Carson's speed for RV's.

I got bored today and got to thinking what is the best speed to fly in my -9 for the least amount of fuel used, which is commonly known as Carson's speed. Fortunately, I fly (or at least, will very soon fly) an RV-9A, for which CAFE performance data is readily available, via their APR flight testing.

The EAA reports Carson's speed is 1.316x the best glide (best L/D) speed, which CAFE measured as 95 MPH for the RV-9A.

Then I got thinking that maybe other RVators could benefit from having their efficiency data in one place, so I did a little digging and came up with the following numbers. Glide speed has been sourced from, in order of preference: Factory data (in the case of the RV12 for example) CAFE APR's where available, finally, averaged from Builder's POH's reports of their phase 1 test results, with the references provided alongside.

So, for the various RV models, we have the theoretical best MPG speeds as follows, normalized to SMPH:

RV-1:
RV-3: 110.5 SMPH - (Avg of 73 KIAS - N223RL POH, only one so far!)
RV-4: 111.1 SMPH - (Avg of 71 KIAS N359DM POH, 82 MPH - C-GFEW POH, 80KIAS - N41RV POH)
RV-6: 139.5 SMPH - CAFE RV-6 APR
RV-7: 120.1 SMPH - (Avg of 90MPH - N2447A POH, 78KIAS - N447RV POH, 85KIAS - HB-YMT AFM, 78KIAS N585RV POH)
RV-8: 140.5 SMPH - CAFE RV-8A APR
RV-9: 125.0 SMPH - CAFE RV-9A APR
RV-10: 131.2 SMPH - (Avg of 80KIAS- N423CF POH, 90KIAS - N961M POH, 90KIAS - N42BU POH)
RV-11: - I'll ask Van when he finishes it. But how do you measure "fuel"?!?
RV-12: 95.4 SMPH - Vans Factory POH. Can't beat that!
RV-13: Classified - Believed to be in excess of 760 SMPH. If I ever feel the need to break into Area 51, I'll update this one!
RV-14: To be added "soon".

Now, these are not meant to be definitive, and may well vary with individual aircraft - look at the variation in best glide speed and reported glide ratio's for the "same" aircraft, but should be enough to get you in the ballpark for your own testing, if you're that way inclined.

Also, bear in mind best glide speed varies with weight though ratio will remain constant, so variation in test conditions among the various owner's will distort the data somewhat, hence using average values.

Finally, don't blame me if you run out of fuel using these numbers to attempt a Hawaii-LAX flight, it's worth what you paid for it!

Interesting information Rob.

I don't plan to fly to Hawaii in an RV but do wonder why the 20 mph spread between RV-7 and RV-8, the aircraft have the same wing just different fuselage.

I suspect if one got serious about establishing the Carson number for the 8 it would be less than 140 mph.

KRviator said:

The EAA reports Carson's speed is 1.316x the best glide (best L/D) speed, which CAFE measured as 95 MPH for the RV-9A.

So, for the various RV models, we have the theoretical best MPG speeds as follows,

Click to expand...

One small clarification - Carson's speed is not the speed for best miles per gallon. The speed for maximum miles per gallon is the best glide speed (ignoring any effect of head or tail winds). No one wants to fly that slowly, unless you absolutely must squeeze every possible mile out of the fuel you have on board.

Update - Ignore the following sentence in my original post, which was complete hogwash: "Carson's speed is the speed that gives the highest ratio of speed to fuel flow". Most people in RVs don't want to fly that slowly either.

Kevin Horton said:

One small clarification - Carson's speed is not the speed for best miles per gallon. The speed for maximum miles per gallon is the best glide speed (ignoring any effect of head or tail winds). No one wants to fly that slowly, unless you absolutely must squeeze every possible mile out of the fuel you have on board.

Carson's speed is the speed that gives the highest ratio of speed to fuel flow. Most people in RVs don't want to fly that slowly either.

Click to expand...

I must be missing something at this early hour. Seems to me that the ratio of speed to fuel flow is:
(miles / hour) divided by (gallons / hour) = miles / gallon

For more than you ever really wanted to know, here's a fun read: http://www.flyingmag.com/very-best-speed-fly

Kevin Horton said:

One small clarification - Carson's speed is not the speed for best miles per gallon. The speed for maximum miles per gallon is the best glide speed (ignoring any effect of head or tail winds). No one wants to fly that slowly, unless you absolutely must squeeze every possible mile out of the fuel you have on board.

Carson's speed is the speed that gives the highest ratio of speed to fuel flow. Most people in RVs don't want to fly that slowly either.

Click to expand...

Wouldn't best glide speed be lowest fuel burn rate to stay in level flight? Different from best mpg.

Best glide, best L/D, would result in best miles per gallon. Not the same as Carson speed which is 1.316 x best L/D. My best L/D is about 100 ktas, so my Carson speed is about 131 ktas

The issue with Carson's speed is that it ignores the efficiency curve of the propulsion system. It assumes that you can create 1 HP as efficiently as 100 HP, and that your prop takes HP and turns it into thrust with the same efficiency at 1000 RPM as it does at 2700 RPM or at 50 Knots or 150 Knots.

Best range comes from best MPG, but MPG is an intersection of lowest drag and the efficiency of your propulsion system to creating thrust from fuel.

Most engines that burn fuel get more efficient at higher power outputs. Thus, while aerodynamically you may have the best efficiency at the Carson speed, if you have your engine at 40% power to do so, you may not have the best MPG possible. You have to add in the effect of the propeller on the plane as well, which will have its own efficiency curve.

Starting a test at the Carsons speed makes sense because it is highly unlikely you need to go slower than that, but it is not surprising if actual peak MPG is achieved at speeds higher than the Carsons speed.

On a practical basis, flying that slow means that you're more susceptible to winds aloft. In fact, Carson's speed, like the L/D speed, must depend on the winds aloft. If you have a headwind, it'll be faster and in a tailwind it'll be slower.

Dave

brad walton said:

Best glide, best L/D, would result in best miles per gallon. Not the same as Carson speed which is 1.316 x best L/D. My best L/D is about 100 ktas, so my Carson speed is about 131 ktas

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Why? It seems some are confusing lowest fuel burn rate with best mpg, which isn't the case.

Not considering prop and engine efficiencies (at various settings), the lowest gal/hr would be at lowest overall drag, or best glide (about). However, for example, if one added 10% more fuel burn, but got 20% more speed, it would result in a higher mpg. I'm not saying these are the numbers, but are for demonstration only.

Carson

Fuel cost are only part of the total cost to go from point to point . Sure you might burn less fuel by going slower but more time on the oil , prop and engine ,etc. Add them up and it's over $20 hour easily before you pay for fuel.
My 8 gets great mileage above 8000' @ 175 knots cruise ( 25 mpg / no wind) seems like the "sweet spot".

Perhaps this picture will be useful:



The vertical axis is power required and the horizontal axis is speed. The blue curve is the power required at any speed. Please note that this is not drag versus speed, but power versus speed.

The left limit to the curve is the stalling speed, Vso. The right limit to the curve is Vmax, the fastest that an airplane can go in level flight for the power available.

The minimum power required is at the bottom of the curve. It corresponds to the speed for the minimum rate of sink. It's not the best glide.

For the best glide, we need the best L/D, and that's where the slanted green line is tangent to the curve.

The same distance faster than the speed for best L/D, compared to the speed for minimum sink, is Carson's speed.

You can look up the math to these, I'm not going into all that here. Just wanted to show you the graphical relationships.

Dave

Ed_Wischmeyer said:

I must be missing something at this early hour. Seems to me that the ratio of speed to fuel flow is:
(miles / hour) divided by (gallons / hour) = miles / gallon

For more than you ever really wanted to know, here's a fun read: http://www.flyingmag.com/very-best-speed-fly

Click to expand...

Ed - thanks for catching my error - that'll teach me to type without thinking.

Maximum range speed is the same as best glide speed, if we ignore the variation of engine efficiency and prop efficiency with speed. In the real world, the best range speed would be a bit faster than the best glide speed.

I went back to Carson's original paper on cruise efficiency, but I find it a pretty tough read. Peter Garrison's summary in your link is more useful.

David Paule said:

On a practical basis, flying that slow means that you're more susceptible to winds aloft. In fact, Carson's speed, like the L/D speed, must depend on the winds aloft. If you have a headwind, it'll be faster and in a tailwind it'll be slower.

Dave

Click to expand...

Only if your using ground speed. The airspeed does not care about head/tail winds.

zilik said:

Only if your using ground speed. The airspeed does not care about head/tail winds.

Click to expand...

In a discussion about miles per gallon, ground speed is the only relevant speed.

-Andy

I remember reading years ago a research paper that found out all these speeds are directly related, by specific amounts, to the calibrated airspeed an airplane needs to maintain for straight and level flight at engine minimum power. Once you determine this minimum power CAS (at a specific weight) then max range, long range cruise and "safe" max endurance (Holding) CAS at different weights are calculated by using a specific multiplying factor.

As fuel is consumed and your aircraft becomes lighter you don't adjust trim to maintain altitude and airspeed you reduce power. In other words you maintain a specific AOA as the aircraft weight changes. By reducing power your fuel consumption drops while maintaining the same airspeed and altitude. This is true no matter the airframe or number of engines.

Once in the real world then for max range and long range cruise you take into account head/tail winds. You increase the airspeed by a certain amount if flying with a headwind and decrease the airspeed by a different amount if flying with a tailwind.

Using these factors, I made up a chart for my airplane, use it on my very long range trips and it seems to be fairly accurate. I will try to find the research paper on this subject and post a link here.