Yes, I know Steve Sloan and his plane very well, and he is definitely a
master at drag reduction (and Rebel construction in general), no doubt about
it. I think it was a lot more than 7 mph improvement. I think he had his
Rebel going over 150 mph on wheels, with the 125 hp IO240 Conti. He also did
a lot of fairing work with his amphibs, which really improved that speed
also, but can't remember those numbers.
I do have the old style fiberglass top and bottom strut fairings on mine,
where many others have the aluminum ones. Perhaps that might help the speed
a bit on mine. Now that I think of it, I do know that the other 160 hp
amphib that I have mentioned here does not have any fairings at all on his
struts.
Also, not really sure what float angle does to drag. I have mine set at 2.5
right now, but would like to try 3. I thought I remember that someone
increased their speed slightly when they did that. That would make sense to
me, as I know floats create lift (which creates drag), and if you increase
the float angle (lower the bows), you would have less lift. This is a bit
like how our reflex works. They decrease lift and therefore drag, and you go
faster.
Walter
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Gary
Gustafson
Sent: January 20, 2009 5:59 PM
To: rebel-builders@dcsol.com
Subject: [rebel-builders] Drag Reduction
Thanks Walter for your further explanation of your comparison. I
can see where a 54 or a 56 pitch Sensenich prop is called for
with floats and that the recommendations provided by Sensenich
are for Rebels on wheels. I also agree that 2" can make a big
difference. Thanks!
I agree entirely with Walter that a side-by-side comparison is the
best way to compare performance. However, the following reference
material should give one a good idea as to where to reduce drag
(particularly if you don't want to land shorter) and get better
performance and more efficient operation at the same time.
Drag reduction can produce considerable gains in performance. The Rebel
is a "draggy" in its original design abd with bungee cords. It will
never be a high performance plane. If we wanted a high performance plane
we would have bought a Venture or an RV. Obviously we were looking for
other characteristics and made trade-offs. However, that does not mean
that improvements can not be made. I first became interested when I
read about Steve Sloan and the performance improvements that he claimed
to have achieved after making drag reduction modifications on his
Rebel. (I recall him saying that he saved over 1 gal an hour or
increased his speed by 7 mph) That intrigued me so I started reading
about drag with the following two articles being the most significant.
The subject is discussed in NACA Report No. 485 titled The Drag of
Airplane wheels, wheel fairings, and Landing Gears. It was written in
1935 (all aerodynamics research at that time is entirely applicable to
the Rebel and physics has not changed much since then) and a copy in pdf
format can be obtained at:
http://naca.central.cranfield.ac.uk/rep ... rt-485.pdf
Figures 24 and 25 indicate that the total drag reduction at 100 mph for
the landing gear design similar to that of the Rebel with coil spring
gear can be reduced from 98 lbs to 27 lbs and it provides information as
to the fairings, wheel pants and fillets that enable that reduction.
Remember that drag increases with the square of the velocity or going
From 100 mph to 120 mph one would have to multiply the figures by 1.44
to get the effect of the drag at the higher speed.
The second is a book which can be obtained at www.aerodrag.com for
free. Just click on down load whole book for free. In it, it explains
the various things that cause drag and the impact of that drag. Based
on these two reference items, I was able to determine the following (in
theory but most likely true). This fellow has based his findings on
actual airplanes and spent much of his life studying drag in airplanes.
Total Drag of the Rebel at 120 mph is 375 pounds
Drag associated with the gap between the flaperons is 7 pounds
Drag of coil spring gear is 94 pounds and can be reduced to 35 pounds
for a reduction 59 pounds,
Total drag of the tail wheel and cross bar is 28.8 pounds which can
be reduced to 18.2 pounds for a saving of 10.6 pounds.
Putting an aerodynamic shape on the two tail supports will save another
3 pounds in drag.
So these 4 items will save close to 80 pounds of drag. This is on the
conservative side a 20% reduction in drag (if the airfoil shapes are
exactly right). That means that your engine will be pulling 89 pounds
less drag through the air and that results in increased speed (7 miles
per hour) or lower fuel usage at the same speed (about 1.1 gph). You
can climb out faster (about 7%). You can glide further (sbout 10%).
Sounds interesting.
Now Walter is right. One has to prove it. After I get my plane flying
(shortly) I will get the numbers and then start to install the above
indicated modifications. I have not proved that this is true. However,
Steve Sloan indicates these ranges of improvements. I recommend that
those who are interested read the referenced article and book and
satisfy yourself that this is possible.
It might be interesting to have Steve Sloan chime in as to what he
actually was able to do.
Gary Gustafson Rebel N52GG (242R)
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Drew
Dalgleish
Sent: Tuesday, January 20, 2009 6:56 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Prop selection
Different prop makers use different methods for calculating pitch so it's
very hard to compare props just using those numbers. Interesting drag
calculations I had no idea that the spring gear made that much difference
to total drag. Sounds like using streamlined tube for the gear legs would
make a noticable difference.
Drew
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[rebel-builders] Drag Reduction
-
Gary Gustafson
[rebel-builders] Drag Reduction
The old style strut fairings are about equal to the later
aluminum sheet metal fairings because of the sharp fillet
between the fuselage and fairing and the wing and the fairing.
If one makes that sharp corner a fillet with a 1" radius, it
will be less than the older fiberglass strut fairings. No fairings
will create the most drag. One can get an idea as to the
relative magnitude of drag of various objects in the following
table.
http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html
Note that if each object has a frontal area of 1 inch the relative
amounts of drag for each. Round cables are normally considered
Cd of 0.45. Square bar stock is about the same drag as a flat plate.
As far as your floats, the float angle is a trade off between
increased lift and drag. Since the shapes are slightly different
you would need to contact the manufacturer and find out what
aerodynamic studies they have done as to the best trade off.
What I have read about floats is " Place the beginning of the step just
behind your CG, and when the aircraft is floating in the water ensure the
rear of the floats are NOT submerged. If they are, slide the floats back
until the rear of the floats are above the water (or opt for a longer set).
Sliding them back will reduce your frontal area, and decrease pitch
stability on the water, so slide them the minimum you need to so your tail
is safely out of the water. You should be able to tap your tail and have it
return to its position without taking a dip. If the tail tap fails (the tail
hits the water and remains) then you should seriously consider a longer set
of floats. Ensure your hard point mountings are stiff and reliable. For
heavier aircraft consider inserting vertical wooden pins or similar devices
to increase the shear strength of your float hard points. The floats must
also be level with your wing (zero Angle of Attack (AoA)) or with a slight
1-3 degree AoA (achieved by the rear float strut being slightly shorter than
the main strut)."
However, about 1/2 half of the drag of floats on a plane are in the struts,
cross members, and exposed cables. If Steve reduced the drag of his floats,
I am sure that he paid quite a bit of attention to eliminating the drag
of the cross struts and cables as well as getting the proper angle of the
floats.
If you know Steve Sloan, maybe you could get him to comment on exactly what
he did and lists the results of these drag reductions. I am sure that will
get a number of Rebel builders thinking about drag reduction as a way of
making drag reduction improvements to their planes. Probably the next step
after they get their plane flying.
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Walter
Klatt
Sent: Tuesday, January 20, 2009 9:23 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Drag Reduction
Yes, I know Steve Sloan and his plane very well, and he is definitely a
master at drag reduction (and Rebel construction in general), no doubt about
it. I think it was a lot more than 7 mph improvement. I think he had his
Rebel going over 150 mph on wheels, with the 125 hp IO240 Conti. He also did
a lot of fairing work with his amphibs, which really improved that speed
also, but can't remember those numbers.
I do have the old style fiberglass top and bottom strut fairings on mine,
where many others have the aluminum ones. Perhaps that might help the speed
a bit on mine. Now that I think of it, I do know that the other 160 hp
amphib that I have mentioned here does not have any fairings at all on his
struts.
Also, not really sure what float angle does to drag. I have mine set at 2.5
right now, but would like to try 3. I thought I remember that someone
increased their speed slightly when they did that. That would make sense to
me, as I know floats create lift (which creates drag), and if you increase
the float angle (lower the bows), you would have less lift. This is a bit
like how our reflex works. They decrease lift and therefore drag, and you go
faster.
Walter
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Gary
Gustafson
Sent: January 20, 2009 5:59 PM
To: rebel-builders@dcsol.com
Subject: [rebel-builders] Drag Reduction
Thanks Walter for your further explanation of your comparison. I
can see where a 54 or a 56 pitch Sensenich prop is called for
with floats and that the recommendations provided by Sensenich
are for Rebels on wheels. I also agree that 2" can make a big
difference. Thanks!
I agree entirely with Walter that a side-by-side comparison is the
best way to compare performance. However, the following reference
material should give one a good idea as to where to reduce drag
(particularly if you don't want to land shorter) and get better
performance and more efficient operation at the same time.
Drag reduction can produce considerable gains in performance. The Rebel
is a "draggy" in its original design abd with bungee cords. It will
never be a high performance plane. If we wanted a high performance plane
we would have bought a Venture or an RV. Obviously we were looking for
other characteristics and made trade-offs. However, that does not mean
that improvements can not be made. I first became interested when I
read about Steve Sloan and the performance improvements that he claimed
to have achieved after making drag reduction modifications on his
Rebel. (I recall him saying that he saved over 1 gal an hour or
increased his speed by 7 mph) That intrigued me so I started reading
about drag with the following two articles being the most significant.
The subject is discussed in NACA Report No. 485 titled The Drag of
Airplane wheels, wheel fairings, and Landing Gears. It was written in
1935 (all aerodynamics research at that time is entirely applicable to
the Rebel and physics has not changed much since then) and a copy in pdf
format can be obtained at:
http://naca.central.cranfield.ac.uk/rep ... rt-485.pdf
Figures 24 and 25 indicate that the total drag reduction at 100 mph for
the landing gear design similar to that of the Rebel with coil spring
gear can be reduced from 98 lbs to 27 lbs and it provides information as
to the fairings, wheel pants and fillets that enable that reduction.
Remember that drag increases with the square of the velocity or going
From 100 mph to 120 mph one would have to multiply the figures by 1.44
to get the effect of the drag at the higher speed.
The second is a book which can be obtained at www.aerodrag.com for
free. Just click on down load whole book for free. In it, it explains
the various things that cause drag and the impact of that drag. Based
on these two reference items, I was able to determine the following (in
theory but most likely true). This fellow has based his findings on
actual airplanes and spent much of his life studying drag in airplanes.
Total Drag of the Rebel at 120 mph is 375 pounds
Drag associated with the gap between the flaperons is 7 pounds
Drag of coil spring gear is 94 pounds and can be reduced to 35 pounds
for a reduction 59 pounds,
Total drag of the tail wheel and cross bar is 28.8 pounds which can
be reduced to 18.2 pounds for a saving of 10.6 pounds.
Putting an aerodynamic shape on the two tail supports will save another
3 pounds in drag.
So these 4 items will save close to 80 pounds of drag. This is on the
conservative side a 20% reduction in drag (if the airfoil shapes are
exactly right). That means that your engine will be pulling 89 pounds
less drag through the air and that results in increased speed (7 miles
per hour) or lower fuel usage at the same speed (about 1.1 gph). You
can climb out faster (about 7%). You can glide further (sbout 10%).
Sounds interesting.
Now Walter is right. One has to prove it. After I get my plane flying
(shortly) I will get the numbers and then start to install the above
indicated modifications. I have not proved that this is true. However,
Steve Sloan indicates these ranges of improvements. I recommend that
those who are interested read the referenced article and book and
satisfy yourself that this is possible.
It might be interesting to have Steve Sloan chime in as to what he
actually was able to do.
Gary Gustafson Rebel N52GG (242R)
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Drew
Dalgleish
Sent: Tuesday, January 20, 2009 6:56 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Prop selection
Different prop makers use different methods for calculating pitch so it's
very hard to compare props just using those numbers. Interesting drag
calculations I had no idea that the spring gear made that much difference
to total drag. Sounds like using streamlined tube for the gear legs would
make a noticable difference.
Drew
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aluminum sheet metal fairings because of the sharp fillet
between the fuselage and fairing and the wing and the fairing.
If one makes that sharp corner a fillet with a 1" radius, it
will be less than the older fiberglass strut fairings. No fairings
will create the most drag. One can get an idea as to the
relative magnitude of drag of various objects in the following
table.
http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html
Note that if each object has a frontal area of 1 inch the relative
amounts of drag for each. Round cables are normally considered
Cd of 0.45. Square bar stock is about the same drag as a flat plate.
As far as your floats, the float angle is a trade off between
increased lift and drag. Since the shapes are slightly different
you would need to contact the manufacturer and find out what
aerodynamic studies they have done as to the best trade off.
What I have read about floats is " Place the beginning of the step just
behind your CG, and when the aircraft is floating in the water ensure the
rear of the floats are NOT submerged. If they are, slide the floats back
until the rear of the floats are above the water (or opt for a longer set).
Sliding them back will reduce your frontal area, and decrease pitch
stability on the water, so slide them the minimum you need to so your tail
is safely out of the water. You should be able to tap your tail and have it
return to its position without taking a dip. If the tail tap fails (the tail
hits the water and remains) then you should seriously consider a longer set
of floats. Ensure your hard point mountings are stiff and reliable. For
heavier aircraft consider inserting vertical wooden pins or similar devices
to increase the shear strength of your float hard points. The floats must
also be level with your wing (zero Angle of Attack (AoA)) or with a slight
1-3 degree AoA (achieved by the rear float strut being slightly shorter than
the main strut)."
However, about 1/2 half of the drag of floats on a plane are in the struts,
cross members, and exposed cables. If Steve reduced the drag of his floats,
I am sure that he paid quite a bit of attention to eliminating the drag
of the cross struts and cables as well as getting the proper angle of the
floats.
If you know Steve Sloan, maybe you could get him to comment on exactly what
he did and lists the results of these drag reductions. I am sure that will
get a number of Rebel builders thinking about drag reduction as a way of
making drag reduction improvements to their planes. Probably the next step
after they get their plane flying.
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Walter
Klatt
Sent: Tuesday, January 20, 2009 9:23 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Drag Reduction
Yes, I know Steve Sloan and his plane very well, and he is definitely a
master at drag reduction (and Rebel construction in general), no doubt about
it. I think it was a lot more than 7 mph improvement. I think he had his
Rebel going over 150 mph on wheels, with the 125 hp IO240 Conti. He also did
a lot of fairing work with his amphibs, which really improved that speed
also, but can't remember those numbers.
I do have the old style fiberglass top and bottom strut fairings on mine,
where many others have the aluminum ones. Perhaps that might help the speed
a bit on mine. Now that I think of it, I do know that the other 160 hp
amphib that I have mentioned here does not have any fairings at all on his
struts.
Also, not really sure what float angle does to drag. I have mine set at 2.5
right now, but would like to try 3. I thought I remember that someone
increased their speed slightly when they did that. That would make sense to
me, as I know floats create lift (which creates drag), and if you increase
the float angle (lower the bows), you would have less lift. This is a bit
like how our reflex works. They decrease lift and therefore drag, and you go
faster.
Walter
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Gary
Gustafson
Sent: January 20, 2009 5:59 PM
To: rebel-builders@dcsol.com
Subject: [rebel-builders] Drag Reduction
Thanks Walter for your further explanation of your comparison. I
can see where a 54 or a 56 pitch Sensenich prop is called for
with floats and that the recommendations provided by Sensenich
are for Rebels on wheels. I also agree that 2" can make a big
difference. Thanks!
I agree entirely with Walter that a side-by-side comparison is the
best way to compare performance. However, the following reference
material should give one a good idea as to where to reduce drag
(particularly if you don't want to land shorter) and get better
performance and more efficient operation at the same time.
Drag reduction can produce considerable gains in performance. The Rebel
is a "draggy" in its original design abd with bungee cords. It will
never be a high performance plane. If we wanted a high performance plane
we would have bought a Venture or an RV. Obviously we were looking for
other characteristics and made trade-offs. However, that does not mean
that improvements can not be made. I first became interested when I
read about Steve Sloan and the performance improvements that he claimed
to have achieved after making drag reduction modifications on his
Rebel. (I recall him saying that he saved over 1 gal an hour or
increased his speed by 7 mph) That intrigued me so I started reading
about drag with the following two articles being the most significant.
The subject is discussed in NACA Report No. 485 titled The Drag of
Airplane wheels, wheel fairings, and Landing Gears. It was written in
1935 (all aerodynamics research at that time is entirely applicable to
the Rebel and physics has not changed much since then) and a copy in pdf
format can be obtained at:
http://naca.central.cranfield.ac.uk/rep ... rt-485.pdf
Figures 24 and 25 indicate that the total drag reduction at 100 mph for
the landing gear design similar to that of the Rebel with coil spring
gear can be reduced from 98 lbs to 27 lbs and it provides information as
to the fairings, wheel pants and fillets that enable that reduction.
Remember that drag increases with the square of the velocity or going
From 100 mph to 120 mph one would have to multiply the figures by 1.44
to get the effect of the drag at the higher speed.
The second is a book which can be obtained at www.aerodrag.com for
free. Just click on down load whole book for free. In it, it explains
the various things that cause drag and the impact of that drag. Based
on these two reference items, I was able to determine the following (in
theory but most likely true). This fellow has based his findings on
actual airplanes and spent much of his life studying drag in airplanes.
Total Drag of the Rebel at 120 mph is 375 pounds
Drag associated with the gap between the flaperons is 7 pounds
Drag of coil spring gear is 94 pounds and can be reduced to 35 pounds
for a reduction 59 pounds,
Total drag of the tail wheel and cross bar is 28.8 pounds which can
be reduced to 18.2 pounds for a saving of 10.6 pounds.
Putting an aerodynamic shape on the two tail supports will save another
3 pounds in drag.
So these 4 items will save close to 80 pounds of drag. This is on the
conservative side a 20% reduction in drag (if the airfoil shapes are
exactly right). That means that your engine will be pulling 89 pounds
less drag through the air and that results in increased speed (7 miles
per hour) or lower fuel usage at the same speed (about 1.1 gph). You
can climb out faster (about 7%). You can glide further (sbout 10%).
Sounds interesting.
Now Walter is right. One has to prove it. After I get my plane flying
(shortly) I will get the numbers and then start to install the above
indicated modifications. I have not proved that this is true. However,
Steve Sloan indicates these ranges of improvements. I recommend that
those who are interested read the referenced article and book and
satisfy yourself that this is possible.
It might be interesting to have Steve Sloan chime in as to what he
actually was able to do.
Gary Gustafson Rebel N52GG (242R)
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Drew
Dalgleish
Sent: Tuesday, January 20, 2009 6:56 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Prop selection
Different prop makers use different methods for calculating pitch so it's
very hard to compare props just using those numbers. Interesting drag
calculations I had no idea that the spring gear made that much difference
to total drag. Sounds like using streamlined tube for the gear legs would
make a noticable difference.
Drew
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-
Walter Klatt
[rebel-builders] Drag Reduction
Steve has talked about his drag reduction mods on this list, and comes on
once in a while. I see him pretty well every year down at Arlington. He and
Dick (the 210 hp Conti 360 Rebel) put on a great BBQ for us Murphy plane
folks the last couple years.
Walter
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Gary
Gustafson
Sent: January 20, 2009 7:01 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Drag Reduction
The old style strut fairings are about equal to the later
aluminum sheet metal fairings because of the sharp fillet
between the fuselage and fairing and the wing and the fairing.
If one makes that sharp corner a fillet with a 1" radius, it
will be less than the older fiberglass strut fairings. No fairings
will create the most drag. One can get an idea as to the
relative magnitude of drag of various objects in the following
table.
http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html
Note that if each object has a frontal area of 1 inch the relative
amounts of drag for each. Round cables are normally considered
Cd of 0.45. Square bar stock is about the same drag as a flat plate.
As far as your floats, the float angle is a trade off between
increased lift and drag. Since the shapes are slightly different
you would need to contact the manufacturer and find out what
aerodynamic studies they have done as to the best trade off.
What I have read about floats is " Place the beginning of the step just
behind your CG, and when the aircraft is floating in the water ensure the
rear of the floats are NOT submerged. If they are, slide the floats back
until the rear of the floats are above the water (or opt for a longer set).
Sliding them back will reduce your frontal area, and decrease pitch
stability on the water, so slide them the minimum you need to so your tail
is safely out of the water. You should be able to tap your tail and have it
return to its position without taking a dip. If the tail tap fails (the tail
hits the water and remains) then you should seriously consider a longer set
of floats. Ensure your hard point mountings are stiff and reliable. For
heavier aircraft consider inserting vertical wooden pins or similar devices
to increase the shear strength of your float hard points. The floats must
also be level with your wing (zero Angle of Attack (AoA)) or with a slight
1-3 degree AoA (achieved by the rear float strut being slightly shorter than
the main strut)."
However, about 1/2 half of the drag of floats on a plane are in the struts,
cross members, and exposed cables. If Steve reduced the drag of his floats,
I am sure that he paid quite a bit of attention to eliminating the drag
of the cross struts and cables as well as getting the proper angle of the
floats.
If you know Steve Sloan, maybe you could get him to comment on exactly what
he did and lists the results of these drag reductions. I am sure that will
get a number of Rebel builders thinking about drag reduction as a way of
making drag reduction improvements to their planes. Probably the next step
after they get their plane flying.
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Walter
Klatt
Sent: Tuesday, January 20, 2009 9:23 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Drag Reduction
Yes, I know Steve Sloan and his plane very well, and he is definitely a
master at drag reduction (and Rebel construction in general), no doubt about
it. I think it was a lot more than 7 mph improvement. I think he had his
Rebel going over 150 mph on wheels, with the 125 hp IO240 Conti. He also did
a lot of fairing work with his amphibs, which really improved that speed
also, but can't remember those numbers.
I do have the old style fiberglass top and bottom strut fairings on mine,
where many others have the aluminum ones. Perhaps that might help the speed
a bit on mine. Now that I think of it, I do know that the other 160 hp
amphib that I have mentioned here does not have any fairings at all on his
struts.
Also, not really sure what float angle does to drag. I have mine set at 2.5
right now, but would like to try 3. I thought I remember that someone
increased their speed slightly when they did that. That would make sense to
me, as I know floats create lift (which creates drag), and if you increase
the float angle (lower the bows), you would have less lift. This is a bit
like how our reflex works. They decrease lift and therefore drag, and you go
faster.
Walter
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Gary
Gustafson
Sent: January 20, 2009 5:59 PM
To: rebel-builders@dcsol.com
Subject: [rebel-builders] Drag Reduction
Thanks Walter for your further explanation of your comparison. I
can see where a 54 or a 56 pitch Sensenich prop is called for
with floats and that the recommendations provided by Sensenich
are for Rebels on wheels. I also agree that 2" can make a big
difference. Thanks!
I agree entirely with Walter that a side-by-side comparison is the
best way to compare performance. However, the following reference
material should give one a good idea as to where to reduce drag
(particularly if you don't want to land shorter) and get better
performance and more efficient operation at the same time.
Drag reduction can produce considerable gains in performance. The Rebel
is a "draggy" in its original design abd with bungee cords. It will
never be a high performance plane. If we wanted a high performance plane
we would have bought a Venture or an RV. Obviously we were looking for
other characteristics and made trade-offs. However, that does not mean
that improvements can not be made. I first became interested when I
read about Steve Sloan and the performance improvements that he claimed
to have achieved after making drag reduction modifications on his
Rebel. (I recall him saying that he saved over 1 gal an hour or
increased his speed by 7 mph) That intrigued me so I started reading
about drag with the following two articles being the most significant.
The subject is discussed in NACA Report No. 485 titled The Drag of
Airplane wheels, wheel fairings, and Landing Gears. It was written in
1935 (all aerodynamics research at that time is entirely applicable to
the Rebel and physics has not changed much since then) and a copy in pdf
format can be obtained at:
http://naca.central.cranfield.ac.uk/rep ... rt-485.pdf
Figures 24 and 25 indicate that the total drag reduction at 100 mph for
the landing gear design similar to that of the Rebel with coil spring
gear can be reduced from 98 lbs to 27 lbs and it provides information as
to the fairings, wheel pants and fillets that enable that reduction.
Remember that drag increases with the square of the velocity or going
From 100 mph to 120 mph one would have to multiply the figures by 1.44
to get the effect of the drag at the higher speed.
The second is a book which can be obtained at www.aerodrag.com for
free. Just click on down load whole book for free. In it, it explains
the various things that cause drag and the impact of that drag. Based
on these two reference items, I was able to determine the following (in
theory but most likely true). This fellow has based his findings on
actual airplanes and spent much of his life studying drag in airplanes.
Total Drag of the Rebel at 120 mph is 375 pounds
Drag associated with the gap between the flaperons is 7 pounds
Drag of coil spring gear is 94 pounds and can be reduced to 35 pounds
for a reduction 59 pounds,
Total drag of the tail wheel and cross bar is 28.8 pounds which can
be reduced to 18.2 pounds for a saving of 10.6 pounds.
Putting an aerodynamic shape on the two tail supports will save another
3 pounds in drag.
So these 4 items will save close to 80 pounds of drag. This is on the
conservative side a 20% reduction in drag (if the airfoil shapes are
exactly right). That means that your engine will be pulling 89 pounds
less drag through the air and that results in increased speed (7 miles
per hour) or lower fuel usage at the same speed (about 1.1 gph). You
can climb out faster (about 7%). You can glide further (sbout 10%).
Sounds interesting.
Now Walter is right. One has to prove it. After I get my plane flying
(shortly) I will get the numbers and then start to install the above
indicated modifications. I have not proved that this is true. However,
Steve Sloan indicates these ranges of improvements. I recommend that
those who are interested read the referenced article and book and
satisfy yourself that this is possible.
It might be interesting to have Steve Sloan chime in as to what he
actually was able to do.
Gary Gustafson Rebel N52GG (242R)
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Drew
Dalgleish
Sent: Tuesday, January 20, 2009 6:56 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Prop selection
Different prop makers use different methods for calculating pitch so it's
very hard to compare props just using those numbers. Interesting drag
calculations I had no idea that the spring gear made that much difference
to total drag. Sounds like using streamlined tube for the gear legs would
make a noticable difference.
Drew
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once in a while. I see him pretty well every year down at Arlington. He and
Dick (the 210 hp Conti 360 Rebel) put on a great BBQ for us Murphy plane
folks the last couple years.
Walter
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Gary
Gustafson
Sent: January 20, 2009 7:01 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Drag Reduction
The old style strut fairings are about equal to the later
aluminum sheet metal fairings because of the sharp fillet
between the fuselage and fairing and the wing and the fairing.
If one makes that sharp corner a fillet with a 1" radius, it
will be less than the older fiberglass strut fairings. No fairings
will create the most drag. One can get an idea as to the
relative magnitude of drag of various objects in the following
table.
http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html
Note that if each object has a frontal area of 1 inch the relative
amounts of drag for each. Round cables are normally considered
Cd of 0.45. Square bar stock is about the same drag as a flat plate.
As far as your floats, the float angle is a trade off between
increased lift and drag. Since the shapes are slightly different
you would need to contact the manufacturer and find out what
aerodynamic studies they have done as to the best trade off.
What I have read about floats is " Place the beginning of the step just
behind your CG, and when the aircraft is floating in the water ensure the
rear of the floats are NOT submerged. If they are, slide the floats back
until the rear of the floats are above the water (or opt for a longer set).
Sliding them back will reduce your frontal area, and decrease pitch
stability on the water, so slide them the minimum you need to so your tail
is safely out of the water. You should be able to tap your tail and have it
return to its position without taking a dip. If the tail tap fails (the tail
hits the water and remains) then you should seriously consider a longer set
of floats. Ensure your hard point mountings are stiff and reliable. For
heavier aircraft consider inserting vertical wooden pins or similar devices
to increase the shear strength of your float hard points. The floats must
also be level with your wing (zero Angle of Attack (AoA)) or with a slight
1-3 degree AoA (achieved by the rear float strut being slightly shorter than
the main strut)."
However, about 1/2 half of the drag of floats on a plane are in the struts,
cross members, and exposed cables. If Steve reduced the drag of his floats,
I am sure that he paid quite a bit of attention to eliminating the drag
of the cross struts and cables as well as getting the proper angle of the
floats.
If you know Steve Sloan, maybe you could get him to comment on exactly what
he did and lists the results of these drag reductions. I am sure that will
get a number of Rebel builders thinking about drag reduction as a way of
making drag reduction improvements to their planes. Probably the next step
after they get their plane flying.
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Walter
Klatt
Sent: Tuesday, January 20, 2009 9:23 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Drag Reduction
Yes, I know Steve Sloan and his plane very well, and he is definitely a
master at drag reduction (and Rebel construction in general), no doubt about
it. I think it was a lot more than 7 mph improvement. I think he had his
Rebel going over 150 mph on wheels, with the 125 hp IO240 Conti. He also did
a lot of fairing work with his amphibs, which really improved that speed
also, but can't remember those numbers.
I do have the old style fiberglass top and bottom strut fairings on mine,
where many others have the aluminum ones. Perhaps that might help the speed
a bit on mine. Now that I think of it, I do know that the other 160 hp
amphib that I have mentioned here does not have any fairings at all on his
struts.
Also, not really sure what float angle does to drag. I have mine set at 2.5
right now, but would like to try 3. I thought I remember that someone
increased their speed slightly when they did that. That would make sense to
me, as I know floats create lift (which creates drag), and if you increase
the float angle (lower the bows), you would have less lift. This is a bit
like how our reflex works. They decrease lift and therefore drag, and you go
faster.
Walter
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Gary
Gustafson
Sent: January 20, 2009 5:59 PM
To: rebel-builders@dcsol.com
Subject: [rebel-builders] Drag Reduction
Thanks Walter for your further explanation of your comparison. I
can see where a 54 or a 56 pitch Sensenich prop is called for
with floats and that the recommendations provided by Sensenich
are for Rebels on wheels. I also agree that 2" can make a big
difference. Thanks!
I agree entirely with Walter that a side-by-side comparison is the
best way to compare performance. However, the following reference
material should give one a good idea as to where to reduce drag
(particularly if you don't want to land shorter) and get better
performance and more efficient operation at the same time.
Drag reduction can produce considerable gains in performance. The Rebel
is a "draggy" in its original design abd with bungee cords. It will
never be a high performance plane. If we wanted a high performance plane
we would have bought a Venture or an RV. Obviously we were looking for
other characteristics and made trade-offs. However, that does not mean
that improvements can not be made. I first became interested when I
read about Steve Sloan and the performance improvements that he claimed
to have achieved after making drag reduction modifications on his
Rebel. (I recall him saying that he saved over 1 gal an hour or
increased his speed by 7 mph) That intrigued me so I started reading
about drag with the following two articles being the most significant.
The subject is discussed in NACA Report No. 485 titled The Drag of
Airplane wheels, wheel fairings, and Landing Gears. It was written in
1935 (all aerodynamics research at that time is entirely applicable to
the Rebel and physics has not changed much since then) and a copy in pdf
format can be obtained at:
http://naca.central.cranfield.ac.uk/rep ... rt-485.pdf
Figures 24 and 25 indicate that the total drag reduction at 100 mph for
the landing gear design similar to that of the Rebel with coil spring
gear can be reduced from 98 lbs to 27 lbs and it provides information as
to the fairings, wheel pants and fillets that enable that reduction.
Remember that drag increases with the square of the velocity or going
From 100 mph to 120 mph one would have to multiply the figures by 1.44
to get the effect of the drag at the higher speed.
The second is a book which can be obtained at www.aerodrag.com for
free. Just click on down load whole book for free. In it, it explains
the various things that cause drag and the impact of that drag. Based
on these two reference items, I was able to determine the following (in
theory but most likely true). This fellow has based his findings on
actual airplanes and spent much of his life studying drag in airplanes.
Total Drag of the Rebel at 120 mph is 375 pounds
Drag associated with the gap between the flaperons is 7 pounds
Drag of coil spring gear is 94 pounds and can be reduced to 35 pounds
for a reduction 59 pounds,
Total drag of the tail wheel and cross bar is 28.8 pounds which can
be reduced to 18.2 pounds for a saving of 10.6 pounds.
Putting an aerodynamic shape on the two tail supports will save another
3 pounds in drag.
So these 4 items will save close to 80 pounds of drag. This is on the
conservative side a 20% reduction in drag (if the airfoil shapes are
exactly right). That means that your engine will be pulling 89 pounds
less drag through the air and that results in increased speed (7 miles
per hour) or lower fuel usage at the same speed (about 1.1 gph). You
can climb out faster (about 7%). You can glide further (sbout 10%).
Sounds interesting.
Now Walter is right. One has to prove it. After I get my plane flying
(shortly) I will get the numbers and then start to install the above
indicated modifications. I have not proved that this is true. However,
Steve Sloan indicates these ranges of improvements. I recommend that
those who are interested read the referenced article and book and
satisfy yourself that this is possible.
It might be interesting to have Steve Sloan chime in as to what he
actually was able to do.
Gary Gustafson Rebel N52GG (242R)
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Drew
Dalgleish
Sent: Tuesday, January 20, 2009 6:56 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Prop selection
Different prop makers use different methods for calculating pitch so it's
very hard to compare props just using those numbers. Interesting drag
calculations I had no idea that the spring gear made that much difference
to total drag. Sounds like using streamlined tube for the gear legs would
make a noticable difference.
Drew
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