-------------------------------------------
I don't know how many people have said that in the last 7 years
and then lived to regret it once they had their airplane flying :)
HP in an airplane is like money. You can never have too much
as long as it does not significantly increases the weight.
Once you change the six port flywheel to a auto tx flex plate there
is no difference in engine weight.
It is a safety thing. You can get in situations all the time
where you need more HP. Climbing over clouds in the flat lands
and climbing over mountains and clouds in the mountains are just
two examples. Taking off on a hot day at something other than
sea level is another. When you get two people and luggage in a
150 HP airplane you will wish you had more HP. A low HP airplane
limits your options.
Sure you get 150 HP at sea level and 67 degrees F but what about
5000 feet or 12,000 feet on a hot day. At 8000 feet you are
down to 120 HP.
You don't have to use the HP if you don't need it.
Drivabilty means nothing in an aircraft engine.
Mazda has a whole different set of requirements than we do. Torque
at less than 5000 RPM means nothing to us but it means everything
in a car engine. One misfire per minute at idle means a lot to
car emissions. It means nothing to us. Apples and Oranges.
Why are you putting a 2.85:1 gear box in there if you don't need
7000 RPM and HP?
Paul Lamar ...No rotor no motor.
Paul,
The reason Van's doesn't want people using engines with more power has to do
with flutter margins. More power or a turbocharger will let the plane reach
a higher true airspeed at altitude, even though indicated airspeed may still
be in the green.
At higher true airspeeds the control surfaces will be more susceptible to
flutter because it is the speed of the air, not the dynamic pressure that
counts when it comes to the excitations that set the vibration in motion.
Van's has an article on this on their website that makes a pretty good case
for staying within the recommended power range.
http://www.vansaircraft.com/pdf/hp_limts.pdf
While I agree that it's hard to have too much power, I also don't like the
idea of an airplane that can exceed flutter margins at altitude. You would
have to exercise real self-control every time you fly up there.
And Van's planes have pretty good power loading anyway -- RV9, even with 150
horsepower will have about 10 pounds per horsepower. That's a lot better
than a lot of small planes.
Regards,
Gordon.
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Yes he has been saying that for years but people like
Dave Anders have gone 256 MPH in his RV4. Tracy Saylor goes
238 MPH in his RV6. Tracy Crook goes 220 MPH. This is mainly
drag speaking. The Harmon Rocket is a six cyl. stuffed into an RV4.
It is not much faster but it sure does climb.
Also Tracy Crook has never had problems with cracks in the
RV tail surface. A lot of other people with Lyc's do. Has to due
with the low vibration levels of the rotary compared to
the Lyc. Engine vibration is also a factor in flutter.
HP has little to do with top speed. It mainly improves
takeoff and climb. Top speed is proportional to the cube
root of HP.
Van is also concerned with weight. A 150 HP A/C engine is lighter
than a 200 HP A/C engine. Van does not like people stuffing
heavier six cylinder engines in four cylinder airplanes.
If you have an engine that weighs 300 pounds no matter what
and it will put out 150 HP or 200 HP where is the big decision?
Nobody is forcing anyone to exceed vne. You exceed vne
at your own peril.
Paul Lamar ...No rotor no motor.
-----------------------------------------
Paul,
Here's one more link with info about how VNE relates to flutter:
http://www.auf.asn.au/groundschool/flutter.html
About a quarter of the way down the page in section 13.2, there is a chart
showing VNE speeds for the Pipstrel Sinus motorglider. Unlike most light
planes that have only one VNE, this little plane can go very high with its
long wings and Rotax turbocharged engine, so it's VNE decreases the higher
it goes.
Notice that VNE goes from 122 knots at zero density altitude to 80 knots at
26,300. That's a pretty big decrease.
The Van's article also has some tables that show how flutter margins
decrease with altitude for its planes.
Yes you could put a bigger engine in the plane and as long as you keep below
about 12,000 feet you should be okay because, as you point out you will not
see a big jump in speed. But if you go higher you will see more true
airspeed and you will have to reduce your VNE -- it would probably be a good
idea to make a placard and put it next to the airspeed indicator.
Regards,
Gordon.
Van is covering his butt just in case someone screws up :)
Paul Lamar
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Paul,
I think Van has come up with conservative flutter margins for his airplanes
just because people can and will screw up somehow, such as more paint than
necessary, etc.
However if you go high and fast, you need to adjust your VNE down. This
isn't an issue with most small planes because they don't go that high, and
they don't carry enough power to go very fast even if they could get up to
altitude. That's why most planes have one VNE speed and that's fine with the
FAA.
But for planes that can go fast and high, like the turbocharged Columbia
400, we see that the FAA has indeed slapped a lower VNE at high altitude.
The Columbia 400's VNE at 12,000 ft. is 235 knots. But at 25,000 feet it is
only 178 knots.
In addition the instrument panel is placarded to reduce VNE by 4.4 knots for
every 1,000 ft. above 12,000.
http://tinyurl.com/kahrj
(If that link doesn't work, I have also attached it as a pdf file.)
Columbia's engineering VP claims the lower VNE at altitude is due to
compressibility effects, and the FAR requirement for Mach number to be
constant in relation to the VN diagram altitude. So as altitude goes up,
Mach number increases at any given indicated airspeed.
In other words, if you fly the same indicated airspeed at 25,000 as you were
flying at 12,000 your Mach number will be a lot higher -- you are going a
lot faster true airspeed.
What Columbia doesn't say is why the higher true airspeed is not allowed,
but I'm guessing it is flutter margins. Structural strength issues are not
affected by true airspeed but by indicated airspeed, or more precisely
dynamic pressure. Columbia probably doesn't want to scare off people so they
aren't mentioning the word flutter.
They address the fact that other fast planes, including the turbocharged
Mooney Bravo, have only one set of speeds, but they claim it is because the
Columbia is so much faster that it pushes them into the rules used for
turboprops and jets. The VNE for the Bravo is 195 kt. at all altitudes up to
its ceiling of 25,000.
In any case, Van's airplanes are pretty slick and fast -- and if you give
them the kind of power that will take them high and fast, flutter margins
should be given some thought.
Regards,
Gordon.
As I lay me down to sleep I pray that Van will not let me hurt
myself. I do know how to hurt myself but please Van, don't let
me do it. Also while you are at it Van... please put a speed
governer on my car.
Paul Lamar ROFL
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