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Decalage, RV-6 vs RV-4

Bob Axsom

Well Known Member
I did some rough calculations last night with a wing tip in the hangar for our RV-6A. The wing tip is a homemade racing tip for when I'm racing with the extra tanks installed (long story - I have many wingtip sets). It is reasonably stable but if I decide to go ahead with this mod I am thinking of I will redo this with one of my metal tip tanks as a pattern.

I traced the airfoil on my 4'x8' workbench top with the leading edge and the trailing edge the same distance from the edge of the surface for general alignment. I drew a straight line (chord line) from the most forward point of the leading edge to the trailing edge of the airfoil drawing and I measured from the trailing edge to locate the point specified in the manual for rigging the wing incidence when setting up to drill the rear spar attach hole (~11.5" - I don't have the measured number with me at the moment but it was 11 3/8" or 11 5/8"). I drew a perpendicular line from the chord line at that point and marked it with a rigging reference point 3 1/32" above the surface of the airfoil. Then I drew a measurement reference line from that point tangent to the high point of the airfoil. Then I measured the vertical distance from the chord line intersection of the nose of the airfoil to the measurement reference line as 4.5" (4 16/32"). Then I measured the vertical distance from the chord line at the rigging reference station to the measurement reference line and found it to be 4 25/32" (4.78125") for a difference of 0.28125" (9/32"). The distance between the two points on the measurement reference line is 47" so the sine of the angle is .28125/47= 0.005984043 and the arc sin = 0.342862429 degrees. Since the semetrical airfoil horizontal stabilizer (assumption) is rigged at 0.0 degrees with respect to the same leveled canopy deck reference plane then the decalage is ~ 0.34 degrees + or - the deformation errors of the wingtip for the airfoil pattern.

I'm interested in speed and I have a friend with an RV-4 that has shimmed his stabilizer leading edge up at the forward mounting point by 1/8". I haven't pulled my tail fairing and started measurement work in there yet but off the top of my head it seems like that would take me past zero-zero on my airplane. In an RV-6 the fuel and the passengers are sitting very close to the CG and a smaller design decalage may be tolerable for pitch stability.

Just out of idle curiosity what is the decalage on an RV-4?

Bob Axsom
 
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Tips

Hello Bob
Im just wondering if you have thought about making a pair of winglets like on other high performance aircraft maybe it would do the trick if your running at jet speed you need jet parts.The blended looks fast.
Bob
 
They just look like a high potential for drag to me but ...

They just look like they have a high potential for drag to me but you can believe if I start getting beat in the RV Blue Class by someone using them I will take a HARD LOOK.

Bob Axsom
 
Decalage

Am I the only one that didn't know this word?

Decalage = the difference in the angle of incidence of the upper and lower wings of a biplane.

What model RV would that be?

Jim Berry
RV-10
 
On a monoplane it's ...

In a monoplane it is the difference in the incidence angle of the wing and horizontal tail.

Bob Axsom
 
I suspect it is not the complete answer

The wing will produce different amounts of lift at different speeds and if you look back at the elevator in cruise and see the elevator perfecty in trail with the horizontal stabilizer at 160 kts should you expect that to be the perfect trim for 190 kts?

If we can trim for level flight with a little up elevator at 70 kts and gradually remove that down force on the tail as the speed increases until we are at maximum velocity at 190 kts, could the speed be further increased if the tail down force were further decreased by raising the leading edge of the horizontal stabilizer, even though intuition says the ideal minimum drag situation is decalage zero and elevator and trim tab perfectly in trail of the horizontal stabilizer.

Can the wing produce so much lift at high speed in a specific planform that the tail has to go into a lifting tail mode to maintain level flight?

If the previous statement is true should the planform be changed by reducing the lifting force at hight speed by cutting the wingspan or as an interim compromise is lifting the leading edge of the stabilizer a way to achieve greater speed as opposed to changine the planform?

This kind of thought is all about achieving maximum level flight speed and not safety or general purpose flying pitch control comfort.

I am a little timid about this but I personally know of two airplanes that have done it (one an RV-6) and they are both perfectly safe in flight and they are faster. I never know first hand what works to increase speed until I personally test a modification.

I have of course tested wing area reduction by changing the original and still travel configuration by reducing the span from 24.5' to 21.5' for race configuration. Both configurations fly perfectly well but the shorter span is approximately 3 kts faster. (late note: I have also flow it with 21', 21.25' and 23' wingspans - so far 21.5' with streamlined tips is the fastest)

This gets into the "DON"T TRY THIS AT HOME" region but for myself I am feeling it out for race speed and only want to know if the standard decalage on a RV-4 is greater than an RV-6.

Bob Axsom

P.S. Recent lookbacks in cruise flight with the short wing, I did not observe any elevator deflection out of the "in trail" position but I have never looked back ther in race configuration at race speeds. I also have not looked back there in travel configuration (long wing). I must admit it is difficult for me to see much back there - this would be a good subject for an inflight camera study.

B.A.
 
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Can the wing produce so much lift at high speed in a specific planform that the tail has to go into a lifting tail mode to maintain level flight?

The center of lift would have to shift forward of the CG. You would certainly have stability issues far before that happened.
trim.gif


Changing the incidence of the tail might reduce drag by improving the airfoil's efficiency at a given speed and loading (lower counterweight drag), but the force generated will remain the same.

If you race with a CG near the aft end of the allowable envelope, you will reduce the amount of down force required, and thus the induced tailplane drag. (it will also be more pitch sensitive, and easier to over-G)
 
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shifting ballast

I recall from my sailplane days that there was some experimentation with pumping water fore and aft. This allowed the cg to be forward for take off and landing, but the CG could be shifted aft for high speed cruise to minimize the required downforce of the horizontal stabilizer and the resultant induced drag.
 
So what has happened in two know cases can't happen

Sometimes it is good to step away from the classroom and static state two dimensional F=MA force thinking. THE SPEED DID INCREASE 5 KNOTS! HELLO, REALITY CHECK!

However, I wish all my competitors follow the do nothing thinking and drone along in hopes of winning because they followed some incomplete rationale.

Bob Axsom
 
In level flight the lift equals the weight of the airplane and is independent of airspeed - Physics.

Lift = weight in constant rate climbing or descending flight also, but not in a level coordinated turn;). Anytime the airplane is in an unaccelerated condtion, lift = weight. The reason trim needs to change with speed is that angle of attack changes, the center of pressure moves, and thus the moment generated by the wing changes, which needs to be offset by changing the lift on the tail.

PS: I'm not an aero engineer, but I did watch a Holiday Inn Express commercial last night...
 
Just out of idle curiosity what is the decalage on an RV-4?

Can the wing produce so much lift at high speed in a specific planform that the tail has to go into a lifting tail mode to maintain level flight?
B.A.

Bob,
The tail is always "lifting" in flight (if the aircraft has a tail). The tail is always providing negative or positive lift as require to counteract the pitching moment about the CG. It is the same concept used on tail rotor blades. 0 or negative AOA would allow the helo to turn opposite direction to the main rotor. Positive AOA would allow the helo to turn in the same direction as the main rotor.

My current RV-4 has a washer between the longeron and the horizontal stab. It does not zeroize the horizontal’s angle of incidence. The incidence angle is still a little negative. For a solo flight in cruise with full fuel, the elevator is dead on level with the horizontal stab. However, it still needs down elevator trimtab.

On my fastback -4 I have zeroized the incidence angle of the horizontal stab by using a 3/16 inch spacer (have to double check my building log at home). Will flight test that configuration and adjust as required for what I consider to be my “standard configuration”.

I was looking at the Panter video during their first flight and saw that the test team (or someone) decided not to have a tail feather fairing installed. I have thought about that too. It would make adjusting the horizontal stab angle of incidence a little less painful however, will change the aerodynamic characteristics when the fairing is installed and the results may not be representative.

If you do decide to change the horizontal stab angle of incidence, please do so in small increments and document the result (just as you do with all your other tests), the original aircraft configuration (fuel state, baggage location, etc.) and atmospheric conditions. Build up in speed and incidence angle, and anticipate the negative effect the change may have and how to mitigate it (prior to flying). I hope some of that helps. Good luck with squeezing more speed out of your -6, I enjoy those posts.
 
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Thanks Axel

That is the kind of information I was looking for. You didn't actually come out and say it but your observations are consistent with what I suspected. I have ordered some 1/8" 2024-T3 to make a shim from but several things have caused me to think this over, and over, and over. I'm not averse to some risk but I want to be certain that I use my best judgement in proceeding with an experiment.

My current thinking is that the RV-4 decalage is probably greater than the RV-6 to deal with the wide CG shift with one and two people on board. Thus I probably should ease into this in smaller increments than the RV-4 experience. It is interesting that you plan a 3/16" shim on you fastback RV-4 because my friend says he could (should) have gone to 1/4" on the RV-4 with the 1/8" shim.

The fellow with the RV-6 showed me with his fingers and numbers that his shim was much less. The numbers he used were on the order of 1/32" but what he showed me with his fingers was on the order of 1/16".

Another friend (my God I have 3 friends, this must all be a big lie) carefully rigged his very fast racer (not an RV) to exactly zero and he said the flight was so squirrely that he landed at a more forgiving airport (big runway) and removed it before flying home.

Bob Axsom
 
decalage

Bob,

I spent considerable time this afternoon looking at my plans and could not find anything that would show the decalage.

This information, I presume is in section 8 ( fuselage construction ) and for some unknown reason, that section is not in my builders manual.

I always enjoy the aerodynamics lessons on the forum, it reminds me of the Air Force Academy Text book on Aerodynamics that was given to me by Capt. Westcott Smith when he returned my designs with his evaluations and suggestions. I was in 6th grade at the time and didn't understand much of it until I was exposed to calculus and physics later on.

Chris M
 
and, once again, I learned my one new thing from Vansaiforce today.....
Decalage will come in handy in "words with friends" ;)
 
''DW'' gets bigger when you go faster

trim.gif


Look at this picture that PaigeHoffart posted. I re-posted it above. The answer to this question lies in that picture.

One key fact that I have not seen you guys bring up yet is that when you go faster, the center of lift moves aft
. (I.e. the faster you go, the bigger "dw" gets in that picture).

This means that, when you take off, the tail needs to produce relatively little down-force, since the center of lift is near the front of the wing and thus very close to the CG. But as you go faster, the center of lift starts moving back, causing your nose to want to drop, so the tail needs to push down more for balance.

If you want a tail with zero incidence during high speed, then that tail would need to have positive incidence (or the elevators would need to be deflected downwards) during slow-speed flight, to keep the nose from rising when the center of lift moves forward. But if the tail incidence gets very high (or if the elevators have to be deployed almost all the way down), then the airplane might become unstable in pitch. And that's a very bad characteristic if you don't have a fly-by-wire system (or some other kind of artificial stability, such as a system that lowers the elevators when the alpha increases) to help you deal with it.

Look at a close-up of the trim markings near the horizontal stabilizer of an airliner (or other large airplane like a B-52). You'll see that just before takeoff, the leading edge of the horizontal stab is all the way up, for minimum down-force. The airplane is basically balanced on the leading edge of the wing with the tail hardly doing anything at all. But as the airplane gains speed, the leading edge of the stab comes down.

But you can't tweak this without also tweaking the CG. If your tail is generating down-force during high-speed flight and you would like to reduce this, the only way is to make the back of the airplane heavier or the front lighter, i.e. move the CG aft. But, again, if you do this too much, the airplane becomes unstable, especially during slow-speed / high-alpha flight when the center of lift moves forward and the horizontal stab needs less downforce.

Or maybe I'm misunderstanding your question. Maybe while going fast, you just want the elevator deflection to be zero (i.e. elevator in line with horizontal stab), rather than wanting the horizontal stab to be generating no force (i.e. horizontal stab and elevator in line with airflow). The first one is definitely achievable - it's achieved in every large jet flying right now. (It's easier for them since they can change the angle of incidence of the horizontal stab "on the fly", not just while on the ground). The second idea (zero force from the horiz stab during fast flight) may or may not be achievable without making the airplane unstable at slow speeds. Depends on how much the center of lift moves (i.e. on how much "dw" changes) between landing speed and racing speed.

(And now I could go into how McDD tried to balance the MD-11 so that the horizontal stab generated as little force as possible during cruise, which makes the airplane borderline-unstable during landing for all the reasons I explained above... but that's a little beyond the scope of the discussion).

For more info: A quick Google search for "Longitudinal static stability" returns some good stuff like this, this, this, this, etc.

And yes, I am an aeronautical engineer :cool: ;) (I do structures work, but I had to take plenty of classes and read plenty of books about balance and dynamics, stability and control, etc).
 
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Bob,

I spent considerable time this afternoon looking at my plans and could not find anything that would show the decalage.

Chris M

I don?t have the plans in front of me right now. I believe the RV-4 drawing (side view) that shows the aft portion of the fuse, the tail wheel spring, and horizontal stab also show the ?spacer? required between the longeron and the horizontal stab. It is very hard to see that spacer because it becomes just another line. The only way I was able to see the spacer was by using highlighters to color each part. Then all of the sudden I could see it. Just as if took the blue pill and became part of the Matrix.

Will post the page number later.

Bob,
I used a simple Excel calculation to determine what size of spacer would zeroize the angle of incidence. You should be able to use trig and calculate the change in angle due to the change in spacer size.
 
decalage

Hey Caveman Bob:

The plans for the -4 call for +.75deg for the wing and zero for the H stab; the -6 plans call for +.5deg on the wing and again zero on the H Stab.

My experience showed that the 6 H stab at zero was not quite what the plane wanted...

Sounds like you are a bit shy on the wing setting, or maybe your wings have some twist built in? If it's working (elevs in trail at race speeds), don't fix it!

If you do adjust the stab, remember to reset the center attach on the fin too - raising the h stab will require a shim at the fin aft spar center attach point.

As for the CP shift on the RV series - not much happens there: the trim acts normally (nose DOWN to go faster; nose UP to go slower).

The prototype F1 Evo has this CP Shift as shown in the diagram - the trim worked opposite the normal direction. Removing some of the undercamber on the ailerons and flaps changed it to a ship that almost needs no trim, as the CP shift is slight, and is matched almost exactly by the H stab setting (+.5deg); wing setting is also +.5deg. Of course, I had Glasair, Lancair, and Cirrus to lead the way with the new foils - they all use flat bottom flaps and ailerons on NLF foils to remove the pitching moment (CP shift).

The 6 you are flying uses the 230XX foil - very good design, and used by a myriad of aircraft.

Carry on!
Mark
 
Thanks guys

I just keeps getting better. I have what I need now. I'll be going to Reno in less than a week tben there is the race in Marysville, OH so I have some time to think on it. As you probably know I'm more of a plodder than a ball of fire but I try very hard to think things out before cutting metal. I have another mod stewing as well.

Bob Axsom
 
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decalage

Bob,

Drawing 30
RV4

1/8 x 1x 2 inch spacer.


Hey Rocket.

Boomer

I have the drawing and the dimension from the stab chord line to the top of the longeron is 1 13/16 and there is a .125 shim ( spacer )

I couldn't find anything in the drawing that gave a relationship between the wing chord line and the longeron. As I recall, establishing the wing and stab incidence was done with a set of spacer blocks of a given dimension and a level. I don't think the exact decalage was ever mentioned.

I think the short wing Pipers ( ie Pacers/Tripacers have a jackscrew that trims the stab up and down. The good thing about the rv series is the relatively large elevator.. I think one would have to work pretty hard at getting outside of its authority....

Chris M
 
Maybe while going fast, you just want the elevator deflection to be zero (i.e. elevator in line with horizontal stab)... The first one is definitely achievable - it's achieved in every large jet flying right now. (It's easier for them since they can change the angle of incidence of the horizontal stab "on the fly", not just while on the ground).

This squares with my engineering experience as well. I see no reason why small, incrementally-tested adjustments to stabilizer incidence could not be used to reach a point where the elevator is faired at neutral position for a desired flight condition. Stability and control margins are maintained; you just minimize the drag associated with deflecting the elevators (exposed mass balance faces, etc) to hold a trim condition. Most of us would probably want this at our cruise speed, but for racing you'd want it up near your maximum speed.


Now, trying to reduce trim downforce is a different ballgame entirely. Flying with an aft CG but still within limits is the best you'll probably be able to do without starting to tread pretty dangerous ground. I think effort trying to do this would instead be better spent on finding other ways to reduce drag.
 
I got the information I was looking for and I will proceed with the experiment

I got the information I was looking for and I will proceed with the experiment. I have done a lot of drag reduction and power enhancement (powered by Red - Red Hamilton that is) while maintaining the 360 cubic inch displacement required by the RV Blue class racing. I have another simple mod I am looking at that I will implement first, then I will shim test at 6,000 ft density altitude using the USAR triangular test method twice in succession and use the NTPS post processing spreadsheet to eliminate the wind effects and get an accurate KTAS then change the shim and repeat (paying attention to the fin & rudder alignment situation Mark).

Bob Axsom
 
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biplane at Reno

Bob,

If you haven't already done so, take a look at the wings on the little white biplane that sits in the front corner of the back hangar at Reno. That little outfit will haul potatoes and it has some very odd looking wings and wingtips. I don't know beans about aerodynamics, but that airplane really drifts from the norm in design. Nevertheless, it smokes all of the conventional stuff with basically the same engine. Just saying.

David Watson
 
I recall from my sailplane days that there was some experimentation with pumping water fore and aft. This allowed the cg to be forward for take off and landing, but the CG could be shifted aft for high speed cruise to minimize the required downforce of the horizontal stabilizer and the resultant induced drag.

IIRC, the tail ballast tanks were also to compensate for the fact that the main wing ballast tanks were usually ahead of the CG.

But the racing pilots would adjust their CG to be on the aft limit for minimum trim drag.
 
Process

Several years ago I went through the process of varying the angle of the horizontal stab on my Tailwind, not to gain speed but to make it more controllable in the landing configuration. Initially we were unable to trim it out sufficiently on finals.

Tailwinds are very like RV in that they have a wide speed envelope, stall was about 57mph and flat out she would do 180mph on an 0-200.

The reason I am commenting is just to emphasize the need for caution as altering the angle has a big effect on stick force and the ability to trim for any given condition.
 
How to make the change

Hey Caveman Bob:

Rule of thumb applies here: shim the LE of the stab (root) about 1/2 the distance and direction the elev horn is deflected.

So, if the elev horn (counterweight) is deflected 1", move the stab LE 1/2" at the root. My expectation is that your measurements will be smaller....

Hey Boomer.

Carry on!
Mark
 
I suspect you are right Mark

Good info Mark. I have a difficult time getting my head turned enough to see the tail - maybe the outboard 10" of the horizontal stabilizer and elevator at ~ 180 kts and it is kind of a snapshot as my head is stretching the limits of neck rotation and snapping back. In that flight state with the autopilot engaged with altitude hold (absolutely steady state) I see zero horn exposure. there may be some horn leading edge exposure below the stab tip but I can't see it. My top speed is a fraction of a knot below 190 kts but I have not looked back there in that state.

Bob Axsom
 
Bob,
Some ideas:
1) Put a piece of yellow tape on your HS next to the elevator horn. The yellow and blue contrast will let you see the difference much easier.

2) Consider mounting a video camera somewhere (inside or out) looking back at the elevator. Sink the time on the camera to gps time. Once you are doing your runs and everything is stable, take the time of your gps. Go back and look at the video around that time frame.

3) A qualified formation pilot looking over your aircraft.

Typical flight test stuff. No need to re-invent the wheel if you can't see back there.
 
Decalage

A few weeks ago I asked if any one has put a level on the upper longerons of a rv8 while in flight. I was curious about this ,as the wheel pants are installed "in trail " when the upper longerons are level ( Sam James Pants).
Now that the tail incidence /decalage discusussion is in full swing , some on might be able to report on the upper longeron/level question .

Tom
 
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