Aeronautical Engineering Help needed

David Lednicer wrote:

To get long range, you want to fly at a speed slower than maximum, near
or at the best L/D point.

snip

Hold it right there, pilgrim. Your premise, which forms the basis for
your entire post, fails the practicality test. When people talk about
the cruise range for aircraft such as a Long EZ or an RV-4, they are
not talking about lumbering along "near or at L/D max" (about 70 kt in
both the Long EZ and the RV-4). Rather, they are talking about the
range at cruise speeds (65% and 75% power at altitude). In a like
manor, the cruise range for piston powered aircraft is typically
specified at 65% and 75% power at altitude, not throttled way back to
max L/D speeds. In truth, the actual cruise range for the Long EZ and
RV-4 are practically identical given the same engine and same fuel
load. That's the difference between reality and an argument based
upon an inappropriate premise and CFD "analysis". The previous
poster's comment that "if you want good range don't choose a canard"
remains laughably absurd in both theory and practice, and his
subsequent post reveals his considerable grudge ax -- no surprise
there.

As for the Voyager, it didn't lumber along "near or at L/D max"
either. The average speed was 122 mph. I find your claim that a
non-canard Voyager would have had better range quite suspect. One
simply can not make such a determination by punching in a few what-if
scenarios into a CFD program, especially for such a highly specialized
aircraft. For example, the Voyager's canard forms a structural box
with the booms and the main wing. Remove the canard and you would
have to add significant structural weight elsewhere to obtain the same
airframe strength.

If a non-canard "Voyager" would indeed have greater range then I will
believe it when I hear it from Burt Rutan himself. I expect that any
realized range difference, one way or the other, would be quite small.
Yes, the new Rutan designed GlobalFlyer will not be a canard
configuration. That design choice, however, could be based solely on
the wishes Fossett/Branson rather than on technical considerations.
The authoritative answer to these questions will come in time but
certainly not here in Usenet (unless Burt himself decides to chime in
as in the old days).

David O -- http://www.AirplaneZone.com

David O wrote:
Hold it right there, pilgrim. Your premise, which forms the basis for
your entire post, fails the practicality test. When people talk about
the cruise range for aircraft such as a Long EZ or an RV-4, they are
not talking about lumbering along "near or at L/D max" (about 70 kt in
both the Long EZ and the RV-4). Rather, they are talking about the
range at cruise speeds (65% and 75% power at altitude). In a like
manor, the cruise range for piston powered aircraft is typically
specified at 65% and 75% power at altitude, not throttled way back to
max L/D speeds. In truth, the actual cruise range for the Long EZ and
RV-4 are practically identical given the same engine and same fuel
load. That's the difference between reality and an argument based
upon an inappropriate premise and CFD "analysis". The previous
poster's comment that "if you want good range don't choose a canard"
remains laughably absurd in both theory and practice, and his
subsequent post reveals his considerable grudge ax -- no surprise
there.

1) I purposely said "long range", not range. For long range, you do
slow down to near L/D max. This is not the result of a 'CFD
"analysis"', this is basic aero. I never mentioned CFD in my post - it
is just one tool that I use in my work.

2) For cruise range like you talk about, you're right, the Long EZ is
actually better than an RV-6. This is due to the Long EZ's low zero
lift drag and reasonable induced drag efficiency.

3) Calling me "Pilgrim" is technically incorrect. My mother's family
came to the Puritan Bay Colony ten years (1632) after the Mayflower
landed (1622).

As for the Voyager, it didn't lumber along "near or at L/D max"
either. The average speed was 122 mph. I find your claim that a
non-canard Voyager would have had better range quite suspect. One
simply can not make such a determination by punching in a few what-if
scenarios into a CFD program, especially for such a highly specialized
aircraft. For example, the Voyager's canard forms a structural box
with the booms and the main wing. Remove the canard and you would
have to add significant structural weight elsewhere to obtain the same
airframe strength.

1) No, the Voyager didn't lumber around near or at L/D max. Early in
the flight, it did, but then Dick got impatient. Average L/D on the
flight was near 22. My analysis (which has nothing to do with CFD)
shows the airplane to have a max L/D, at the average flight condition,
closer to 26.

2) Stop grinding your ax - I don't just "punch in a few what-if
scenarios into a CFD program". For highly specialized aircraft, CFD is
the only way to get a handle on such things as the stability and control
(Burt used a very crude code called Tanwing to design the VariEze,
Voyager, Long EZ, etc.). However, you need to do a lot more than run a
CFD code to analyze an aircraft.

3) Your average speed for Voyager is incorrect. They covered 24,986.73
statute miles in 216 hours, 3 minutes and 44 seconds. This works out to
115.6 mph ground speed. I have been told that this is a pretty good
approximation of the average true airspeed.

4) Yes, the Voyager benefited from the structural layout. However, it
is not the only way to skin the cat.

If a non-canard "Voyager" would indeed have greater range then I will
believe it when I hear it from Burt Rutan himself. I expect that any
realized range difference, one way or the other, would be quite small.
Yes, the new Rutan designed GlobalFlyer will not be a canard
configuration. That design choice, however, could be based solely on
the wishes Fossett/Branson rather than on technical considerations.
The authoritative answer to these questions will come in time but
certainly not here in Usenet (unless Burt himself decides to chime in
as in the old days).

Oh, so only Burt knows anything about designing airplanes. I guess the
rest of us aero engineers might as well go quit and go home. My
firsthnad experience is that Burt is a very skillful designer, but there
are many designers, equally skillful, who get little or no press.

David Lednicer wrote:

1) I purposely said "long range", not range. For long range, you do
slow down to near L/D max.

Your response to Richard's claim that canards have good range began
with, "I've got to disagree with your assertion that canards are good
for range." Others can judge for themselves whether Richard was
talking about cruise range or about range at max L/D. You already
know my take on both Richard's meaning and your response.

2) For cruise range like you talk about, you're right, the Long EZ is
actually better than an RV-6. This is due to the Long EZ's low zero
lift drag and reasonable induced drag efficiency.

Q.E.D.

3) Calling me "Pilgrim" is technically incorrect. My mother's family
came to the Puritan Bay Colony ten years (1632) after the Mayflower
landed (1622).

Relax, cowboy, it's just an expression.

3) Your average speed for Voyager is incorrect. They covered 24,986.73
statute miles in 216 hours, 3 minutes and 44 seconds. This works out to
115.6 mph ground speed. I have been told that this is a pretty good
approximation of the average true airspeed.

Your 115.6 mph is the FAI accredited average ground speed based on the
FAI accredited distance flown. It is not, however, the actual average
ground speed flown. The 122 mph figure I cited is the actual average
ground speed flown based on the actual distance flown (26,366 statute
miles).

Oh, so only Burt knows anything about designing airplanes.

No, but with all due respect, I would better trust Burt on this
particular subject.

David O -- http://www.AirplaneZone.com

And my family met Columbus at the dock. So what?

Jim



David O
shared these priceless pearls of wisdom:

-David Lednicer wrote:

-
-3) Calling me "Pilgrim" is technically incorrect. My mother's family
-came to the Puritan Bay Colony ten years (1632) after the Mayflower
-landed (1622).
-
-Relax, cowboy, it's just an expression.




Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com