Van's Air Force
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Easy speed mods?
- Thread starter Tram
- Start date
pressure recovery
Basically, it's this. The drag of a body is the difference between the force on the front pushing back, and the force on the back pushing forward. If you can get the pressure on the back side of the body to increase without separating, it will push forward on the back-slope of the body. The concave "pressure-recovery" shape is intended to bring the airflow around each surface back together without separation.
Basically, it's this. The drag of a body is the difference between the force on the front pushing back, and the force on the back pushing forward. If you can get the pressure on the back side of the body to increase without separating, it will push forward on the back-slope of the body. The concave "pressure-recovery" shape is intended to bring the airflow around each surface back together without separation.
rv7charlie
Well Known Member
Earlier, someone mentioned airflow on the backside of our canopies. I'm curious about whether anyone has done tuft or oil dot testing of the canopy aft end area. The pressure back there is quite high on every canopy type plane I've seen (the Thorp T-18 has its cockpit fresh air *inlet* at the bottom rear of the sliding canopy). I suspect that it's quite turbulent, too. My 1st (purchased) RV-4 had an inexpensive wire whip ELT antenna mounted a few inches behind the canopy. At some point early in the plane's life, it broke off at the point of its mounting (long cone with the coax fitting & gland nut on the other side of the skin. I suspect that constant whipping in the turbulent flow fatigued the wire whip.
I've often wondered if vortex generators somewhere near the back 1/3 of the canopy would reduce the turbulent flow back there.
Charlie
I've often wondered if vortex generators somewhere near the back 1/3 of the canopy would reduce the turbulent flow back there.
Charlie
pierre smith
Well Known Member
The fastback mod...
....addresses that problem very effectively. The earlier P-51's were fastback but the problem was that the pilots couldn't 'check their six', so the bubble came....with turbulence
Best,
....addresses that problem very effectively. The earlier P-51's were fastback but the problem was that the pilots couldn't 'check their six', so the bubble came....with turbulence
Best,
Alan Carroll
Well Known Member
I've done this with yarn tufts on my RV-8, and didn't see any large-scale turbulence (ie the tufts were all pointing aft). There was some movement of the tufts though; I'm not sure if due to disturbance by the canopy or just the general movement of the turbulent boundary layer.
Tom Martin
Well Known Member
The back of my RV4 canopy would lift a little in flight. Cold air came in and hit the passenger right on the back of the neck. I added a glass flare/faring to the metal shirt to blend it into the turtle deck and the canopy no longer lifted. It was not that big, probably only extending an inch or two aft of the metal skirt but it did seem to smooth the airflow in that area. This was confirmed by using the CAIBONS system of cabin air pressure measurement (cold air in back of neck scale)
Christopher Murphy
Well Known Member
canopy air flow
After first flight and about 8 hours I installed the wheel pants and gear leg fairings and with the increased speed came a noticeable and disconcerting vibration. After many flights and investigating the wrong areas I found it to be the aft canopy skirt. A readjustment of the rubber seal stopped that but I believe that to be a source of turbulent flow and drag.
Chris M RACE34
After first flight and about 8 hours I installed the wheel pants and gear leg fairings and with the increased speed came a noticeable and disconcerting vibration. After many flights and investigating the wrong areas I found it to be the aft canopy skirt. A readjustment of the rubber seal stopped that but I believe that to be a source of turbulent flow and drag.
Chris M RACE34
Canopy lift
And well it should. There's a low pressure on the top of the canopy that is an extension of the low pressure on top of the wing that goes right on up and over the canopy. The canopy accounts for a very large percentage of the total lift on your plane, since the low pressure is close to the greatest over it. On mine it is about 400 lb in un-accelerated flight. I have a friend who found that one of his canopy latches was not closed after takeoff. He thought that by slowing down he could relieve the force on his gull-wing door and open and close it. Bad Move! Bye, bye door! The lift is on the canopy no matter what your flight speed, but goes up with g-load. I found that on a friend's plane and mine that the pressure within the cockpit is about 0.1" below static, and it will cause cold air to enter the cockpit from a higher-pressure area, such as at the tail-cone. If you use the cockpit for an alternate static source, be prepared to see the ROC, altitude, and IAS jump up when you select it!
And well it should. There's a low pressure on the top of the canopy that is an extension of the low pressure on top of the wing that goes right on up and over the canopy. The canopy accounts for a very large percentage of the total lift on your plane, since the low pressure is close to the greatest over it. On mine it is about 400 lb in un-accelerated flight. I have a friend who found that one of his canopy latches was not closed after takeoff. He thought that by slowing down he could relieve the force on his gull-wing door and open and close it. Bad Move! Bye, bye door! The lift is on the canopy no matter what your flight speed, but goes up with g-load. I found that on a friend's plane and mine that the pressure within the cockpit is about 0.1" below static, and it will cause cold air to enter the cockpit from a higher-pressure area, such as at the tail-cone. If you use the cockpit for an alternate static source, be prepared to see the ROC, altitude, and IAS jump up when you select it!
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rv7charlie
Well Known Member
Is that 400 lb number at cruise? At 100-120 kts, it's nowhere near that high, because I can hold an RV-4 canopy (almost) shut with one hand. Yes, I actually know that, due to involuntary data collection. Caused by an aviation syndrome known as 'failure to latch'.
Charlie
Charlie
Your plane has 110 sq.ft. wing area with a Hershey bar wing, whereas I have 77 sq ft with a tapered wing. My wing loading at 1400 lb runs 18.2 psf averaged over the total wing, higher near the fuselage, lower outboard; yours at the same weight runs 12.7 psf so mine is about 50% higher than yours. The wing loading is independent of the speed; it still takes the same amount of lift to keep the plane in the air. Both of them should have an approximately elliptically-shaped (luv that word) lift distribution which for yours is more natural, mine requires wing-twist to reduce the apparent lift near the tip. (See the Schrenk method of determining lift distribution). As a result, the lift over the fuselage is much higher than the average loading. Just take your wing loading and multiply it times the canopy area, then, in your case, add an additional 11% to get the apparent canopy load. Keep in mind that when the canopy came open, airflow around and under the "slightly" open canopy rduced the load you were holding, which you also shared with any canopy hinges. The load I'm referring to is if you had to hold the canopy competely shut, not "almost", without any other structure such as the hinges helping to hold it. So there! Gotcha!
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http://www.airportnac.com/images/F1 Racing/Invictus @ Reno.JPG
Funny you should mention that. The formula 1 winner last year at Reno(Invictus) certainly gave the concept a shot.
George
Bill Wightman
Well Known Member
The rectangular wing planform produces the highest Cl values at the plane of symmetry. Unfortunately, that's also where the fuselage is so the theoretical lift over the fuselage is spoiled by wing/fuselage interference. Cl's drop from there to the tip. Shrenk's approximation simply super imposes an elliptical lift plot over the planform's area distribution. Its not really based on hard science, but is only an empirically derived method of analysis that gets close to reality.
A tapered planform is MUCH better at producing more efficient spanwise lift distribution. A tapered planform will produce maximum spanwise Cl's somewhere in mid-span, depending primarily on taper ratio and aspect ratio. Twist in tapered planforms became the norm due to designers habitually using thinner airfoil sections at the tip than those used at the root, which will obviously lead to tip stalling problems. On a properly designed wing, twist might only be used to tailor lift distribution when analyzing the wing-tail system for least drag in a balanced flight condition, or to force the root to stall well ahead of the tips. Its not used to dump lift outboard, which only hurts efficiency: the tapered planform maintains lift in the outboard portion of the wing simply due to its shape.
Although Shrenk would predict max Cl at the fuselage centerline on the hershey bar wing, the lift carry-over across the fuselage is most likely quite a bit lower than analysis might suggest for the wing by itself. The fuselage isn't a shape normally designed to produce lift, and only acts as an interference factor in the spanwise distribution. There are some nice gains to be had in "marrying" the fuselage's pressure distribution with that of the wing, but we can't easily do that on our RV's. Instead, we get what's called "interference drag" between the two, and a corresponding loss of lift over the fuselage.
Canopy lift / flow separation: I sincerely doubt anybody who tuft tests their RV will find any evidence of flow instability or separation on the afterbody of the canopy. The canopy will generate some lift, that's true, but it duck-tails up in cold conditions primarily due to the extreme shrinkage of the material its made out of (lexan?) when subjected to cold. My RV4 would lift its canopy aft edge up enough to slide your fingers under at high altitude in the winter time, but not hardly at all at full speed on a hot summer day at low altitude. So, this is what I think is happening - its due to shrinkage and not so much the lift forces on it.
**********************
"Pressure recovery": the term is used to denote that region in a flow field where pressure is going from a relatively low pressure to a relatively higher pressure in the flow direction. On an airfoil, we see the lowest pressure point roughly on top of the wing spar. From there back, pressure must build back up to static pressure at the trailing edge. So, that part of the airfoil - from low pressure point to the trailing edge - is called the "pressure recovery" side of the airfoil. Its also called an "adverse pressure gradient" because the flow is moving against nature; going from low pressure to higher pressure. It must rely on momentum to make this happen. This is one fundamental idea in understanding momentum theory boundary layer analysis.
rvmills
Well Known Member
Tom, I believe that south of the border its the WOBON scale (wind on back of neck)
Kinda like degrees F/C! The discussion of canopy lift reminded me of the fact that the Tomcat's critical mach was determined by the canopy, as the air accelerating over it reached supersonic before airflow anywhere else on the airframe, including the wings. Air definitely accelerates up and over the canopy. Now with the prop in front of the canopy, it may accelerate even more...or it could be more turbulent...hard to say.
But Tom, sounds like your fairing had positive results. Were you doing speed testing back then, and if so, what change did you see when you got rid of the CAIBON/WOBON?
George, any idea if Invictus attributed any speed increase to the v. gens? Hmmm, wonder how you'd put those on a side-by-side slider...any thoughts AX?
Cheers,
Bob
Bill Wightman
Well Known Member
RV8 Span Distribution
Hi Paul,
Well there certainly is lift on our canopy due to its high curvature; but how much I couldn't even offer a guess. I understand and 100% agree with your observation, but trying to quantify the value is actually extremely difficult to do. I have a couple aero texts that wave their hand at this issue, but no solution is offered as to how wing lift gets distributed over the fuselage. There are ways to approximate a solution to this, but they're beyond my ability to analyze.
FWIW, here's a wing/fuselage/tail model I did of the RV8 for the purpose of finding span distribution and balanced flight loads. This is how the model geometry looks:
Although the fuselage, tail and propeller effects are included here, the wing/fuselage interaction is not addressed at all. The lift is handled with a vortex-lattice type of analysis; so is the tail. Downwash is predicted on the tail, fuselage effects are included by themselves and the prop's influence on pitch is included.
Here's how the lift distribution looks, at an angle of attack of 1.0 degree which is close to level cruise flight for the RV8:
Shrenk's analysis, for a rectangular wing, would be close to this.
Hi Paul,
Well there certainly is lift on our canopy due to its high curvature; but how much I couldn't even offer a guess. I understand and 100% agree with your observation, but trying to quantify the value is actually extremely difficult to do. I have a couple aero texts that wave their hand at this issue, but no solution is offered as to how wing lift gets distributed over the fuselage. There are ways to approximate a solution to this, but they're beyond my ability to analyze.
FWIW, here's a wing/fuselage/tail model I did of the RV8 for the purpose of finding span distribution and balanced flight loads. This is how the model geometry looks:
Although the fuselage, tail and propeller effects are included here, the wing/fuselage interaction is not addressed at all. The lift is handled with a vortex-lattice type of analysis; so is the tail. Downwash is predicted on the tail, fuselage effects are included by themselves and the prop's influence on pitch is included.
Here's how the lift distribution looks, at an angle of attack of 1.0 degree which is close to level cruise flight for the RV8:
Shrenk's analysis, for a rectangular wing, would be close to this.
rv7charlie
Well Known Member
Not a problem; I'm pretty hard to offend, anyway. Actually, I was a little concerned after I sent my email that my question might have been taken as offensive. Emails are often mis-interpreted because the receiver can't read the sender's face, voice, body language, etc.
It would really help info exchange if everyone would treat a challenge to their idea as simply that, instead of an attack on them personally.
As a data point, I was able to hold the -4's canopy almost completely shut; just shy of being able to set the latch pins. It wasn't easy, but strength-wise I'm a wimp. Especially with the arm position required to hold it.
FWIW, I've read a couple of articles indicating that designers try to avoid net lift on the fuselage, because the very low aspect ratio causes high lift-induced drag. Can't prove that; it's just what I read in the papers. On a -4, 400 lbs of lift would be over a quarter of the gross weight of the plane.
Charlie
It would really help info exchange if everyone would treat a challenge to their idea as simply that, instead of an attack on them personally.
As a data point, I was able to hold the -4's canopy almost completely shut; just shy of being able to set the latch pins. It wasn't easy, but strength-wise I'm a wimp. Especially with the arm position required to hold it.
FWIW, I've read a couple of articles indicating that designers try to avoid net lift on the fuselage, because the very low aspect ratio causes high lift-induced drag. Can't prove that; it's just what I read in the papers.
On a -4, 400 lbs of lift would be over a quarter of the gross weight of the plane.Charlie
Bill,
At the risk of wandering off topic bit, would you mind expanding your thoughts a bit on your statements above. As someone who is into efficient composite planes, I'd love to hear your thoughs regarding aerodynamic twist straight tapered wing planforms and the "marrying of the fuselage's pressure distrubution with that of the wing". By that I assume you are referring to things like having the max canopy height inline with the trailing edge of the wings and not retracting the fuselage shape until at or near the trailing edge as well. Anyway, you hit on a subject or two that really interests me regarding high efficiency aircraft design.
rvmills,
The Invictus race plane had the VGs on the canopy for a fairly short time. I believe the entire canopy and teardrop trailing afterbody that blends it into the rear of the fuse was changed out when Thom Richards was given the go ahead to pilot the plane at Reno last year. He is on the tall side for a Formula 1 pilot, and the plane was way too tight for him to fit as it was. The new canopy was raised a bit and the trailing fairing was given a longer and more gradual return shape to the after fuse. I don't believe the VGs were deemed necessary with the new shape.
George
Clean and wax
I remember reading somewhere that a C-182 with a full wax job is good for 4-5 kts. Not sure if that'll do much for an RV since its already a clean wing (flush rivets) but its worth a shot and its probably the cheapest 'mod'.
I just wonder out of curiosity how a good wax or paint sealant on the upper surface only would affect high altitude cruise. My though is that this will clean up the airflow and create an even lower pressure on the upper surface which will then allow a lower AOA at higher altitudes. Speaking of AOA, what does an RV fly like in the 8's and above?
Some say that a 9 will out cruise a 6/7/8 since it has a more generous wing. Just a thought. If it doesn't work at least you'll have a clean shiny airplane.
I remember reading somewhere that a C-182 with a full wax job is good for 4-5 kts. Not sure if that'll do much for an RV since its already a clean wing (flush rivets) but its worth a shot and its probably the cheapest 'mod'.
I just wonder out of curiosity how a good wax or paint sealant on the upper surface only would affect high altitude cruise. My though is that this will clean up the airflow and create an even lower pressure on the upper surface which will then allow a lower AOA at higher altitudes. Speaking of AOA, what does an RV fly like in the 8's and above?
Some say that a 9 will out cruise a 6/7/8 since it has a more generous wing. Just a thought. If it doesn't work at least you'll have a clean shiny airplane.
eric_marsh
Well Known Member
I've read this entire thread and find it very interesting, especially in regards to how significant gains can be made with the correct minor changes.
I doubt that any of us have access to a wind tunnel, but has anyone tried experimenting with some sort of a home made tunnel using large commercial fans to test small components such as wheel pants or perhaps some sort of fairing on the landing gear struts using smoke tests? Right now this more or less falls into the "inquiring minds want to know" category for me.
On a slightly different tangent, I've noticed that the air intake for the induction looks rather small. Ram air induction is now mainstream with motorcycles though the gains are small - a percentage point or two. Clearly there is a tradeoff between the increased amount of power one would see with a ram air intake vs the amount of drag a larger opening would create. Has anyone ever experimented with that?
I doubt that any of us have access to a wind tunnel, but has anyone tried experimenting with some sort of a home made tunnel using large commercial fans to test small components such as wheel pants or perhaps some sort of fairing on the landing gear struts using smoke tests? Right now this more or less falls into the "inquiring minds want to know" category for me.
On a slightly different tangent, I've noticed that the air intake for the induction looks rather small. Ram air induction is now mainstream with motorcycles though the gains are small - a percentage point or two. Clearly there is a tradeoff between the increased amount of power one would see with a ram air intake vs the amount of drag a larger opening would create. Has anyone ever experimented with that?
Kyle Boatright
Well Known Member
The CAFE foundation found a 2.8 mph increase in speed after waxing a Mooney M20E with lightly oxidized paint. Given comparable speeds (the Mooney tested was a 200-205 mph aircraft) and the relative size of the aircraft, the increase from waxing an RV would probably be slightly less.
http://cafefoundation.org/v2/pdf_cafe_reports/Wax Job with PICT.pdf
Bill Wightman
Well Known Member
George, you're right on target. The wing and fuselage, when put together have mutual effect on each other. I think the book "speed with economy" talks about this, but I haven't read it (I plan on getting it).
The fuselage's shape - how it accelerates the flow around the cockpit/canopy can have significant effect on the flow over the wing. Details such as canopy position, fuselage lofting, root fairings, etc can have really significant effect on the flow field. In my undergraduate study, I did several wind tunnel tests on the interaction between a body such as a fuselage, and the wing. To my amazement, the tests showed flow over the wing being affected up to three fuselage diameters away! For us, that's like half way out on the wing, but I didn't test an RV type of shape which has a much higher fineness ratio. I ran the tests with multiple angles of attack, and multiple angles of incidence between the fuselage and the wing. It was an eye opening study.
But... this was done in a wind tunnel. I currently do not have sufficient analytical horsepower to do a CFD analysis of something like this.
The models I posted were produced using a software product called "ACES", produced by a Cessna engineer, if I remember right. He's out of the biz now, I think but had a company called Digital Aerodynamics. Its just matrix mathematic evaluation of multiple equations - to solve a flow system around the wing. Its known popularly as a "vortex lattice" analysis, which you can find descriptions of in most good aero texts.
I also have some very good software from an ex-Beech engineer, Hal McMaster for doing superb loads analysis and performance studies. Good stuff!
Gotta go... thx for your input.
The fuselage's shape - how it accelerates the flow around the cockpit/canopy can have significant effect on the flow over the wing. Details such as canopy position, fuselage lofting, root fairings, etc can have really significant effect on the flow field. In my undergraduate study, I did several wind tunnel tests on the interaction between a body such as a fuselage, and the wing. To my amazement, the tests showed flow over the wing being affected up to three fuselage diameters away! For us, that's like half way out on the wing, but I didn't test an RV type of shape which has a much higher fineness ratio. I ran the tests with multiple angles of attack, and multiple angles of incidence between the fuselage and the wing. It was an eye opening study.
But... this was done in a wind tunnel. I currently do not have sufficient analytical horsepower to do a CFD analysis of something like this.
The models I posted were produced using a software product called "ACES", produced by a Cessna engineer, if I remember right. He's out of the biz now, I think but had a company called Digital Aerodynamics. Its just matrix mathematic evaluation of multiple equations - to solve a flow system around the wing. Its known popularly as a "vortex lattice" analysis, which you can find descriptions of in most good aero texts.
I also have some very good software from an ex-Beech engineer, Hal McMaster for doing superb loads analysis and performance studies. Good stuff!
Gotta go... thx for your input.
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Thanks for the thoughts. I have been trying to absorb every bit of information I can locate on subjects such as these. Kent Paser's book is great book. A true experimenter, with an aeronautical engineering background.
On the subject of wing/fuselage interference, Mike Arnold(world record setting AR-5 and Reno formula 1 winning AR-6) has gone so far as stating that he fully believes that area ruling functionally applies to subsonic aircraft(in his case, in the 200-300mph range). Certainly not the mainstream view, but since he only designed and built two aircraft total and they were both record setters, it's hard to dismiss.
George
fodrv7
Well Known Member
Modern computing power.
Amazing what you can do now.
When I worked at ARL (Aeronautical Research Laboratories) in Australia in the late 60s we had a DEC PDP10. It cost a zillion dollars, took up a huge room, but couldn't do anything like that.
The wind tunnel ruled supreme.
According to Wiki; The KI10 and later processors offered paged memory management, and also supported a larger physical address space of 4 megawords.
Pete.
Amazing what you can do now.
When I worked at ARL (Aeronautical Research Laboratories) in Australia in the late 60s we had a DEC PDP10. It cost a zillion dollars, took up a huge room, but couldn't do anything like that.
The wind tunnel ruled supreme.
According to Wiki; The KI10 and later processors offered paged memory management, and also supported a larger physical address space of 4 megawords.
Pete.
David Paule
Well Known Member
I just learned that Hoerner's excellent classic book on drag, "Fluid Dynamic Drag," is still available. While you can get it through Amazon it's a lot cheaper from Hoerner Fluid Dynamics, Box 21992, Bakersfield, CA 93390. It's $95 plus $4.20 shipping and, in CA, sales tax.
I don't know how old that price information is, sorry. There was a slip of paper with this info on it in with the AR-5 tapes.
I've got a copy of the book, had it a while, and it's definitely recommended.
Incidentally, video 1, "Why it goes so fast" was fascinating. I recommend it.
http://www.ar-5.com/tapes.html
I don't have any connection to these except that I like both the book and the video, and felt that anyone wanting to make their airplane go faster might want to check these out.
I don't know how old that price information is, sorry. There was a slip of paper with this info on it in with the AR-5 tapes.
I've got a copy of the book, had it a while, and it's definitely recommended.
Incidentally, video 1, "Why it goes so fast" was fascinating. I recommend it.
http://www.ar-5.com/tapes.html
I don't have any connection to these except that I like both the book and the video, and felt that anyone wanting to make their airplane go faster might want to check these out.
