Today's Message Index:
----------------------
1. 07:10 AM - Subject: changing props (why?) ()
2. 10:22 AM - Re: Subject: changing props (why?) (LessDragProd@aol.com)
3. 11:30 AM - re[2]: Subject: changing props (why?) (Greg Carnforth)
4. 05:12 PM - Re: changing props (why?) (why MT?) ()
5. 07:33 PM - Re: changing props (why?) (why MT?) ()
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| Subject: | changing props (why?) |
>" I initially fit my RV7 cowling for a fixed pitch Sensenich prop
and spinner and
>later upgrade to an MT?"
Once you have the Sensenich on your plane, flying, going real fast and having
fun, I know you will be hard justify a $8060 (two blade) or $9390 (three blade)
MT prop, plus the $1150 for prop governor and misc. parts. I know MT enthusiast
will tell you how smoother they are, no doubt, and if that is your only
criteria and want a constant speed prop, than the MT may be the way to go, but
you will go slower. A fixed pitch Sensenich is faster.
If I where in the market for constant speed prop I would put a Hartzell Blended
Airfoil PROP C2YR-1BF/F7496. The Hartzell IS about 8 mph faster at 8000 ft.
Despite claims this speed gap is closer at altitudes just does not stand-up.
The Hartzell will be faster (more efficient) at all altitudes.
Van's Aircraft did a test of the MT against the Hartzell (both the F7666 and
new Blended AF F7496). The blended airfoil was 8 mph faster. This was a controlled
test at 8000ft, 2500rpm, WOT, published in the RVator about a year and 1/2
ago. The Sensenich was only 1 mph slower than the Hartzell C2YK/F7666. So you
can expect to be about 7 mph faster than the MT.
MT dealer Jim at Less Drag did a flight test of 4 props (Hartzell 2-blade metal,
MT-12 (3-blade) composite, MT-15 (2-blade) metal and (2-blade) composite.
Jim tested these props at 5 altitudes and 4 RPMs (2300, 2400, 2500 and 2700).
This was an ambitious effort and in no way a put down, however I have major disagreement
with this data which Jim presents on his web site.
http://www.lessdrag.com/lycomingpropeller.html
Now here is some of Jim's data plotted either by prop model or by altitude.
http://img203.imageshack.us/my.php?image=jimspropdata2qe.jpg
Jim runs "Less Drag" and is a MT prop dealer and says MT prop's gets more efficient
at altitude, well that is true, but ALL props tend to get more efficient
with altitude (with in the normal range we fly at). A Hartzell, also gets more
efficient with altitude. In fact lets say the Hartzell is approx 0.80 efficient
at 8000ft, than at 12,500ft it's efficiency is .832. This is just a function
of air density, true airspeed changes, which effects all props. Prop efficiency
if affected by RPM, airspeed and air density. You are going slower at 12,500
because .....you are making less HP overall. It's a piston engine thing.
Also air density is less. So efficiency goes up a little because of these factors,
less power, less airspeed and less air density.
(BTW the .80 and .822 efficiency for the Hartzell is not a guess, it is derived
from actual data from Hartzell, for the C2YK/F7666-2 data and RV-7 aero data.)
Also Jim claims that at higher RPM speed will drop off. Well efficiency for all
props tends to drop with higher RPM at cruise. (Efficiency may increase with
RPM at slow air speeds, such as in a climb.) It's possible speed could drop,
but its very much unlikely from my experience and the actual Hartzell prop data.
When you increase engine 100 RPM power will increase about 2-5 HP. More HP
means more thrust and thus more speed. However it's true that ANY prop's efficiency
falls off a little with increased RPM (in cruise). In our case, even at
12,500' a 200 RPM increase will increase speed about 1%. HP alone results in
a speed increase of 1.45%, however prop efficiency drops about say 1/2%. So the
net gain is small, about 1%. Speed increases despite the props reduced efficiency,
because HP overrides the efficiency loss. At worst speed will remain almost
the same. These effects apply to ALL props.
Why is the MT slower. One is blade thickness. Since the blades are near supersonic
speeds (mach .70-.90) thicker blades are less efficient. Wood core composite
blades are thicker than metal blades. Second is number of blades. The only
reason to go to more blades is to absorb big HP on high speed planes (P-51,
C-130, Turboprops).
http://en.wikipedia.org/wiki/Propeller
To keep tip speed down, while having the needed prop area for the HP, designers
add more blades and reduce the diameter. Diameter is critical to prop tip speed
obviously. Designers try to minimize blade numbers for performance as much
as possible. In general more blades is less efficient (but look cool). When
you see 3, 4 or 6 blades there is a reason, big HP and high speeds.
In Jim's plotted data, you can see that there is "scatter" in the data. Also
the data at 12,500 ft shows the Hartzell drops 6 mph from 2500 to 2700 RPM!! That
is just not correct. First you will make about 5 hp more at going from 2500
to 2700 rpm. More HP means more thrust and thus speed. 5 hp will result in about
1.5% more speed. However more RPM at cruise speed will result in slight reduction
of prop efficiency, about 0.5%. So the net result will be a very small
increase in speed. NEVER a 6 mph drop. Also I am not sure most people fly 2700
rpm in cruise, except to race. I am into racing and believe me, I run full
high RPM. The fastest RV, Dave Anders 250 mph RV-4 was turning 2900 rpm, on his
Hartzell C2YK/F7666.
Some of Jim's results are invalid, no offense to Jim. Some data looks OK. Due
to the difficulty in doing flight test on different days and conditions, measuring
small speed changes consistently is hard to do. To Jim's credit he took
over 100 data points. He tried a wide range of conditions (altitude, RPMs) and
several props. However unfortunately inspection of the data plotted shows trend
lines crossing, large deviations, reversial of curve slopes (several time)
and no consistency between altitudes or RPMs. It just does not compare favorably
with the predicted theoretical data and experience of others, at lest for the
Hartzell.
Van only had to do one altitude (8000) and one RPM (2,500), which is representative of normal flight conditions. Also other builders have observed the same performance loss when switching from a Hartzell to MT. He also had the advantage of running two RV-8's side by side as a cross check. In the future it would be good to just pick the "standard" 8000 density altitude, at 2500 rpm and WOT (wide open throttle). Speed should be calculated with GPS ground speed and using constant TRACKS about 120 degrees apart. (A spread sheet on http://www.kilohotel.com/rv8/ can reduce the data for no wind ground speed.)
I am not anti-MT just anti-inaccurate data. I think builders should have
the accurate data to use. Jim's data does not test the Hartzell blended airfoil
which is 3.5 mph faster than the Hartzell C2YK/F7666. Jim does show at 2,500
feet, compared to the MTV-12, the C2YK/F7666 design is 6mph faster at 2300 rpm
and 4 mph faster at 2700 rpm. That is fine, but some how he got a total reversal
at 12,500 feet, where the MTV-12 is now 6 mph faster at 2700 and 4 mph faster
at 2300. This represents a total change of 10 mph? What changed? Air density,
engine HP and airspeed (TAS). Please that makes no sense and a 6% change?
You measure change in porp's in fractions of a present. 6% is crazy change, plus
at the intermediate altitudes Jim tested (5,000 / 7,500 / 10,000 ft) the trends
flop back and forth.
My only point is if you use a MT for smoothness be happy with that, than fine,
but the speed difference is a real factor and should not be discounted and you
should know what to realistically expect, about 8 mph across the board. Speed
difference between the HC and MT may be a little less at higher altitudes,
but you will not see a speed advantage, ever, with a MTV-12, with all do respect
to Jim's data. The Sensenich was only 1 mph slower than the Hartzell (F7666)
so you will be about 7 mph than the MT.
George
PS: You did not mention the engine you have, but with a stock O-360XP there are
NO restrictions. On Lycoming, stock or modified with electronic ignition:
1) above 22" manifold pressure below 2350 RPM is limited (not an operational issue);
2) above 2600 RPM is limited to takeoff, reduce after T/O (which is normal
operation anyway).
From: BTomm <fvalarm@rapidnet.net>
--> RV7-List message posted by: BTomm <fvalarm@rapidnet.net>
Can anyone tell me if there are cowling (or other ) issues if I
initially fit my RV7 cowling for a fixed pitch sensenich prop and spinner and
later upgrade to an MT? I suspect that the spinner and backing plate will
make for a difference in spacing from the cowling. I would want to build in
this difference so I don't have to modify the cowling later. Anybody
know? Bevan RV7A Fuse
---------------------------------
Message 2
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| Subject: | Re: changing props (why?) |
Hi George and All,
It amazes me, sometimes, on how much can be said just on an opinion, and a
scattering of engine and propeller theory.
Let's try taking one single point I had hoped to get across from the
empirical data obtained.
To quote you: "Van only had to do one altitude (8000) and one RPM (2,500),
which is representative of normal flight conditions."
Regardless of the propeller, my data shows that 2,500 RPM is NOT the best
propeller speed to obtain the best performance from the aircraft.
2,300 RPM was 4 mph slower. However, the aircraft was consuming 2 gph less
fuel.
What do you really want?
203 mph at 10 gph? Or 199 mph at 8 gph?
To put this into perspective, after flying for 3 hours at 199 mph, you will
have flown 12 miles less distance with 6 gallons of additional fuel on board.
2,500 RPM is an engine thing, not an aircraft system thing.
Best Regards,
Jim Ayers
PS I can hardly wait for your response. :-)
In a message dated 12/08/2005 8:25:46 AM Pacific Standard Time,
gmcjetpilot@yahoo.com writes:
>" I initially fit my RV7 cowling for a fixed pitch Sensenich prop and
spinner and
>later upgrade to an MT?"
Once you have the Sensenich on your plane, flying, going real fast and
having fun, I know you will be hard justify a $8060 (two blade) or $9390 (three
blade) MT prop, plus the $1150 for prop governor and misc. parts. I know MT
enthusiast will tell you how smoother they are, no doubt, and if that is your
only criteria and want a constant speed prop, than the MT may be the way to
go, but you will go slower. A fixed pitch Sensenich is faster.
If I where in the market for constant speed prop I would put a Hartzell
Blended Airfoil PROP C2YR-1BF/F7496. The Hartzell IS about 8 mph faste r at 8000
ft. Despite claims this speed gap is closer at altitudes just does not
stand-up. The Hartzell will be faster (more efficient) at all altitudes.
Van's Aircraft did a test of the MT against the Hartzell (both the F7666 and
new Blended AF F7496). The blended airfoil was 8 mph faster. This was a
controlled test at 8000ft, 2500rpm, WOT, published in the RVator about a year
and
1/2 ago. The Sensenich was only 1 mph slower than the Hartzell C2YK/F7666.
So you can expect to be about 7 mph faster than the MT.
MT dealer Jim at Less Drag did a flight test of 4 props (Hartzell 2-blade
metal, MT-12 (3-blade) composite, MT-15 (2-blade) metal and (2-blade)
composite. Jim tested these props at 5 altitudes and 4 RPMs (2300, 2400, 2500
and
2700). This was an ambitious effort and in no way a put down, however I have
major disagreement with this data which Jim presents on his web site.
_http://www.lessdrag.com/lycomingpropeller.html_
(http://www.lessdrag.com/lycomingpropeller.html)
Now here is some of Jim's data plotted either by prop model or by altitude.
_http://img203.imageshack.us/my.php?image=jimspropdata2qe.jpg_
(http://img203.imageshack.us/my.php?image=jimspropdata2qe.jpg)
Jim runs "Less Drag" and is a MT prop dealer and says MT prop's gets more
efficient at altitude, well that is true, but ALL props tend to get more
efficient with altitude (with in the normal range we fly at). A Hartzell, also
gets
more efficient with altitude. In fact lets say the Hartzell is approx 0.80
efficient at 8000ft, than at 12,500ft it's efficiency is .832. This is just a
function of air density, true airspeed changes, which effects all props. Pr
op efficiency if affected by RPM, airspeed and air density. You are going
slower at 12,500 because .....you are making less HP overall. It's a piston
engine thing. Also air density is less. So efficiency goes up a little because
of
these factors, less power, less airspeed and less air density.
(BTW the .80 and .822 efficiency for the Hartzell is not a guess, it is
derived from actual data from Hartzell, for the C2YK/F7666-2 data and RV-7 aero
data.)
Also Jim claims that at higher RPM speed will drop off. Well efficiency for
all props tends to drop with higher RPM at cruise. (Efficiency may increase
with RPM at slow air speeds, such as in a climb.) It's possible speed could
drop, but its very much unlikely from my experience and the actual Hartzell
prop data. When you increase engine 100 RPM power will increase about 2-5 HP.
More HP means more thrust and thus more speed. However it's true that ANY
prop's efficiency falls off a little with increased RPM (in cruise). In our case,
even at 12,500' a 200 RPM increase will increase speed about 1%. HP alone
results in a speed increase of 1.45%, however prop efficiency drops about say
1/2%. So the net gain is small, about 1%. Speed increases despite the props
reduced efficiency, because HP overrides the efficiency loss. At worst speed
will remain almost the same. These effects apply to ALL props.
Why is the MT slower. One is blade thickness. Since the blades are near
supersonic speeds (mach .70-.90) thicker blades are less efficient. Wood core
composite blades are thicker than metal blades. Second is number of blades. The
only reason to go to more blades is to absorb big HP on high speed planes
(P-51, C-130, Turboprops).
< DIV>_http://en.wikipedia.org/wiki/Propeller_
(http://en.wikipedia.org/wiki/Propeller)
To keep tip speed down, while having the needed prop area for the HP,
designers add more blades and reduce the diameter. Diameter is critical to prop
tip
speed obviously. Designers try to minimize blade numbers for performance as
much as possible. In general more blades is less efficient (but look cool).
When you see 3, 4 or 6 blades there is a reason, big HP and high speeds.
In Jim's plotted data, you can see that there is "scatter" in the data. Also
the data at 12,500 ft shows the Hartzell drops 6 mph from 2500 to 2700 RPM!!
That is just not correct. First you will make about 5 hp more at going from
2500 to 2700 rpm. More HP means more thrust and thus speed. 5 hp will result
in about 1.5% more speed. However more RPM at cruise speed will result in
slight reduction of prop efficiency, ab out 0.5%. So the net result will be a
very small increase in speed. NEVER a 6 mph drop. Also I am not sure most
people fly 2700 rpm in cruise, except to race. I am into racing and believe me,
I
run full high RPM. The fastest RV, Dave Anders 250 mph RV-4 was turning 2900
rpm, on his Hartzell C2YK/F7666.
Some of Jim's results are invalid, no offense to Jim. Some data looks OK.
Due to the difficulty in doing flight test on different days and conditions,
measuring small speed changes consistently is hard to do. To Jim's credit he
took over 100 data points. He tried a wide range of conditions (altitude, RPMs)
and several props. However unfortunately inspection of the data plotted
shows trend lines crossing, large deviations, reversial of curve slopes (several
time) and no consistency between altitudes or RPMs. It just does not compare
favorably with the predicted theoretical data and experience of oth ers, at
lest for the Hartzell.
Van only had to do one altitude (8000) and one RPM (2,500), which is
representative of normal flight conditions. Also other builders have observed
the
same performance loss when switching from a Hartzell to MT. He also had the
advantage of running two RV-8's side by side as a cross check. In the future
it
would be good to just pick the "standard" 8000 density altitude, at 2500 rpm
and WOT (wide open throttle). Speed should be calculated with GPS ground
speed and using constant TRACKS about 120 degrees apart. (A spread sheet on
_http://www.kilohotel.com/rv8/_ (http://www.kilohotel.com/rv8/) can reduce the
data for no wind ground speed.)
I am not anti-MT just anti-inaccurate data. I think builders should have the
accurate data to use. Jim's data does not test the Hartzell blended airfoil
which is 3.5 mph faster than the Hartzell C2YK/F7666. Ji m does show at 2,500
feet, compared to the MTV-12, the C2YK/F7666 design is 6mph faster at 2300
rpm and 4 mph faster at 2700 rpm. That is fine, but some how he got a total
reversal at 12,500 feet, where the MTV-12 is now 6 mph faster at 2700 and 4 mph
faster at 2300. This represents a total change of 10 mph? What changed? Air
density, engine HP and airspeed (TAS). Please that makes no sense and a 6%
change? You measure change in porp's in fractions of a present. 6% is crazy
change, plus at the intermediate altitudes Jim tested (5,000 / 7,500 / 10,000
ft) the trends flop back and forth.
My only point is if you use a MT for smoothness be happy with that, than
fine, but the speed difference is a real factor and should not be discounted and
you should know what to realistically expect, about 8 mph across the board.
Speed difference between the HC and MT may be a little less at higher
altitudes, but you will not se e a speed advantage, ever, with a MTV-12, with
all do
respect to Jim's data. The Sensenich was only 1 mph slower than the Hartzell
(F7666) so you will be about 7 mph than the MT.
George
PS: You did not mention the engine you have, but with a stock O-360XP there
are NO restrictions. On Lycoming, stock or modified with electronic ignition:
1) above 22" manifold pressure below 2350 RPM is limited (not an
operational issue); 2) above 2600 RPM is limited to takeoff, reduce after T/O
(which is
normal operation anyway).
--------------------------------
From: BTomm <_fvalarm@rapidnet.net_
(http://us.f300.mail.yahoo.com/ym/Compose?To=fvalarm@rapidnet.net&YY=83194&order=down&sort=date&pos=0&view=a&head=b) >
--> RV7-List message posted by: BTomm <_fvalarm@rapidnet.net_
(http://us.f300.mail.yahoo.com/ym/Compose?To=fvalarm@rapidnet.net&YY=83194&order=down&sort=da
te&pos=0&view=a&head=b) >
Can anyone tell me if there are cowling (or other ) issues if I
initially fit my RV7 cowling for a fixed pitch sensenich prop and spinner
and
later upgrade to an MT? I suspect that the spinner and backing plate will
make for a difference in spacing from the cowling. I would want to build in
this difference so I don't have to modify the cowling later. Anybody
know? Bevan RV7A Fuse
Message 3
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| Subject: | changing props (why?) |
--> RV7-List message posted by: Greg Carnforth <greg@chesterpools.com>
Whoo-Hoo
Here we go!
Greg Carnforth, Louisville RV7 Slow, just about to choose a prop.
Message 4
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| Subject: | Re: changing props (why?) (why MT?) |
>" I initially fit my RV7 cowling for a fixed pitch
Sensenich prop and spinner and
>later upgrade to an MT?"
Once you have the Sensenich on your plane, flying, going real fast and having
fun, I know it will be hard to justify a $8060 (two blade) or $9390 (three blade)
MT prop, plus the $1150 for prop governor and misc. parts. I know MT enthusiast
will tell you how smoother they are, no doubt, and if that is your only
criteria and want a constant speed prop, than the MT may be the way to go, but
you will go slower. A fixed pitch Sensenich will cruise is faster than a MT
by 7 mph.
If I where in the market for constant speed prop I would put a Hartzell Blended
Airfoil PROP C2YR-1BF/F7496. The ($5800) Hartzell IS 8 mph faster at 8000
ft. Despite claims that the MT to Hartzell speed gap is closer than that or less
at altitudes, it just doesn't or stand-up. The Hartzell will be faster (more
efficient) at all altitudes.
Van's Aircraft did a test of the MT against the Hartzell (both the F7666 and
new Blended AF F7496). The blended airfoil was 8 mph faster. This was a controlled
test at 8000ft, 2500rpm, WOT, published in the RVator about a year and 1/2
ago. The Sensenich was about 7 mph faster than the MT.
MT dealer Jim at Less Drag did a flight test of 4 props: Hartzell 2-blade metal,
MT-12 (3-blade) composite, MT-15 (2-blade) metal and MT-15 (2-blade) composite.
Jim tested these props at 5 altitudes and 4 RPMs (2300, 2400, 2500 and 2700)
on one RV, changing props. This was an ambitious effort and in no way a put
down, however I have major disagreement with this data which Jim presents on
his web site, and find too many discrepancies to be considered reliable or usable.
http://www.lessdrag.com/lycomingpropeller.html
Now here is some of Jim's data plotted either by prop model or by altitude.
http://img203.imageshack.us/my.php?image=jimspropdata2qe.jpg
Jim at "Less Drag" says MT prop's gets more efficient at altitude, well that
is true, but ALL props tend to get more efficient with altitude (with in the normal
range we fly at). A Hartzell, also gets more efficient with altitude on
a RV. This is just a function of air density, true airspeed changes, which effects
all props. Prop efficiency if affected by engine power, RPM, airspeed and
air density. Your TAS is slower at 12,500 because .....you are making less HP
overall. It's a piston non-turbo engine. Also air density is less. So efficiency
goes up a little because of these factors, less power, less airspeed and less
air density. Even though efficiency goes up speed goes down, because of HP;
however the % of HP converted to thrust is higher, thus more prop efficiency.
Also Jim claims that at higher RPM speed will drop off. Well efficiency for all
props tends to drop with higher RPM at cruise. Prop tips speeds are in the
area of +0.80 mach (mach = tip speed / speed of sound). Efficiency may increase
with RPM at slow air speeds, such as in a climb. It's possible speed could drop
RPM, but its very much unlikely from my experience and the actual Hartzell
prop data. When you increase engine 100 RPM power will increase about 2-5 HP.
More HP means more thrust and thus more speed. However it's true that ANY prop's
efficiency falls off a little with increased RPM (in cruise). In our operations
HP will outweigh the loss in prop efficiency. Remember we are talking about
speed not best gas mileage or range.
Why is the MT slower. One is blade thickness. Since the blades are near supersonic
speeds (mach .70-.90) thicker blades are less efficient. Wood core composite
blades are thicker than metal blades. Second is number of blades. The only
reason to go to more blades is to absorb big HP on high speed planes (P-51,
C-130, Turboprops).
http://en.wikipedia.org/wiki/Propeller
To keep tip speed down, while having the needed prop area for the HP, designers
add more blades and reduce the diameter. Diameter is critical to prop tip speed
obviously. Designers try to minimize blade numbers for performance as much
as possible. In general more blades is less efficient (but look cool). When
you see 3, 4 or 6 blades there is a reason, big HP and high speeds.
In Jim's plotted data, you can see that there is "scatter" in the data. Also
the data at 12,500 ft shows the Hartzell drops 6 mph from 2500 to 2700 RPM!! That
is just not correct. First you will make about 5 hp more at going from 2500
to 2700 rpm at 8,000 feet. More HP means more thrust and thus speed. 5 hp will
result in about 1.5% more speed. HP overrides the efficiency loss which is
about 1/2%. Jim in his data shows a 6 mph drop in speed? That is like speed brakes.
This is just not going to happen unless there is an engine problem and the
engine drops HP with RPM. May be a 100-200 RPM will produce negligible speed
increase but NEVER a 6 mph drop, may be no net change but not a drop, much less
6 mph.
Also I am not sure most people fly 2700 rpm in cruise, except to race. I am into
racing and believe me, I run full high RPM. The fastest RV, Dave Anders 250
mph RV-4 was turning 2900 rpm, on his Hartzell C2YK/F7666. Reno formula racers
with O200 continentals, turn into the high 3000's.
Some of Jim's results are invalid, no offense to Jim. Some data looks OK. Due
to the difficulty in doing flight test on different days and conditions, measuring
small speed changes consistently is hard to do. To Jim's credit he took
over 100 data points. He tried a wide range of conditions (altitude, RPMs) and
several props. However unfortunately inspection of the data plotted shows trend
lines crossing, large deviations, reversial of curve slopes (several time)
and no consistency between altitudes or RPMs. The data is faulty and of no real
use, except as a sales tool. It just does not compare favorably with the predicted
theoretical data and experience of others, at least for the Hartzell.
Van only concentrated on one altitude (8000) and one RPM (2,500), which is representative of more typical normal flight conditions. Also other builders have observed the same performance loss when switching from a Hartzell to MT. Several MT customers who value speed have switched back to Hartzell. http://www.lazy8.net/proptest.htm
Van also had the advantage an engineering degree/background and +35 years of aircraft design, flight test and pilot experience. plus a crew of talented pilots and mechanics. Also running two like RV-8's side-by-side as a cross check was an advantage. In the future it would be good for pilots/builders to just pick one test parameters, the "standard" 8000 density altitude, at 2500 rpm and WOT (wide open throttle) to compare performance. Speed should be calculated with GPS ground speed and using constant TRACKS about 120 degrees apart. (A spread sheet on http://www.kilohotel.com/rv8/ can reduce the data for no wind ground speed.)
I am not anti-MT just anti-inaccurate data. I think builders should have
the accurate data to use. Jim's data is not accurate. Also it does not show the
Hartzell blended airfoil which is 8 mph faster than the MT and the Hartzell
C2YK/F7666 is 4 MPH faster than the MT.
Jim does show at 2,500 feet, compared to the MTV-12, the C2YK/F7666 design is
6mph faster at 2300 rpm and 4 mph faster at 2700 RPM. At 7,500 ft Jim's data
shows the MTV-12 tied or 1 MPH slower from 2,300 to 2,700 RPM. That is fine,
but some how he got a total reversal at 12,500 feet, where the MTV-12 is now 6
mph faster at 2700 and 4 mph faster at 2300. This represents a total change of
10 mph? What changed? Air density, engine HP and airspeed (TAS). Please that
makes no sense a 6% change? You measure change in prop's by fractions of a percent
NOT 6%, which is crazy change; plus at the intermediate altitudes Jim tested
(5,000 / 7,500 / 10,000 ft) the trends between the C2YK and MTV-12 flop back
and forth? Makes no sense and does not match the design data, nor does it
match the flight experience of myself or others.
My only point is if you use a MT for smoothness be happy with that, than fine,
but the speed difference is a real factor and should not be discounted and you
should know what to realistically expect, about 8 mph across the board. Speed
difference between the HC and MT may be a little less at higher altitudes,
but you will not see a speed advantage, ever, with a MTV-12, with all do respect
to Jim's data. The Sensenich will be about 7 mph than the MT and cost $8,000
less!. The Hartzell is of course is over $2,000 cheaper and American made and
supported.
No matter how you wish it you can't have it all. Yes the MT is smoother, great,
but it comes at a cost, performance. As long as MT uses wood core composite
blades and chooses to market large diameter 3-bladed props to RV'ers they will
suffer less performance. The MT is great on a Pitt special where weight is more
important and cruise performance is not. "Total Performance" is the mission
RV planes are designed to. Don't get me wrong the German's are good engineers
and the workmanship is fine, just in my opinion the Hartzell or Sensenich is
a better value and fit on the RV's. Both Sensenich and Hartzell have invested
much effort in making products tailored to the RV's, designed and tested on RV's.
Both companies in development of their props have tested them on RV's. Hartzell
even tested their props with different engines (Lycoming, Superior, both
stock, modified and FADEC). Just make an informed choice.
George
PS: You did not mention the engine you have, but with a stock O-360XP there are
NO restrictions. On Lycoming, stock or modified with electronic ignition:
1) above 22" manifold pressure below 2350 RPM is limited (not an operational issue);
2) above 2600 RPM is limited to takeoff, reduce after T/O (which is normal
operation anyway).
From: BTomm <fvalarm@rapidnet.net>
--> RV7-List message posted by: BTomm <fvalarm@rapidnet.net>
Can anyone tell me if there are cowling (or other ) issues if I
initially fit my RV7 cowling for a fixed pitch sensenich prop and spinner and
later upgrade to an MT? I suspect that the spinner and backing plate will
make for a difference in spacing from the cowling. I would want to build in
this difference so I don't have to modify the cowling later. Anybody
know? Bevan RV7A Fuse
---------------------------------
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| Subject: | Re: changing props (why?) (why MT?) |
>" I initially fit my RV7 cowling for a fixed pitch
Sensenich prop and spinner and
>later upgrade to an MT?"
Once you have the Sensenich on your plane, flying, going real fast and having
fun, I know it will be hard to justify a $8060 (two blade) or $9390 (three blade)
MT prop, plus the $1150 for prop governor and misc. parts. I know MT enthusiast
will tell you how smoother they are, no doubt, and if that is your only
criteria and want a constant speed prop, than the MT may be the way to go, but
you will go slower. A fixed pitch Sensenich will cruise is faster than a MT
by 7 mph.
If I where in the market for constant speed prop I would put a Hartzell Blended
Airfoil PROP C2YR-1BF/F7496. The Hartzell IS 8 mph faster at 8000 ft. Despite
claims that the MT to Hartzell speed gap is closer than that or less at altitudes,
it just doesn't or stand-up. The Hartzell will be faster (more efficient)
at all altitudes.
Van's Aircraft did a test of the MT against the Hartzell (both the F7666 and
new Blended AF F7496). The blended airfoil was 8 mph faster. This was a controlled
test at 8000ft, 2500rpm, WOT, published in the RVator about a year and 1/2
ago. The Sensenich was about 7 mph faster than the MT.
MT dealer Jim at Less Drag did a flight test of 4 props: Hartzell 2-blade metal,
MT-12 (3-blade) composite, MT-15 (2-blade) metal and MT-15 (2-blade) composite.
Jim tested these props at 5 altitudes and 4 RPMs (2300, 2400, 2500 and 2700)
on one RV, changing props. This was an ambitious effort and in no way a put
down, however I have major disagreement with this data which Jim presents on
his web site, and find too many discrepancies to be considered reliable or usable.
http://www.lessdrag.com/lycomingpropeller.html
Now here is some of Jim's data plotted either by prop model or by altitude.
http://img203.imageshack.us/my.php?image=jimspropdata2qe.jpg
Jim at "Less Drag" says MT prop's gets more efficient at altitude, well that
is true, but ALL props tend to get more efficient with altitude (with in the normal
range we fly at). A Hartzell, also gets more efficient with altitude on
a RV. This is just a function of air density, true airspeed changes, which effects
all props. Prop efficiency if affected by engine power, RPM, airspeed and
air density. Your TAS is slower at 12,500 because .....you are making less HP
overall. It's a piston non-turbo engine. Also air density is less. So efficiency
goes up a little because of these factors, less power, less airspeed and less
air density. Even though efficiency goes up speed goes down, because of HP;
however the % of HP converted to thrust is higher, thus more prop efficiency.
Also Jim claims that at higher RPM speed will drop off. Well efficiency for all
props tends to drop with higher RPM at cruise. Prop tips speeds are in the
area of +0.80 mach (mach = tip speed / speed of sound). Efficiency may increase
with RPM at slow air speeds, such as in a climb. It's possible speed could drop
RPM, but its very much unlikely from my experience and the actual Hartzell
prop data. When you increase engine 100 RPM power will increase about 2-5 HP.
More HP means more thrust and thus more speed. However it's true that ANY prop's
efficiency falls off a little with increased RPM (in cruise). In our operations
HP will outweigh the loss in prop efficiency. Remember we are talking about
speed not best gas mileage or range.
Why is the MT slower. One is blade thickness. Since the blades are near supersonic
speeds (mach .70-.90) thicker blades are less efficient. Wood core composite
blades are thicker than metal blades. Second is number of blades. The only
reason to go to more blades is to absorb big HP on high speed planes (P-51,
C-130, Turboprops).
http://en.wikipedia.org/wiki/Propeller
To keep tip speed down, while having the needed prop area for the HP, designers
add more blades and reduce the diameter. Diameter is critical to prop tip speed
obviously. Designers try to minimize blade numbers for performance as much
as possible. In general more blades is less efficient (but look cool). When
you see 3, 4 or 6 blades there is a reason, big HP and high speeds.
In Jim's plotted data, you can see that there is "scatter" in the data. Also
the data at 12,500 ft shows the Hartzell drops 6 mph from 2500 to 2700 RPM!! That
is just not correct. First you will make about 5 hp more at going from 2500
to 2700 rpm at 8,000 feet. More HP means more thrust and thus speed. 5 hp will
result in about 1.5% more speed. HP overrides the efficiency loss which is
about 1/2%. Jim in his data shows a 6 mph drop in speed? That is like speed brakes.
This is just not going to happen unless there is an engine problem and the
engine drops HP with RPM. May be a 100-200 RPM will produce negligible speed
increase but NEVER a 6 mph drop, may be no net change but not a drop, much less
6 mph.
Also I am not sure most people fly 2700 rpm in cruise, except to race. I am into
racing and believe me, I run full high RPM. The fastest RV, Dave Anders 250
mph RV-4 was turning 2900 rpm, on his Hartzell C2YK/F7666. Reno formula racers
with O200 continentals, turn into the high 3000's.
Some of Jim's results are invalid, no offense to Jim. Some data looks OK. Due
to the difficulty in doing flight test on different days and conditions, measuring
small speed changes consistently is hard to do. To Jim's credit he took
over 100 data points. He tried a wide range of conditions (altitude, RPMs) and
several props. However unfortunately inspection of the data plotted shows trend
lines crossing, large deviations, reversial of curve slopes (several time)
and no consistency between altitudes or RPMs. The data is faulty and of no real
use, except as a sales tool. It just does not compare favorably with the predicted
theoretical data and experience of others, at least for the Hartzell.
Van only concentrated on one altitude (8000) and one RPM (2,500), which is representative of more typical normal flight conditions. Also other builders have observed the same performance loss when switching from a Hartzell to MT. Several MT customers who value speed have switched back to Hartzell. http://www.lazy8.net/proptest.htm
Van also had the advantage an engineering degree/background and +35 years of aircraft design, flight test and pilot experience. plus a crew of talented pilots and mechanics. Also running two like RV-8's side-by-side as a cross check was an advantage. In the future it would be good for pilots/builders to just pick one test parameters, the "standard" 8000 density altitude, at 2500 rpm and WOT (wide open throttle) to compare performance. Speed should be calculated with GPS ground speed and using constant TRACKS about 120 degrees apart. (A spread sheet on http://www.kilohotel.com/rv8/ can reduce the data for no wind ground speed.)
I am not anti-MT just anti-inaccurate data. I think builders should have
the accurate data to use. Jim's data is not accurate. Also it does not show the
Hartzell blended airfoil which is 8 mph faster than the MT and the Hartzell
C2YK/F7666 is 4 MPH faster than the MT.
Jim does show at 2,500 feet, compared to the MTV-12, the C2YK/F7666 design is
6mph faster at 2300 rpm and 4 mph faster at 2700 RPM. At 7,500 ft Jim's data
shows the MTV-12 tied or 1 MPH slower from 2,300 to 2,700 RPM. That is fine,
but some how he got a total reversal at 12,500 feet, where the MTV-12 is now 6
mph faster at 2700 and 4 mph faster at 2300. This represents a total change of
10 mph? What changed? Air density, engine HP and airspeed (TAS). Please that
makes no sense a 6% change? You measure change in prop's by fractions of a percent
NOT 6%, which is crazy change; plus at the intermediate altitudes Jim tested
(5,000 / 7,500 / 10,000 ft) the trends between the C2YK and MTV-12 flop back
and forth? Makes no sense and does not match the design data, nor does it
match the flight experience of myself or others.
My only point is if you use a MT for smoothness be happy with that, than fine,
but the speed difference is a real factor and should not be discounted and you
should know what to realistically expect, about 8 mph across the board. Speed
difference between the HC and MT may be a little less at higher altitudes,
but you will not see a speed advantage, ever, with a MTV-12, with all do respect
to Jim's data. The Sensenich will be about 7 mph than the MT and cost $8,000
less!. The Hartzell is of course is over $2,000 cheaper and American made and
supported.
No matter how you wish it you can't have it all. Yes the MT is smoother, great,
but it comes at a cost, performance. As long as MT uses wood core composite
blades and chooses to market large diameter 3-bladed props to RV'ers they will
suffer less performance. The MT is great on a Pitt special where weight is more
important and cruise performance is not. "Total Performance" is the mission
RV planes are designed to. Don't get me wrong the German's are good engineers
and the workmanship is fine, just in my opinion the Hartzell or Sensenich is
a better value and fit on the RV's. Both Sensenich and Hartzell have invested
much effort in making products tailored to the RV's, designed and tested on RV's.
Both companies in development of their props have tested them on RV's. Hartzell
even tested their props with different engines (Lycoming, Superior, both
stock, modified and FADEC). Just make an informed choice.
George
PS: You did not mention the engine you have, but with a stock O-360XP there are
NO restrictions. On Lycoming, stock or modified with electronic ignition:
1) above 22" manifold pressure below 2350 RPM is limited (not an operational issue);
2) above 2600 RPM is limited to takeoff, reduce after T/O (which is normal
operation anyway).
From: BTomm <fvalarm@rapidnet.net>
--> RV7-List message posted by: BTomm <fvalarm@rapidnet.net>
Can anyone tell me if there are cowling (or other ) issues if I
initially fit my RV7 cowling for a fixed pitch sensenich prop and spinner and
later upgrade to an MT? I suspect that the spinner and backing plate will
make for a difference in spacing from the cowling. I would want to build in
this difference so I don't have to modify the cowling later. Anybody
know? Bevan RV7A Fuse
---------------------------------
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