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RV-8 Fastback Aerodynamics

Can anyone give me a comparison of the behaviour in flight of a fastback RV8 vs the conventional canopy. It certainly looks better (subjective) but does it stall differently or fly faster?
 
Probably won't make any difference in speed or stall characteristics, UNLESS the plane is built in Texas. (or maybe Oklahoma)
 
No real air data, but: Here are the fastback advantages in my opinion;
-No roll bar / canopy joint in the pilots field of vision.
-Easier access to the rear of the inst. panel.
-No blast of cold air on the back of your neck in winter :)
- looks cool

Tom D
RV8 - Rock Hill, SC
 
Stick Force...

I've often thought that the canopy profile and position on the -8 may be a contributing factor to the unusual stick forces experienced on the -8 at low speed and increasing AOA (landing). There have been past discussions and some conclusions that the cause of this phenomenon was the position of the gear leg intersection at the fuselage. While I don't disagree, I'm not at all certain it's the major cause.

The -8 fast back could well be the platform to easily prove or disprove my theory and possibly contribute to a fix for those of us 'not so fortunate' owners of a standard model. A simple comparison could be accomplished in a short period of time... same pilot (well qualified and experienced in the -8 standard model), 2-aircraft, one standard and one fast back balanced to same CG (tests at fwd, mid and aft CG), a few air and landing test regimens, and some cool ones afterward to complete the debrief.

At the very least, the test would be a great excuse to go fly... Any additional thoughts? Anyone up to the task?
 
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How about Scott and Jon?

Scott Hersha and Jon Thocker have nearly identical -8's except that Scott's is fastback - so an obvious choice for any comparisons. I think they have posted some speed comparisons already.

As for the unusual stick forces....If you mean the funny buffet at stall, its pretty clear that's from the gear intersection - different fairings make different feel, different buffet. If you mean the neutral to negative stick force gradient when at aft c.g. : the number one contributor to this is the low aspect ratio wing. The downwash flow on the tail reduces its stability contribution significantly. The fancy name for the term in the stability equations is d_epsilon/d_alpha. It would be interesting to compare the stick force gradients and 'feel' on -8's with the bigger tips compared to the early tips with 23' span. I bet the 23' span tips are on the planes that people comment the most on the unusual stick feel at aft c.g.

One might wonder why the RV-4 and -6 don't show the same trait. Part of it is that they are lighter - less lift req'd, less downwash. The -4 has roughly the same canopy lines, so I doubt that the canopy-aft fuselage shape is a contributing factor.
 
Thanks Steve...

Yes, I was referring to the neutral to negative stick force gradient when at aft c.g., and I agree with almost everything that you have said.

You stated "the -4 has roughly the same canopy lines, so I doubt that the canopy-aft fuselage shape is a contributing factor".

You may well be correct, and my observation may be the result of my age, 8 year old glasses, and no engineering background. Some time ago however I had the opportunity to stack a friend's -4 and my -8 together in the hangar and my visual observation at that time was that the -8 canopy was considerably taller, somewhat wider, and that the rear profile differed slightly. I didn't take measurements, and of course my observation was without the benefit of any engineering background or data... it just appeared that it may be a contributing factor. I'm just a lazy opportunist searching for answers.
 
Its possible...

Hmm, thats interesting, I didn't realize that it was much bigger/taller.

It might be that the -8 canopy is just enough bigger to cause an issue at high angle of attack, I don't know. We do know that the flow is well attached at normal flight conditions - note the oft-complained draft in from the rear skirt blowing onto the rear pax neck.

If the canopy flow did 'stall', it could conceivably reduce tail effectiveness. An easy way to study this would be with some yarn 'tufts'.

Sounds like a fun project for Kevin H.
 
Re: Stick Force...

I've often thought that the canopy profile and position on the -8 may be a contributing factor to the unusual stick forces experienced on the -8 at low speed and increasing AOA (landing). There have been past discussions and some conclusions that the cause of this phenomenon was the position of the gear leg intersection at the fuselage. While I don't disagree, I'm not at all certain it's the major cause.

The -8 fast back could well be the platform to easily prove or disprove my theory and possibly contribute to a fix for those of us 'not so fortunate' owners of a standard model. A simple comparison could be accomplished in a short period of time... same pilot (well qualified and experienced in the -8 standard model), 2-aircraft, one standard and one fast back balanced to same CG (tests at fwd, mid and aft CG), a few air and landing test regimens, and some cool ones afterward to complete the debrief.

At the very least, the test would be a great excuse to go fly... Any additional thoughts? Anyone up to the task?

[I'm not an -8 guy, and this issue is new to me, so forgive me if I'm repeating stuff that's already been discussed in the past.]

The question of canopy vs. gear geometry as the cause of this characteristic behavior could be answered without involving the fastback variable. A readily available control group is the standard -8A model. If the canopy is the culprit (or any other feature shared by both -8 and -8A), then both -8 and -8A models should exhibit the same characteristic behavior. If the -8 gear geometry is the culprit, then -8A should not exhibit the same characteristic behavior as the -8. So, does it?
 
The 8 VS 8A comparison would be valid as far as the gear question goes, but the fuselage question would still remain unanswered until a fastback A model were built.
I strongly believe in fuselage lift. Bellanca believed in it and so did Burnelli.
Zivi Nadivi flew an F-15 without the right wing for 10 miles and ended with a successful landing. (at 260 knots):eek:
So I find it possible that the shape of the back of my stock 'old style' RV-8 canopy could cause flow characteristics that effect the feel of the tail plane.
I'm not planning to do anything about it just yet. I'm more interested in the other end of my plane. ;)
 
If my memory is correct, one of the speed mods that Dave Anders made to his 4 was a fastback.
 
The 8 VS 8A comparison would be valid as far as the gear question goes, but the fuselage question would still remain unanswered until a fastback A model were built.
I strongly believe in fuselage lift. Bellanca believed in it and so did Burnelli.
Zivi Nadivi flew an F-15 without the right wing for 10 miles and ended with a successful landing. (at 260 knots):eek:
So I find it possible that the shape of the back of my stock 'old style' RV-8 canopy could cause flow characteristics that effect the feel of the tail plane.
I'm not planning to do anything about it just yet. I'm more interested in the other end of my plane. ;)

I agree with everything you said, except that there might not be (likely not be) a need for an -8 or -8A fastback experiment for the purpose of investigating the stick force issue. If the 8 vs. 8A comparison proves that it isn't the gear, then as you said we still don't know if it's the canopy or some other shared attribute of the 8/8A design, and then further testing with other variables such as with a fastback would be called for. But if the 8 vs. 8A comparison proves that it is the gear, then we have the answer right there and there's no need to look further. I think that makes it a sensible first step in investigating this issue -- testing the most suspect variable first and either proving it or eliminating it as a suspect. It's also relatively easy to do in this case because there is a large control population available (hundreds of standard -8A's flying). But yes, all of this of course does assume that the effect is attributed primarily to a single design feature (i.e. the gear geometry, canopy geometry, etc.) and not a more complex interaction between two or more such variables.
 
Oops!

I think I forgot to eliminate the roll bar. :rolleyes:

view.php


Oh well, back to the old drawing board. :)

Happy Holidays
Steve
 
Yes, I was referring to the neutral to negative stick force gradient when at aft c.g., and I agree with almost everything that you have said.

You stated "the -4 has roughly the same canopy lines, so I doubt that the canopy-aft fuselage shape is a contributing factor".

You may well be correct, and my observation may be the result of my age, 8 year old glasses, and no engineering background. Some time ago however I had the opportunity to stack a friend's -4 and my -8 together in the hangar and my visual observation at that time was that the -8 canopy was considerably taller, somewhat wider, and that the rear profile differed slightly. I didn't take measurements, and of course my observation was without the benefit of any engineering background or data... it just appeared that it may be a contributing factor. I'm just a lazy opportunist searching for answers.

Ernie,
I have copies of the CAD files that Rian Johnson (one of Van's Engineers) gave to Doug some years back covering all models from the 3 to the 10. As a professional draftsman, the first thing I did was to check the accuracy of the CAD drawings vs. the preview plans for the 4 and the 8 (the two I'm most interested in). I can tell you with certainty that the 8 canopy is longer, taller, wider and a different shape (taller in back) than the 4 canopy. Whether this has any affect on the low speed handling of the 8 or not I don't know, but the canopies are different.

Looking forward to seeing you at LOE again,
 
What's next...

sez the lazy opportunist. I do need to give full credit to Gary Hunter, aka The Shell Answer Man for resins and the Glu-Guru for all things plastic, for making the initial canopy observation.

I considered tufting, but decided to pass after considering the time and expense of setting up cameras etc to record the results. I also considered adding some VGs to the bottom of the horizontal stab, but never got around to it.

I do believe that the canopy is suspect and deserves further consideration. To that end, I also believe that a controlled flight comparison between the standard -8 and the -8 fast back is a cheap, fun, and productive starting point to gather some quantified data. Anyone up to the task?
 
Anyone up to the task?

I'd love to, except I refuse to make my -8 a fast back since I'm the only person in the world that finds them ugly, try-hard and me-too. (no disrespect intended). Also, that my wings are still cleco'd together will put me at an aerodynamic and structural disadvantage! :D
 
Actually I kinda agree with Andy, but I'm building a fastback anyway. The reasons are mostly practical. And you can't just say 'fastback" since there are two varieties. Personally I don't think the slider fastback has any practical advantage compared to a tip-over.

In the context of speed, two advantages for the tip-over; better sealing at the canopy perimeter (reduced leakage drag) and reduced canopy drag due to the one-piece bubble. Raymer suggests canopy form factor drag is increased by 40% with a seamed two-piece canopy.

However, speed considerations pale compared to the Missus Factor. Most of us are happily paired with lovely ladies of a certain age. A lot of those ladies have a very practical problem with RV entry and exit; it's hard to be graceful when there's nothing good to grab. The relocated roll bar assembly is a huge handhold.

 
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"However, speed considerations pale compared to the Missus Factor. Most of us are happily paired with lovely ladies of a certain age. A lot of those ladies have a very practical problem with RV entry and exit; it's hard to be graceful..."




Great, not only does he build with the skill of an artist, now he puts us to shame by writing with the prose of a Pulitzer Prize winner. Dan H....Renaissance Man/Silver-Tongued Devil :rolleyes: :) ;)


Joe
 
I'd love to, except I refuse to make my -8 a fast back since I'm the only person in the world that finds them ugly, try-hard and me-too. (no disrespect intended).


You're not completely alone... I too prefer the standard -8 appearance. I guess i'm a "D model" kinda guy...:D
 
UGLY

I'd love to, except I refuse to make my -8 a fast back since I'm the only person in the world that finds them ugly, try-hard and me-too. (no disrespect intended). :D

You are not the only one after all. I greatly prefer the look of the std 8 over the fastback, might as well build a rocket. To each his own, as they say.
 
Ugly was early WWII Italian fighters. They had a bubble canopy on a hunchback fuselage to give ultimate pilot visibility.
Funny how successful fighters soon followed the practice....minus the ugly.:D
I'm OK with fastbacks, Kent Paser did the ultimate expiriment by fastbacking his already built standard Mustang II and documenting the resultant change.
So I build toward completion, a long and grinding road. Maybe after a few years I'll take some tips from Kent's fine book; "Speed with Economy".
 
Fast Back?

In the context of speed, two advantages for the tip-over; better sealing at the canopy perimeter (reduced leakage drag) and reduced canopy drag due to the one-piece bubble. Raymer suggests canopy form factor drag is increased by 40% with a seamed two-piece canopy.

However, speed considerations pale compared to the Missus Factor. Most of us are happily paired with lovely ladies of a certain age. A lot of those ladies have a very practical problem with RV entry and exit; it's hard to be graceful when there's nothing good to grab. The relocated roll bar assembly is a huge handhold.


I have never cared for the look of the fast back. In addition to demonstrating a golden tongue, Dan has shown us another reason for the fast back. If it eliminates the draft on the neck of the afore mentioned lovely ladies, I would consider the ugly fast back.
 
Looks great and MORE filling!

As one who thinks Fastback -4's and -8's DO look much better, mostly becausew they get rid of that bulbuous giant canopy look on the -8 especially, with no loss of pilot headroom, another BIG benefit for the Fastback is the considerably larger cubic foot area of the rear baggage compartment. While -8's in general have adequate baggage space with the front and rear baggage areas, adding the Fastback mod makes for a higher baggage space-more room in the veritical-because of the higher bulkheads. In my -4 Fastback I can put in standard sized airline overhead bags in the aft baggage compartment. Also, on X-C flights, my wife brings a pillow and can lean her head back on the aft cockpit Fastback bulkhead and 'sleep' her way to our destination.

Yes, I vote they are better looking but also more practical. I can't say for sure aerodynamically they are more efficient, but the old axiom "Form follows function" would seem to apply, at least for me:rolleyes:

Ugly???? I don't think so: http://img33.imageshack.us/img33/3724/n88xl0341.jpg

http://img695.imageshack.us/img695/6756/n234jj.jpg


Tailwinds!
 
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As one who thinks Fastback -4's and -8's DO look much better, mostly becausew they get rid of that bulbuous giant canopy look on the -8 especially,

IMO, the D model P-51 Mustang with it's full bubble canopy, was one of the best looking aircraft ever designed. It's my all time favorite, and happily I had a friend who owned one. Therefor, I was able to have the pleasure of riding in a 1944 P51D as a passenger where the fuselage fuel tank once resided.

However, I also think the full canopy on the 8 is out of porportion with the rest of the aircraft. The 8 is just much smaller than a P-51D, yet we're cramming two people in it. I too, prefer the looks of the fastback. However, the "standard" 8 and full canopy, still emulate the look of the P-51D from certain angles. I guess that's what counts! :)

L.Adamson ---- RV6A (fastback by original design)
 
Fastback probably only refers to appearance for all practical purposes. Jon and I have nearly identical RV-8's as someone here said and mine is 2-3 knots faster, but the 'nearly' needs clarification. I have a Whirlwind 200RV prop and electronic ignition. Jon has the Hartzell blended airfoil prop and 2 mags. Mine weighed in about 45 pounds lighter than Jon's on the same day - mostly because of those two items just mentioned. We have the same stuff in our panels, same upholstery kits, similar paint jobs. Weight is partly responsible for the speed difference, and electronic ignition may account for something too, because in our speed comparison, we were in loose formation with wide open throttles, and about 2550 RPM. Asthetics should be the main reason you want a Fastback. It's a matter of taste. Some of us like blondes, some like brunnetts. Don't ask your wife to relate to this because they are more practical. And there are advantages and disadvantages practically speaking with a Fastback compared to the standard canopy. Visibility is the major winner in the practicality area in a Fastback. Entry/exit is also easier - in both seats. Baggage area behind the back seat is bigger and easier to load. Being able to easily access the behind the panel area is wonderful when you need it. You probably won't need it very often - maybe never - after you're done building, so I wouldn't weigh that advantage too heavily. If you like to work with fiberglass (I don't mind it), then you'll have fun with the Showplanes kit. It was easy to work with because it's so well made and if you know what you're doing, or are willing to learn through practice, you can get a truly tight fit all the way around. But let me warn you about the air blowing down your wife's neck. It still happens unless you seal it somehow. My canopy lays down very tight on the turtledeck. I can barely get a fingernail under it when it's sitting on the ground. When you're airborne, there's enough lift on top to open up a very small area and that's all it takes to get a small amount of ice cold air moving vertically down the back bulkhead between the fiberglass and metal. It's very easy to seal though. I used foam weather seal from Lowes that is sticky-backed and it fits snugly between the fiberglass/honeycomb aft canopy bulkhead and the metal turtledeck bulkhead when closed (I stuck it on the aft side of the fiberglass bulkhead). When you are building the canopy, you bond this bulkhead in place by clamping it to the metal turtledeck bulkhead bulkhead with small wood spacer blocks (1/2" I think), and then 'glue' it to the canoppy frame with epoxy/flox by lowering the frame down onto it. This leaves a space after it's glassed in that is the best place to seal this area. A similar situation exists up front too - also easily sealed with soft foam.
Disadvantages to a Fastback? Appearance if you don't like fastbacks. Taxiing - you loose the cool looks of a fighter pilot after a formation arrival at an airshow with your arm hanging over the side waving at the crowd.... I can still do that (left side only) with my canopy in the taxi position but it doesn't look as cool. Temperature - wise it is cool though because in the taxi position you get a really good blast of fan-forced air through the cabin (hang on to your hat!). You sacrifice visibility while taxiing though. In the taxi position, the glareshield/canopy front blocks the bottom of your field of view and you need to sit taller in your seat to see clearly over the nose. This may be do to seat cushion geometry. I tried to make my canopy as low as possible for appearance purposes. I didn't like the 'bulbuous' look of the canopy and kept trimming and moving the canopy forward until I had the look I liked. I still have plenty of head room (I'm 6'1") and I can see straight over the nose normally with no problem, but with the canopy in the taxi position, I have to stretch a bit or do some zig-zagging. I may drill an intermediate hole/taxi position to give me more vis and less air. I'll 'sperriment with that next spring. If there's anything about this you don't like, don't do the Fastback for 3 knots. I don't think it's worth that.
Am I happy with my Fastback? I love this airplane. I fly formation with several other RV-8's (Cincinnati River Rats) and when we are sitting on the ground, I just look at mine and I'm so happy I did it this way. In my opinion (the one that counts), I have the best looking one by far... The formation visibility is incomparible too.

Scott
RV-8 FB
 
Scott - that is one of the best, most objective write-ups on the topic I've ever seen - thanks for taking the time!

I am personally in the "I like the looks of the bubble because that is what a P-51D has, and that's always been my favorite airplane, so there!:p" camp, but I don't deny the fact that others live in the other camp very happily, and it is great to have a choice!

Paul
 
"Fastback by original design"

L.Adamson,

"Fastback by original design" I never thought of that! Very clever!

The P-51 is beautiful, in every flavor. The ability to adequately 'check six' was enormously beneficial and the pilots of yore were grateful for the change from Fastback to bubble canopy, just as we are today. However, as in the P-51D, there was an aerodymanic penalty to be paid for the bubble canopy, but they continued to add more horsepower and as it increased on the P-51, it was the aerodynmaic compressability and parasitic drag of the entire airframe, not just the canopy, that became the limiting performance factor, things which mostly are not a problem for us peace loving RV drivers!

Just to be clear, I like all RV's, stock and modded. Just like the FB more! Even the 'original design'
 
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Scott - Paul took the words right out of my mouth!
Scott - that is one of the best, most objective write-ups on the topic I've ever seen - thanks for taking the time!
Also, I believe your statement "In my opinion (the one that counts), I have the best looking one by far." summarizes the standard vs fastback discussion very appropriately.

PS - Earlier in this thread, I questioned the size and position of the standard -8 canopy as being a possible 'culprit'. Any more thoughts on this?
 
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great write-up Scott! Both styles look great. I seriously considered the Fastback from showplanes. The deciding factor for me in sticking with the standard was simply the roll bar. I think Dan's looks great and will do what it needs to do, but IMHO that big wide, solid rollbar in front is the better of the two with respect to protection..that was my only reason. I was also a little concerned about the kind of stuff that might get blown into your face and/or the cockpit during taxi with the canopy open. Any issues with this?

Ken
 
Don't forget about the Tom Clark fastback...it is a slider with a front roll bar just like standard 8's (actually, the front roll bar is canted forward about 14 degrees).

With this approach, you get a fastback without sacrificing the slider and roll bar.
 
Everyone mentions D model, and F-15's come to mind. If I was only ever going to fly solo, I'd think about an F-15C shaped canopy. :)

I dunno, just not a P-51 fan, nor really any other warbirds other than P-38's. Course I think F-4E/G Phantom's are beautiful so there may be something wrong with me. :D
 
Naa;
Nothing wrong with you Shockwave.
I like Fokker D-XXI fixed gear pre-WWII fighters too.
I think the Curtiss CW-19 is a nice "pre Van's" RV-4 or RV-8.
They both have greenhouse canopies that form into 'half a fastback'.
All kinds of aircraft have 'purdy spots'. There's no telling when you will be struck by an aspect of an aircraft. :eek:
 
I don't see a P51D when I see a RV8.

I like everything about the Show Planes fastback, form and function; mostly no roll bar in the face.

My favorite plane on the planet is this, but a RV8FB is better for my use; not to mention the extra 300 grand.

extra300.jpg
 
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Just stirring the pot

Probably won't make any difference in speed or stall characteristics, UNLESS the plane is built in Texas. (or maybe Oklahoma)

So would it be faster if built in Texas and slower if built in Oklahoma? Or faster if built in Oklahoma and slower if it's built in Texas?

Charlie Kuss

The definition of an instigator is someone who goes downtown to start a riot. Once it starts, he leaves and goes home to watch it on TV! :p
 
Ken,
I've never had anything blow up into the cockpit while taxiing with the canopy in the taxi position. But you do get a strong blast of air, especially with the power above idle. Like I said, I'll be working on an intermediate taxi position hole for the mid cabin brace. All I have to do is drill a hole. I think my roll bar which sit right behind my head and is slightly above it offers pretty good protection if I'm unfortunate enought to end up upside down. Somebody asked for some pics

N184SB


Scott
 
Pictures

This thread is GREATLY lacking in photos. Here are a few photos of the different style RV-8 fastbacks, from links on this thread and others and a brief internet search. None of these are my photos.

scotthershasrv8fastbackyi0.jpg


RV8_Fastback_N525TC.jpg

2qj9mqv.jpg


I have a definite preference.
 
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