrs really fast and loose 1-2 hrs with Procedures and ground time…
Ferry pilots typically quote fixed price so are desperate to reduce fuel and landing costs. This actually leads to quite different economic drivers from normal GA pilots.
that it’s faster to fly 5-6 hrs slow than two times 2hrs really fast
It is obviously quicker to fly 5hrs at speed X kt than 2x 2hrs at speed Y kts, for any value of X and Y, unless you can be assured the stop time will be less than 1hr.
A stop time of over 1hr will waste the entire gain of the faster flight, not to mention the extra risk of one approach and landing and one departure, wx below you etc etc.
And it is hard to get the ground time under 1hr. I normally plan for 2hrs, though at well sorted airports one could do it a lot faster than 1hr.
I still don’t think they fly at Vy. That is really slow.
