The Voyager Constant

[Bill James]

When Dick and Jeana flew the Voyager around the world nonstop unrefuled, there was one constant. What was it?
...Mark Z, please wait as least 0.003 seconds before you nail it 
Or better, until there are a couple of good replies.
Thanks
Bill James

[magnum]

A.O.A.

[Bill James]

Ha!
Yes,
Why?

Bill   

[Joe Person]

Scratch-scratch-scratch (gray hairs falling from the noggin)... Max Endurance AOA = minimum thrust required...

[magnum]

Bingo! And it also keeps you alive, IE the Kennedy crash a while back at night.

[Marc Zeitlin]

Max Endurance AOA = minimum thrust required...

Tsk, Tsk, Tsk.  I'm embarrassed for you, Joe :-).  Max _ENDURANCE_ AOA?  Max. Endurance AOA <> Max. Range AOA (i.e., best L/D) for propeller driven aircraft, which Voyager was.  You're stuck in Boeing's jet world :-).  You got the "min thrust required" right, but that's not "max endurance".

[Marc Zeitlin]

Bingo! And it also keeps you alive, IE the Kennedy crash a while back at night.

Please elaborate.  How would maintaining a constant AOA (any particular AOA - whether the max L/D AOA or not) have affected Kennedy flying in marginal VFR conditions at night with no horizon and not using the autopilot or staying near shore so as not to become disoriented and enter a spiral dive?

[Joe Person]

Hey, c'mon Marc, I prefaced the whole bit with the "gray hairs falling from head" qualifier...  You wait until you work in the aluminum (& now carbon) overcast biz, and you also go gray, you'll see I tell you! 

In rapidly-increasing senitlity,

[Joe Person]

"Senility", that is, and my typing abilities are diminishing as well...

[Bill James]

So.
AOA.
We are one step along the path here. AOA is significant.
True or false, an airfoil (or aircraft) always stalls at the same angle/AOA.
If true, what would be a cause for, a Cessna 120 for example, to stall on one flight at 50 mph, and 85 on another flight?
Bill

[Vandy12]

True!

That AOA is the 'Critical Angle of Attack'-

A fixed-wing aircraft, by definition, is stalled at or above this critical angle of attack, not at or below a particular airspeed.

The airspeed at which the aircraft the stalls varies with the weight of the aircraft, the load factor, bank angle, the center of gravity of the aircraft, etc.

However...the aircraft always stalls at the same critical angle of attack.

[Bill James]

  "The aircraft always stalls at the same critical angle of attack." Thanks Vandy12!

I like it. A profound, useful statement. A truth that you can sink your teeth into and use.
   ...I am perusing the three pages of notes here in front of me. Which way to go...?
How about this-
How did the pilots of the Voyager determine that AOA. Did they have an AOA indicator? Did they use a speed schedule?
What power schedule did they use?

(At the end of my three pages of notes there are a couple of practical ways to use this info. But hopefully we won' skip too many interesting factors along the way - Like the airspeed thing you mentioned Brian)
- How did they determine how to hold that AOA, hour to hour, day to day?
- How did power play into the plan?
- What was the initial cruise speed?
- What was the final cruise speed

Bill
Thanks for playing   

[Bill James]

A little more info on this thread-
A few years ago i flew the VariEze non-stop 1400 sm in 8 hours. I determined the cruise rpm to use by seeing how much the plane slowed with each 50 rpm and then 20 rpm reductions. The first big speed droop was when the rpm was reduced below 2400. So 2400 it was. It worked fine, averaging about 3.8 gallons an hour. Now we all know that others have greatly surpassed that, but it was an exciting event for me. The plane had no electrical system and just the two mags, plain and simple. Hand held com and GPS. If I couldn’t have made it non-stop I had scouted out the airports for good refueling spots. I stayed in my fuel discipline because I already started the descent into Truckee before the going into the 5 gallon tank under the aft thigh support. I used 31 gallons and landed with 4 gallons, a little over an hour remaining.

I often think of doing the Reno run again now with electronic ignition and an auto pilot. I'll have to check with Tim LoDolche to see if he will still let me sleep on his kitchen floor during the Races. 
The question is, how much would it benefit to try to optimize the speed (AOA) and power similar to the Voyager. I know the detail-itus is a little overkill but it is still interesting to ruminate about.
If i remember reading the book correctly, the Voyager initially cruised at about 132 knots with both engines at full power. They gradually reduced power and around the 4th day shut down the front engine and on the last run back into Mojave were at a low power setting at about 92 knots. Those speeds gave them that AOA but I don’t know how they knew what speed to hold or if they were using an AOA indicator. I understood that after 9 days they landed with about 12 gallons and wouldn’t have made it if not holding that min-drag AOA.
I know there’s a real name for it. There are several distinct AOAs like best L/D etc. and I hope someone will enumerate a few of them here along the way, in particular those that are meaningful for us.
So that’s my motivation. But after we digest these interesting morsels, there is still an approach issue related to AOA and speed and stuff.
I know we are all spiffing up the birds for big O. Good spiffing!
Bill

[Marc Zeitlin]

Those speeds gave them that AOA but I don’t know how they knew what speed to hold or if they were using an AOA indicator.

There was no AOA indicator.  In fact, there was a "deck angle" indicator (which is not an AOA gauge, but serves as something similar in 1G flight), but it broke during the flight.  See:

   http://www.oshkosh365.org/saarchive/eaa_articles/1987_02_01.pdf

for a description of the flight.

The object, in Voyager's case (or any distance goal) was to fly at the max L/D AOA, which is constant, or the max L/D speed, which changes with weight.  If you know how much you weigh, then you know what speed to fly (since L = W). Since Voyager knew how much fuel they were burning, they could estimate their weight at any given time, and thereby know what speed to fly to maximize range.  More work, but just as effective, IF you know your weight.

I know there’s a real name for it. There are several distinct AOAs like best L/D etc...

For Voyager (or Global Flyer, or any commercial airliner attempting to maximize range), it's Max. L/D AOA or speed.

With respect to your previous posting's questions, there are really three meaningful AOA's/speeds (and all this is for prop planes, NOT jets - they're slightly different, since their thrust different):

1) Max Endurance speed.  This occurs when the power required to stay in the air is a minimum (and so the fuel burn is lowest, so the fuel lasts the longest)

2) Max L/D speed.  This occurs at 1.32 * Max Endurance Speed, and is the speed to fly for maximum range. This occurs at the speed at which drag is the lowest.  You need more power than the Max. Endurance speed (obviously, since you're going faster), but you pick up a more range by going faster than you lose by burning fuel faster.

3) Carson speed. This occurs at 1.32 * Max L/D speed. This is the "Optimum" cruising speed to fly which optimizes the fuel flow/kt of speed.

For a COZY MKIV (which I fly), the Max Endurance speed is about 75 - 80 kts.  The Max L/D (range) speed is about 100 kts.  The Carson speed is about 132 kts.  Now, these speeds do change with weight, but since I always fly between 1700 - 2100 lb, they don't change much (not like Voyager or Global Flyer, which burned 80-90% of their weights during their flights, or a commercial airliner that might burn 1/2 it's MGW on a flight).  It might be a few kts slower at lower speeds to keep the same AOA.  Remember that lift is proportional to velocity squared, so a 25% decrease in weight will lead to an 11% decrease in speed for the same AOA.

So my max endurance speed might drop to 70 kts at light weight, etc.

Interestingly enough, since I almost always fly above 7500 ft, my IAS is generally in the 145 kts - 125 kt. range (between 7500 - 13.5K ft).  This puts me squarely in the Carson speed area.  I'd have to fly a lot higher (with lower IAS's) to get near the Max L/D speed.

If you fly low, you have to slow down a lot to get to the Carson speed, and a LOT to get anywhere near the Max L/D speed.

Here are three articles that explain what's going on in a lot more detail, and with some math if you want it.

http://www.eaa1000.av.org/technicl/perfspds/perfspds.htm

http://cafefoundation.org/v2/pdf_tech/MPG.engines/AIAA.1980.1847.B.H.Carson.pdf

http://www.flyingmag.com/very-best-speed-fly


PS - still waiting to hear how all of this applies to the Kennedy crash...

[MikeD]

This takes me back to my cross country gliding days.

See http://en.wikipedia.org/wiki/Lift-to-drag_ratio

Note the Glide ratio section. If you are still interested, read the link 'Water ballast' in that section.

Miked (U.K.)