Peter Garrison wrote a very good piece on this. The 1.32 times Vbg, Vmd or L/Dmax is the tangent from the origin to the power required curve – I think, hopefully someone corrects me if I am wrong. As induced drag increases much faster than parasite drag, increasing speed by 1% to 5% above 1.32*Vmd, has little penalty in real terms, and your piston engine power efficiency improves at Full Throttle, hence most GA normally aspirated aircraft are happiest at 55%-75% power and full throttle altitude, and coarse or cruise pitch.
Peter wrote:
That’s not true. Nothing stands in the way of LOP cruise climbs, except a much reduced VSI.
It IS true.To reach the operating ceiling, by definition you must use the most power available.
For you and for me, that means ROP.
I mis-understood, removed the remark but not before you copied it !
Peter wrote:
That’s an interesting observation i.e. you are saying that as one climbs (non turbocharged scenario) the IAS falls, so one ends up closer to Vbg?
Yes. On the Twin Comanche for example, the best L/D occurs at about 85 KIAS. Which creeps into triple-digit KTAS at about 10,000 ft.
As you move further away from best L/D, it gets worse quadratically, so if you would probably find that for 120 KTAS, equivalent to 100 KIAS at 10,000 ft and 120 KIAS at sea level, there’s a considerable difference in L/D.
Chapters 19 and 20 of Stick and Rudder are entitled The Working Speeds of an Airplane and Thin Air. There is a section entitled ‘Something for Nothing’ that’s still good a million years after publication, and explains how you can go faster over the ground and closer to best L/D at the same time, assuming still air or favorable winds.
I had a bit of a look about this Carson-Speed thing. In my plane, Carson Speed would be 138.6 mph.
I find that if i try to match this to the performance tables, that this would conform to pretty much 62% power with 8.8 GPH fuel flow. This yields an operational range of around 650 NM.
When I look at the tables however, I find that this is not the best range regime, which comes at slightly different values. The best LRC values come at around 57% power and a TAS slightly below Carson Speed with 717 NM at 10’000 ft, FF of 7.8 GPH (a full gallon below the 62% value) and a TAS of around 130 kt(145 mph).
I’d have to test fly the airplane to see how close these values come to reality, wanted actually to do some test runs yesterday but it’s too choppy at the moment.
What is absolutely clear is that there is an abundance of ways to optimize the cruise performance of your plane regarding range and speed, and not many of them are listed per se in your POH. It takes going out there and test fly them to find the sweet spot for each individual airframe.
When on a long flight I often fiddle with the power settings, to see if I can get more MPG by changing power.
(Always peak-EGT or LOP – anything else is obviously a waste of time)
On the KLN94/KMD550 I get an “MPG” readout. Also I get the LFOB (landing fuel on board) readout. These are updated about every second.
What is curious is how little difference the power setting makes – if you wait a few minutes for the aircraft to recover steady state.
I may be able to change the LFOB from say 55USG to 56USG. And obviously as you progress down the route, the power setting makes a reducing difference to the LFOB anyway.
So the curve must be very flat. The Carson speed is a fair way up the drag curve so it is a largely arbitrary figure.
RPM makes a bigger difference.
The above is different for high altitude flying (FL300+) and jets because there you are flying close to Vbg (best L/D) and e.g. weight makes a difference.
Mooney_Driver wrote:
I had a bit of a look about this Carson-Speed thing. In my plane, Carson Speed would be 138.6 mph.I find that if i try to match this to the performance tables, that this would conform to pretty much 62% power with 8.8 GPH fuel flow. This yields an operational range of around 650 NM.
When I look at the tables however, I find that this is not the best range regime
Carson speed was never intended to be the best range speed. Carson speed is the ‘optimal’ cruise speed, meaning that up from this speed the increase in fuel consumption is more ‘relevant’ that the speed gain that it produces. In economic terms it is a maximum in the ‘marginal cost curve’ of the speed. It is slow compared with the cruise speed at the usual 65-75 % power, but better than the maximun range speed. I don’t think anybody uses it actually, because it’s a slow cruise.
Correct. Max range will occur at L/D max, or best glide speed normally.
Here’s a great article I’ve posted before about Klaus Savier who gets some almost insane range out of his VariEze. Close to 100mpg at his best setting. So if you have the time and are on a budget…
Another useful link, this from the EAA
Coolhand wrote:
Carson speed is the ‘optimal’ cruise speed, meaning that up from this speed the increase in fuel consumption is more ‘relevant’ that the speed gain that it produces. In economic terms it is a maximum in the ‘marginal cost curve’ of the speed. It is slow compared with the cruise speed at the usual 65-75 % power, but better than the maximun range speed. I don’t think anybody uses it actually, because it’s a slow cruise.
Thanks, I had to revisit the pages to understand more about this.
Actually, that speed is not exactly slow for my aircraft. I ran some calcs on it and for the sweet spot of 10’000 ft it pretty much is what the airplane will do.
Carson Speed Cruise (138.6 mph IAS)
Altitude TAS/KT Range NM
2500 125 631
5000 130 645
7500 134 656
10000 140 676
15000 150 681
As tables can not be displayed in this forum properly, I’ve kicked all the intermediate values out. Range is with 45’ reserve at cruise fuel flow of 8.9 GPH, 62% power.
I have planned to do some performance test flying with the Aspen soon and will verify those figures. In low altitude, the speed penalty definitly is there, but at normal cruise height, it is minimal. I’ll let you know once I get to it.
