Stab and Elevator size

[Sidney Raiford]

As I look at the modern stunter designs, I can't help but wonder when we started making stab-elev area so incredibly big.. I saw one this afternoon that looks like it was easily 45% of the wing area. What is driving this trend?

[Dave_Trible]

As I look at the modern stunter designs, I can't help but wonder when we started making stab-elev area so incredibly big.. I saw one this afternoon that looks like it was easily 45% of the wing area. What is driving this trend?

Wow that seems huge!  The stab/elevator percentage has grown over the years from something like 16-18% in the 50s to mid 60s to the average 24-26% today.  Some approach 30%.  The main reason is to allow for a more rearward CG in the effort to increase the square turning ability and still maintain smoothness and predictable tracking with the rearward CG.  There is probably a point of lost returns in this and we might be near that point.  Larger tail surfaces begin to exponentially increase construction and finish weight to a point of needing to add nose weight or length which then is at cross purposes to what was being attempted.

Dave

[Howard Rush]

Robby Hunt thinks it should be 400%.

[Brett Buck]

As I look at the modern stunter designs, I can't help but wonder when we started making stab-elev area so incredibly big.. I saw one this afternoon that looks like it was easily 45% of the wing area. What is driving this trend?

  To provide sufficient torque to maneuver the airplane through tight corners, and to provide sufficient damping that it comes out of the corner straight. A typical size is 25%, and a few are larger than that.

   Another difference is that the tolerable CG range gets larger with larger tail volume coefficients.

   BTW, making it longer OR larger works, but has different effects. For a typical full stunt airplane (40-60 sized, 650 square inches) the hinge-to-hinge distance is about 18".

    Brett

[Sidney Raiford]

Hmmmm, OK, then why not just take an inch or two off of those seemingly overly long noses? Tail moments, I can understand.. BUT, coming from a combat background, I can see no reason (other than appearance) for the nose moments to be so long- and is that why these things need flaps? To haul that preponderous proboscis through 120° of arc because of too short of a tail moment?

[Brett Buck]

Hmmmm, OK, then why not just take an inch or two off of those seemingly overly long noses? Tail moments, I can understand.. BUT, coming from a combat background, I can see no reason (other than appearance) for the nose moments to be so long- and is that why these things need flaps? To haul that preponderous proboscis through 120° of arc because of too short of a tail moment?

   You need to balance the airplane, and, you need to fit all the parts inside. My nose is about as short as it can be and fit in the engine, fuel tank in front of the wing. That's with a 6" long fuel tank that is 1 1/4" thick to fit the required 7 ounces of fuel. It's tight enough that the front of the tank has to have the corner knocked off to clear the header. Others fit >8 ounces using still thicker tanks.  My airplanes tend to be nose-heavy nonetheless.

   For electric the noses tend to be longer because they tolerate a further-forward CG and the motors are relatively light.

    Brett

[Sidney Raiford]

Yes, to balance, of course. So I'm thinking maybe these concourse beauties are coming out a little tail heavy, then? Sorry, not trying to be a noodge, but I've seen my fair share of stunt tanks and where they tend to get stuffed, and always wondered why a more appropriately shaped tank isn't used- just convenience? With current materials, those long engine bearers that have to be squeezed between can be sized down tremendously behind the firewall.. Sorry, just thinking out loud.

[Brett Buck]

Yes, to balance, of course. So I'm thinking maybe these concourse beauties are coming out a little tail heavy, then?

    No, not usually, while mine is not a councours winner, they tend to come out nose-heavy. I don't see a lot of other people adding tons of nose weight, either, at least not any more. I can't easily see a way to make the tank a lot shorter without also making the nose wider. I am not sure what you mean about beam mounts - you need the mount to be particularly sturdy to get a proper engine run, particularly with larger engines. My bearers go all the way back to intersect the wing to tie them together for more rigidity.

   I am trying to get all the lighest stuff in the nose I can (in this case, a lighter prop washer and carbon spinner) to try to get 1/2 ounce weight I have on the tail even after a significant repair.  Other people use magnesium spinner and engine parts (head/clamp ring, backplate, spinner).

    Electrics can be built a lot lighter because of no vibration and the CG wants to be further forward, the motor is way lighter than an engine so the noses end up a lot longer.
   Brett


   

[Dave Hull]

Tank shapes are not arbitrary. As one example, you need to the center of volume of the tank to line up with the pee hole in the spraybar as a starting point. Many engines have a "standard" height (offset) between the mounting surface and the spraybar. This led to a standard commercial stunt tank height of 1". After you get things mounted in the center, then you can start "tuning" the tank. A deep tank does not always work well as it creates more head issues in hard corners. Muffler pressure can only make up for so much of that. An engine with more offset between the lugs and the spraybar gives you the opportunity to see if it can handle a deeper tank. But just because there is room, doesn't mean that the fatter tank will run well. The resistance to draw due to tank length is much less of an issue than tank height (thickness). So you tend to see long skinny tanks because they cause the least problems for fuel draw in maneuvers and as the fuel level changes.

If you go back to old combat designs with hard tanks, you start to see similar conventions. Running case pressure helped. And switching to a bladder helped.

[Brett Buck]

Tank shapes are not arbitrary. As one example, you need to the center of volume of the tank to line up with the pee hole in the spraybar as a starting point.

   We have run tanks with the wedge, pickup, and uniflow vent 1/4" from the top, and 3/4" from the bottom at the front, and 1.5" to the bottom at the back, and tapered in two dimensions - depth is 1 inch at the front, 1.75 at the back, width is 1.75 at the front and 2.25 at the rear. Ran exactly like symmetrical tank. This for clearance on a short nose, clears the header at the front, and gets really fat at the back, to try to get 7 ounces.

   Brett

[Dave_Trible]

Yes, to balance, of course. So I'm thinking maybe these concourse beauties are coming out a little tail heavy, then? Sorry, not trying to be a noodge, but I've seen my fair share of stunt tanks and where they tend to get stuffed, and always wondered why a more appropriately shaped tank isn't used- just convenience? With current materials, those long engine bearers that have to be squeezed between can be sized down tremendously behind the firewall.. Sorry, just thinking out loud.

There aren't really many cases where a tank is set between engine bearers,  just resting on top (really below.)  I use 1/2" square motor mounts which gives me a little more room.  Then my tanks don't have a wedge.  I simply pinch the rear corner together on the centerline (1 3/8" thick tank). Finally the front is sloped at about 45 degrees to clear the header.  With this I get the nose under 9" with the RO Jetts.  Tanks end up about 7 1/2 ounces.  Transmitting vibration out of the nose and back into the rest of the airplane to mitigate it is important .  You don't want to cut those 'RR tracks' down.  I have seen where a plastic clunk tank is used and the mounts relieved some to allow it to set in lower since they are so thick.  In those cases I guess it works ok but I'm not a fan. 

Dave

[Robby Hunt]

Robby Hunt thinks it should be 400%.

[Sidney Raiford]

You don't want to cut those 'RR tracks' down.  I have seen where a plastic clunk tank is used and the mounts relieved some to allow it to set in lower since they are so thick.  In those cases I guess it works ok but I'm not a fan.
Dave

Actually that is what I was thinking- Like molded in carbon mounts in a mostly carbon plus wood front end. the engine bearers would be solid carbon with their aft ends thinning and spreading to distribute the load back into the upper shell & crutch. Anyway, I appreciate the perspective and insight, thanks to all for your input.

           Gus

[Dave Hull]

"We have run tanks with the wedge, pickup, and uniflow vent 1/4" from the top, and 3/4" from the bottom at the front, and 1.5" to the bottom at the back, and tapered in two dimensions - depth is 1 inch at the front, 1.75 at the back, width is 1.75 at the front and 2.25 at the rear. Ran exactly like symmetrical tank. This for clearance on a short nose, clears the header at the front, and gets really fat at the back, to try to get 7 ounces."

Interesting. I have not seen this, not having run a pipe. Not many (any?) of these in our club, either. So the real issue is the fuel mass distribution relative to the spraybar. I can't quite visualize from my sketch based on your description what the maximum and minimum head actually is.

[Dave Moritz]

Dave

Jolly good laughing point that "peehole in the spray bar"!

Moe...

[Brett Buck]

"We have run tanks with the wedge, pickup, and uniflow vent 1/4" from the top, and 3/4" from the bottom at the front, and 1.5" to the bottom at the back, and tapered in two dimensions - depth is 1 inch at the front, 1.75 at the back, width is 1.75 at the front and 2.25 at the rear. Ran exactly like symmetrical tank. This for clearance on a short nose, clears the header at the front, and gets really fat at the back, to try to get 7 ounces."

Interesting. I have not seen this, not having run a pipe. Not many (any?) of these in our club, either. So the real issue is the fuel mass distribution relative to the spraybar. I can't quite visualize from my sketch based on your description what the maximum and minimum head actually is.

    The tank is defined by the uniflow vent position, not by the mass distribution of the fuel. Put that in the right place to get the right outlet pressure for the engine (notionally the spraybar, but whatever the engine needs - frequently offset a bit toward the cylinder head) and it doesn't make any difference where the rest of the fuel is stored relative to it. The one caveat is that the wedge *angle* needs to be the same above and below the pickup. This only matters for the cutoff characteristics.

  The tank described above was used for one of Jim Aron's airplanes which was intended to be flown with a 40 or 46VF, for whatever reason, his particular engine wanted to use close to 7 ounces of fuel (as I recall - it was 25 years ago) instead of what we usually do, instead of 5ish. So I drew up and built the tank in a hurry to get to the NATs in time. He ended up switching to a PA65, which naturally uses about that much, so it worked out. Looked pretty weird but worked fine. I think he finished 4th in Advanced with it after a few problems in the flyoff.

    I also note that the tank needs to be adjustable up/down in extremely fine increments - like on the order of .010", a few layers of masking tape. It's not always exactly in line with the spraybar, almost always, it has to be offset "down" as the airplane sits on the wheels. This is due to the "internal ballistics" of the engine, and the tendency of many engines to run leaner on outside turns than inside. Shimming the tank to get it roughly even over the range of g loading is common. Some engines are much more prone to this than others, some venturi types reduce or effectively eliminate the problem, others make it much worse.

     Brett

[Mike Griffin]

As I look at the modern stunter designs, I can't help but wonder when we started making stab-elev area so incredibly big.. I saw one this afternoon that looks like it was easily 45% of the wing area. What is driving this trend?

I have noticed that as well Brett. They are beginning to look like Wild Bill's Doodlebug.

Mike

[Brett Buck]

I have noticed that as well Brett. They are beginning to look like Wild Bill's Doodlebug.

   I think you were replying to Sidney, not me. But...

  The "25% stab" goes back to the early 70's with Bill Simmons "Gemini" (which heavily influenced the much-better-known Genesis by Bob Hunt).

   The early 70s are more than *50 years ago*, that is, more than *half a century*. I am not criticizing anyone, or accusing anyone of anything, this is just an observation, but - how in the world can we be doing something for 50+ years and not notice it until now ?!

    It's the same thing with engines - people are still fiddling with 4-2 break engines 40ish years after they became obsolete, and didn't notice or care about it. We now have electric, people are amazed at the performance. Great - but you could have had something nearly equivalent in 1988!

   All this has been well-documented for decades, and we now have a way of directly communicating with most of the people involved for free any time you want. I and almost anyone else are right here to answer any question asked. Most of these airplanes have plans available (some, like the Imitation and Impact, to the nth detail so that absolutely anyone can see what is there and why). Are we doing something wrong as a group that this sort of information just disappears or is not obvious?

   Brett

p.s. BTW, the Doodlebug was far from the first airplane to have that enormous high-aspect ratio stab - the most obvious being the original Humbug from the late 60s. The idea was the same - make sure you have enough pitch control authority, fast enough, to generate the AoA necessary fast enough to minimize the turn radius. The Doodlebug had the effect of trying to get the pitch rate so high you could stall it easily, like the BiSlob.   I would note that while it definitely does what Bill intended, this sort of high-aspect ratio stab is very much not preferred for stunt competition because of the touchy control response. For (again) many years, several of us have done the inverse - low-aspect ratio stabs with a 60/40 or lower spit. This was inspired by Gid Adkisson's Bud Light Lazer, which was noted for it's excellent and very consistent corner exits.

[BYU]

" A typical size is 25%, and a few are larger than that."

Does the 25% include flap area?

[Brett Buck]

" A typical size is 25%, and a few are larger than that."

Does the 25% include flap area?

   Yes.

   Brett

[Ken Culbertson]

The "25% stab" goes back to the early 70's with Bill Simmons "Gemini" (which heavily influences the much-better-known Genesis by Bob Hunt).

It goes back even further.  I was flying one in '63.  No clue why, I was 16 and it just looked cool.

Ken

[Brett Buck]

The "25% stab" goes back to the early 70's with Bill Simmons "Gemini" (which heavily influences the much-better-known Genesis by Bob Hunt).

It goes back even further.  I was flying one in '63.  No clue why, it just looked cool.

   I am sure people did this and many other things over the years. Bill Simons and the search for the "0-radius corner" was the point at which it became essentially standard. Interestingly, the intent was to make it possible to have the CG far forward (which was the predominate trimming technique at the time, e.g. "I only got 12 appearance points, so I added an ounce of nose weight") and still have enough power to shove the nose around.

   This was one big step along the path to the other, more comprehensive solutions leading to the best/definitive IC airplanes, particularly, the Impact.

     Brett

[Trostle]

To provide sufficient torque to maneuver the airplane through tight corners, and to provide sufficient damping that it comes out of the corner straight. A typical size is 25%, and a few are larger than that.

    Brett

Interesting comments in this thread on the "evolution" of "modern" design for our stunt models.  I thought some might find it interesting about an article in the January 1960 issue of the Aeromodeller magazine (hard to grasp that was only 66 years ago).  The article included a table of 20  dimensions for 16 designs (all from the United States), all recognized designs, even today (thanks to Mike Keville for inventing the Classic event for VSC).  At the bottom of that chart is a line for the "Trend".  The "Trend" for total wing area was 620 sq in.  The "Trend" for flap area was 15% of the total wing area.  The "Trend" for for what I understood as total "tailplane" area was 16% of the wing area, with the elevators being just less than half of that - 7.3% of the wing area.  Those numbers are what I used for my Semi-Scale Focke Wulf that I flew in the mid 1960's, winning my first stunt trophy and eventually placing in three Nationals with that design.  For the standards of the day, it flew OK but I often heard that Trostle did not have any corners.  Indeed, there were other designs in the 70's that had better corners, but I am sure most had flaps approaching 20% and tails well over 20% including Charles Mackay's Gableswantz (and Bill Simons and a few others).  The blinding corners with the piped ships and electrics of today with compartively huge stabilizers had not yet been invented.

Keith

[Dan McEntee]

Interesting comments in this thread on the "evolution" of "modern" design for our stunt models.  I thought some might find it interesting about an article in the January 1960 issue of the Aeromodeller magazine (hard to grasp that was only 66 years ago).  The article included a table of 20  dimensions for 16 designs (all from the United States), all recognized designs, even today (thanks to Mike Keville for inventing the Classic event for VSC).  At the bottom of that chart is a line for the "Trend".  The "Trend" for total wing area was 620 sq in.  The "Trend" for flap area was 15% of the total wing area.  The "Trend" for for what I understood as total "tailplane" area was 16% of the wing area, with the elevators being just less than half of that - 7.3%.  Those numbers are what I used for my Semi-Scale Focke Wulf that I flew in the mid 1960's, winning my first stunt trophy and eventually placing in three Nationals with that design.  For the standards of the day, it flew OK but I often heard that Trostle did not have any corners.  Indeed, there were other designs in the 70's that had better corners, but I am sure most had flaps approaching 20% and tails well over 20% including Charles Mackay's Gableswantz (and Bill Simons and a few others).  The blinding corners with the piped ships and electrics of today with compartively huge stabilizers had not yet been invented.

Keith

  I have the plans for your Focke Wulf, and who knows, someday I might get the gumption to attempt building it!! Given what you know now, how critical is balance in this mix? I think some don't pay enough attention to this. I didn't ever get to see that model fly, but what it may have lacked in corner, I'm sure it compensated well for that in other areas of the pattern , and your hand on the handle sure helped!! And with the stab area increasing to the 20+ percent range, thickness and shape become more important, or all that extra weight and area could become a detriment to achieving a better, smoother corner?
  At the bottom rung of this ladder, I have a Netzeband Dootlebug with a Norvel .061 that was built by the late Jim Wilson. As he built it, it was pretty sensitive, and had a quick elevator and a lot of throw. I don't think it needed all of that and cut the throw in half, slowed the controls down to a more manageable speed with a longer elevator horn and a better handle, then started test flying and moved the balance point back until I liked the turn and then moved on from there. It's fun, and does the entire pattern well enough for me, but when all was said and done, I just don't think that big stab was necessary. I'm sure it added weight and also drag, and I wonder how it would perform with a stab/elevator half the area? But then it wouldn't be a Doodlebug!
  Type at you later,
   Dan McEntee

[Sidney Raiford]

I asked the initial question because as I calculate the stab & elevator area on the the plane I'm building now, I see that it's 22.5% of the wing area, but it just looks huge. The wing area is 749.5 sq. in.