Today's Message Index:
----------------------
1. 04:01 AM - Re: drag conundrum (Thom Riddle)
2. 06:35 AM - Re: Re: drag conundrum (Dana Hague)
3. 06:54 AM - Re: drag conundrum (Thom Riddle)
4. 07:08 AM - Re: Re: drag conundrum (Jack B. Hart)
5. 09:13 AM - Re: Re: drag conundrum (Eugene Zimmerman)
6. 09:23 AM - Re: drag conundrum (Thom Riddle)
7. 10:52 AM - Re: Re: drag conundrum (robert bean)
8. 12:55 PM - Re: drag conundrum (John Hauck)
9. 02:26 PM - Re: Re: drag conundrum (John Hauck)
10. 03:23 PM - Re: Re: drag conundrum (Jack B. Hart)
11. 04:10 PM - Re: Re: drag conundrum (John Hauck)
12. 06:03 PM - Re: Re: drag conundrum (Jack B. Hart)
13. 07:13 PM - Re: Re: drag conundrum (John Hauck)
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Subject: | Re: drag conundrum |
Jack,
Thanks for your input.
I know and expected that the loss of drag at the lowest point of the aircraft in
flight will(did) necessitate the addition of nose down trim.
However your assumption that the CG would move forward with the loss of weight
from lighter tires is incorrect, since the main wheels are well forward of the
CG range. My empty weight CG location moved aft by .63 inches and my typical
loaded CG moved aft by .25 inches.
My yaw string behavior did not change.
I think that my fuel consumption rate is down after changing tire size but won't
be certain until after I've flown a few hours at my normal cruise rpm. My surprise
was that the same rpm did not result in higher airspeed at same fuel burn
rate, rather than reduced fuel consumption at the same airspeed. But the more
I think about it, I can see how reduced drag could have either result or some
combination of the two. Why one and not the other is still a mystery to me,
but if I get less fuel consumption at same speed, then my original goal has been
accomplished.
--------
Thom Riddle
Buffalo, NY (9G0)
Kolb Slingshot SS-021
Jabiru 2200A #1574
Tennessee Prop 64x32
I am always doing that which I can not do, in order that I may learn how to do
it.
- Pablo Picasso
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=299246#299246
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Subject: | Re: drag conundrum |
At 08:58 PM 5/28/2010, Thom Riddle wrote:
>Dana,
>
>As I read your post I was almost convinced you were right when you said...
>
>At the same rpm and the same airspeed, the engine should be putting out
>the same amount of power, since the same amount of air is moving through
>the prop, at the same speed.
>
>Then I started thinking about the amount of work being done. If the drag
>is less then the work being done by the prop at a certain rpm and
>traveling at the constant airspeed, must be less. Less work means less
>power and less fuel consumption. If I had a very precise vernier throttle
>with a precise scale, I would speculate that I'm getting the same airspeed
>with the same rpm but at a smaller throttle opening.
I almost thought your were right too, at first :)
Consider: At constant airspeed and constant rpm. the same amount of air is
flowing through the prop, at the same speed, as you say. This means a
constant blade AOA, thus constant blade drag, constant torque on the prop
shaft, and thus constant HP... and constant thrust. Change any one thing
and at least one other thing has to change; they're all interrelated.
If the drag is less, then the thrust is also less... but it can't be, under
the same conditions, as described above. Either the aircraft accelerates
until the drag increases until it equals thrust (which by this time will be
a little less since at constant RPM the blade AOA will decrease with
increasing airspeed), or you have to reduce thrust (RPM) until it equals
drag, to maintain a constant airspeed.
Less drag = go faster at the same rpm, or less thrust (less RPM) at the
same airspeed. 5% drag reduction gives 5% lower fuel consumption with a
reduced power setting at the same airspeed, but only 2.5% higher airspeed
at the same (original) power setting.
I still suspect in this case instrument error in the ASI or tach or both,
or varying atmospheric conditions, or all three.
-Dana
--
A .44 magnum is the world's only usable point-and click interface.
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Subject: | Re: drag conundrum |
Dana,
I agree with your explanation. Perhaps my measurement accuracy is off enough to
not be able to detect the change, whatever it is. As John Hauck noted, he could
not detect any difference in cruise speed when he changed from 600x6 to 800x6
tires. The change, though real, is probably within the measurement inaccuracy
range or differences in atmospheric conditions.
However, if over the next few hours of flight at my normal cruise rpm I see measurably
lower fuel consumption at similar density altitudes, then I'm inclined
toward thinking my throttle opening for this condition is less than with the
larger tires. The significantly lower drag has to show up somewhere.
I wish I had a fuel flow monitor or at least a manifold pressure gauge which is
a better proxy for power than rpm alone on a fixed pitch prop.
--------
Thom Riddle
Buffalo, NY (9G0)
Kolb Slingshot SS-021
Jabiru 2200A #1574
Tennessee Prop 64x32
I am always doing that which I can not do, in order that I may learn how to do
it.
- Pablo Picasso
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=299256#299256
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Subject: | Re: drag conundrum |
At 04:00 AM 5/29/10 -0700, you wrote:
>
>However your assumption that the CG would move forward with the loss of weight
from lighter tires is incorrect, since the main wheels are well forward of the
CG range. My empty weight CG location moved aft by .63 inches and my typical
loaded CG moved aft by .25 inches.
>
>My yaw string behavior did not change.
>
Thom,
In my case I have to plead a brain fart. Once in a while humble pie is good
for me.
When flying gliders, I used yaw strings. But one must remember that yaw
strings indicate the direction of the local air flow over a small portion of
the plane. In our case of a pusher powered aircraft with a little power
factor present, one side of the prop swing will give a greater bite that the
other side. Therefore the yaw string will indicate no slip when the plane is
actually slipping. Also, if you are used to using a yaw string and always
fly with the string centered, why would you expect for it to change
indication?
A ball slip indicator indicates what the total plane is doing. It might be
worth your time to borrow a ball indicator from someone and recheck your
trim.
I hope this makes better sense.
Jack B. Hart F004
Winchester, IN
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Subject: | Re: drag conundrum |
Hey guys,
It appears that you guys are not getting the the appropriate dose of
air-time to prevent the airplane hypochondria syndrome. ; )
|
_____|_____
*=======================R=======================*
\ / ^
\ /
( /---\ )
\___/
/ \
() ()
Eugene Zimmerman
On May 29, 2010, at 9:30 AM, Dana Hague wrote:
> At 08:58 PM 5/28/2010, Thom Riddle wrote:
>> Dana,
>>
>> As I read your post I was almost convinced you were right when you
>> said...
>>
>> At the same rpm and the same airspeed, the engine should be putting
>> out the same amount of power, since the same amount of air is
>> moving through the prop, at the same speed.
>>
>> Then I started thinking about the amount of work being done. If the
>> drag is less then the work being done by the prop at a certain rpm
>> and traveling at the constant airspeed, must be less. Less work
>> means less power and less fuel consumption. If I had a very precise
>> vernier throttle with a precise scale, I would speculate that I'm
>> getting the same airspeed with the same rpm but at a smaller
>> throttle opening.
>
> I almost thought your were right too, at first :)
>
> Consider: At constant airspeed and constant rpm. the same amount of
> air is flowing through the prop, at the same speed, as you say.
> This means a constant blade AOA, thus constant blade drag, constant
> torque on the prop shaft, and thus constant HP... and constant
> thrust. Change any one thing and at least one other thing has to
> change; they're all interrelated.
>
> If the drag is less, then the thrust is also less... but it can't
> be, under the same conditions, as described above. Either the
> aircraft accelerates until the drag increases until it equals thrust
> (which by this time will be a little less since at constant RPM the
> blade AOA will decrease with increasing airspeed), or you have to
> reduce thrust (RPM) until it equals drag, to maintain a constant
> airspeed.
>
> Less drag = go faster at the same rpm, or less thrust (less RPM) at
> the same airspeed. 5% drag reduction gives 5% lower fuel
> consumption with a reduced power setting at the same airspeed, but
> only 2.5% higher airspeed at the same (original) power setting.
>
> I still suspect in this case instrument error in the ASI or tach or
> both, or varying atmospheric conditions, or all three.
>
> -Dana
>
>
> --
> A .44 magnum is the world's only usable point-and click interface.
>
>
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Subject: | Re: drag conundrum |
Gene,
I'm on honey-do duty today but tomorrow BB and I are planning on meeting up and
fly to a not-very-distant b-fast, in a flight of two Kolbs, probably arriving
to great yawns and someone hollering "Look at them ultralights coming here.
Sure hope they know how to land without hitting my real airplane." That would
be from someone whose real airplane is not actually flying because it costs too
much to feed and maintain.
When is the last time you flew your Kolb to a fly-in and the crowds flocked around
the 8th C172 to land? I love Kolbing.
do not archive
--------
Thom Riddle
Buffalo, NY (9G0)
Kolb Slingshot SS-021
Jabiru 2200A #1574
Tennessee Prop 64x32
I am always doing that which I can not do, in order that I may learn how to do
it.
- Pablo Picasso
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=299274#299274
Message 7
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Subject: | Re: drag conundrum |
quite right Eugene. And I'm one of the deprived. Strange how a guy can
be retired with no responsibilities but a dog and
STILL doesn't get around to flying something he spent hours/weeks/years
building.
Thom, that new float valve appears to have done the trick.. No more
fluctuating idle. I reset the mixture and will live with
the 1400 rpm idle. (divide by 2.26 for prop) Should be good for
launch tomorrow morning.
re: prop rpm..... I don't know how many of you guys are so used to
grass that when you get on busy pavement, especially
if it's downhill slightly, that you wish your brakes were slightly
better :( If I get in a dicey situation I shut down the motah
to zero out any thrust. I usually like to park away from the crowd.
-and while I'm rambling here.... it sure bugs me when folks at a fly
in seem to think it's ok to open the door on your plane
and even let kids climb in. -with no invite. Lotta nerve.
Follow them back to their car and climb in the back seat while they're
leaving.
BB
grump mode off
do not archive
On 29, May 2010, at 12:13 PM, Eugene Zimmerman wrote:
> Hey guys,
>
> It appears that you guys are not getting the the appropriate dose of
air-time to prevent the airplane hypochondria syndrome. ; )
>
> |
> _____|_____
> *=======================R=
======================*
> \ / ^ \
/
> ( /---\ )
> \___/
> / \
> () ()
>
> Eugene Zimmerman
>
>
>
> On May 29, 2010, at 9:30 AM, Dana Hague wrote:
>
>> At 08:58 PM 5/28/2010, Thom Riddle wrote:
>>> Dana,
>>>
>>> As I read your post I was almost convinced you were right when you
said...
>>>
>>> At the same rpm and the same airspeed, the engine should be putting
out the same amount of power, since the same amount of air is moving
through the prop, at the same speed.
>>>
>>> Then I started thinking about the amount of work being done. If the
drag is less then the work being done by the prop at a certain rpm and
traveling at the constant airspeed, must be less. Less work means less
power and less fuel consumption. If I had a very precise vernier
throttle with a precise scale, I would speculate that I'm getting the
same airspeed with the same rpm but at a smaller throttle opening.
>>
>> I almost thought your were right too, at first :)
>>
>> Consider: At constant airspeed and constant rpm. the same amount of
air is flowing through the prop, at the same speed, as you say. This
means a constant blade AOA, thus constant blade drag, constant torque on
the prop shaft, and thus constant HP... and constant thrust. Change any
one thing and at least one other thing has to change; they're all
interrelated.
>>
>> If the drag is less, then the thrust is also less... but it can't be,
under the same conditions, as described above. Either the aircraft
accelerates until the drag increases until it equals thrust (which by
this time will be a little less since at constant RPM the blade AOA will
decrease with increasing airspeed), or you have to reduce thrust (RPM)
until it equals drag, to maintain a constant airspeed.
>>
>> Less drag = go faster at the same rpm, or less thrust (less RPM) at
the same airspeed. 5% drag reduction gives 5% lower fuel consumption
with a reduced power setting at the same airspeed, but only 2.5% higher
airspeed at the same (original) power setting.
>>
>> I still suspect in this case instrument error in the ASI or tach or
both, or varying atmospheric conditions, or all three.
>>
>> -Dana
>>
>>
>> --
>> A .44 magnum is the world's only usable point-and click interface.
>>
>>
>>
href="http://www.matronics.com/Navigator?Kolb-List">http://www.matronics
.com/Navigator?Kolb-List
>> href="http://forums.matronics.com">http://forums.matronics.com
>>
href="http://www.matronics.com/contribution">http://www.matronics.com/co
ntribution
>>
>
>
>
>
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Subject: | Re: drag conundrum |
roger/gang
save some fun for me.
i use the the yaw string to calibrate my slip/skid indicator. basic
adjustment can be made with airframe, on the ground in level flight
attitude, but the yaw string in flight is the instrument to fine tune
the slip/skid indicator.
typed left handed, index finger. ;-)
john h
Roger @ the Rock House
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Subject: | Re: drag conundrum |
jack,
Also, if you are used to using a yaw string and always
> fly with the string centered, why would you expect for it to change
> indication?
>
i don't know. why?
> A ball slip indicator indicates what the total plane is doing. It might
> be
> worth your time to borrow a ball indicator from someone and recheck your
> trim.
>
> Jack B. Hart F004
yaw string indicates relative wind. mount the string bottom center
windshield. i used a short piece of safety wire for standoff and no lateral
drag.
if you are concerned of prop influencing string, shut down the engine.
out of trim situations have insignificant effect on kolbs. i flew 17,400
mile flight in my mkiii at 1/2 ball out of trim. later doubled size of
rudder trim tab. mkiii flies ball centered, feet off the rudder pedals.
flew 10,000+ mile flight trimmed up, ball centered.
performance figures for both flights nearly identical.
yaw string, properly mounted, more accurate than slip/skid indicator.
take care,
john h
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Subject: | Re: drag conundrum |
At 04:25 PM 5/29/10 -0500, you wrote:
>
>jack,
>
> Also, if you are used to using a yaw string and always
>> fly with the string centered, why would you expect for it to change
>> indication?
>>
>
>
>i don't know. why?
John, I thought I was asking the question.
>
>
>yaw string indicates relative wind. mount the string bottom center
>windshield. i used a short piece of safety wire for standoff and no lateral
>drag.
>
>if you are concerned of prop influencing string, shut down the engine.
It is a little difficult to maintain altitude following this procedure.
>
>out of trim situations have insignificant effect on kolbs. i flew 17,400
>mile flight in my mkiii at 1/2 ball out of trim. later doubled size of
>rudder trim tab. mkiii flies ball centered, feet off the rudder pedals.
>flew 10,000+ mile flight trimmed up, ball centered.
>
>performance figures for both flights nearly identical.
>
Since you compensated for built in skid or slip by use of a tab on the
rudder, one would not expect to see any drag reduction or any performance
improvement. My Kolb FireFly, must be an exception, in that by trimming out
p-factor, and offsetting the thrust line to fly straight in level flight and
not using trim tabs, it has shown definite improvement.
>yaw string, properly mounted, more accurate than slip/skid indicator.
The slip/skid indicator indicates coordinated, lower drag, and more
economical flight. A yaw string indicates as you said relative wind and I
use it the most on taxiing out to detect what the cross wind is doing.
Jack B. Hart FF004
Winchester, IN
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Subject: | Re: drag conundrum |
> The slip/skid indicator indicates coordinated, lower drag, and more
> economical flight. A yaw string indicates as you said relative wind and I
> use it the most on taxiing out to detect what the cross wind is doing.
>
>
> Jack B. Hart FF004
> Winchester, IN
jack,
yaw string and slip/skid indicator provide identical information, whether
the aircraft is trimmed in yaw, is slipping or skidding.
when trimmed in yaw, the aircraft is trimmed into relative wind. when i
line up the string with the center line of the airframe, it indicates i am
trimmed in yaw.
how does a slip/skid indicator indicate coordinated, lower drag, and more
economical flight?
realize you were asking question. i don't have an answer. reason i asked
why.
only time i have ever been concerned with pitch trim was shooting 2,75 inch
rockets from the AH-1G Cobra in VN.
i use the wind sock when taxiing.
how precise is your instrumentation? probably same as mine.
a slip/skid indicator with ball centered indicates relative wind.
flying from my bed which needs a yaw string to keep me trimmed with the meds
i am taking. ;-)
john h
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Subject: | Re: drag conundrum |
At 06:09 PM 5/29/10 -0500, you wrote:
>
>
>jack,
>
>yaw string and slip/skid indicator provide identical information, whether
>the aircraft is trimmed in yaw, is slipping or skidding.
>
The yaw string only gives an indication of relative wind over one spot of
the aircraft. This works very well on a sailplane or glider, because one
does not have to counter propeller torque, thrust and gyroscopic effects.
When under power and if one trims for the most used or a desired flight
speed, one must trim the roll to counter the propeller torque and adjust the
horizontal stabilizer to establish hands off level flight. Under this
condition, the plane more than likely will skid due to the fact the roll
trim requires one wing to carry a little more load than the opposite side.
In this case the ball will be out. Before slapping a tab on the rudder to
bring it in, it is best to washer the engine to check for p-factor and
adjust to bring the ball in as far as it will go. Then what ever ball is
left can be brought in by sliding the thrust line side to side or a tab on
the rudder. To ignore the p-factor means that you are just tossing money
away due to inefficient use of thrust.
If you do all of the above and you want to check it with a yaw string, then
I will agree that the yaw string is as good as a slip/skid indicator.
>
>when trimmed in yaw, the aircraft is trimmed into relative wind. when i
>line up the string with the center line of the airframe, it indicates i am
>trimmed in yaw.
>
If you have not addressed cruise p-factor you will be using excessive trim
on the rudder and creating an overall higher drag condition.
>
>how does a slip/skid indicator indicate coordinated, lower drag, and more
>economical flight?
>
If the plane is properly trimmed out, the ball centered position will always
give the lowest drag configuration.
>
>i use the wind sock when taxiing.
>
Not all fields have wind socks.
>
Jack B. Hart FF004
Winchester, IN
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Subject: | Re: drag conundrum |
> The yaw string only gives an indication of relative wind over one spot of
> the aircraft.
that is all you need. the rest of the airplane is permanently connected to
that spot.
when you center the yaw string you are trimmed for yaw and roll.
when you center the ball, you are trimmed for yaw and roll.
both instruments accomplish equal roles.
p factor at cruise is probably not measureable.
how do you calibrate your slip/skid indicator?
what kind of slip/skid indicator do you have?
we have adverse yaw as the result of the way the prop wash hits the tail
section, especially high horse power and large prop combos. this is quite
evident on my mkiii. only the left side of the upper and lower vertical
stabilizers are hit. right sides stay clean.
top of left horizontal stab and elevator are also hit by prop wash. there
is a lot going on back there.
remember when you were going to tuft the tail section to see if prop wash
was hitting elevators? i knew it was and i recommended standing behind the
airplane with the engine running, stick your hand down there and feel the
air? lot quicker than tufting.
23 years ago i experimented with thrust line of my firestar. up, down,
left, and right. no change in performance, yaw, or pitch.
I have experimented with leading edge upper vertical stab, leading edges
horizontal stabs when i built and tested my mkiii 18 years ago. found the
sweet spot for horizontal stabs. vertical stab a waste of time.
i have an airplane that performs well throughout its entire performance
band.
for off site landings with no wind sock i use basic pilot skills to
determine wind direction. there's lots of ways to do that. if i had an
open cock pit, the wind on my face. if not, the airplane gives me very
definite feed back when taxiing.
john h still flying my bed, but no adverse yaw or roll.
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