Attitude issue.
 

I did the maiden on my Pitts today and it took about 4° of down elevator to fly level. The plane is a tad nose heavy so this should not be. The H stab is a set 2°, the wings are set supposedly at 0°, with no down thrust on the engine. Which area should I be looking into for adjustment to correct the elevator?

Here’s a 50cc biplane I have with everything set at zero. No trim is necessary. Getting your wings to zero incidence will help. Dan.



http://www.rcuniverse.com/forum/atta...mentid=2098587

Hi Ace,

Why do you think that's a problem? The trim is what it is. There is no rule that says the elevator must be lined up with the horizontal stabiliser (I presume this is what you are seeing as wrong ?).

There are some theoretical advantages in having it faired, but these are only minor drag effects. A pylon racer might be interested in it or a full size designer looking for best possible range, but we would never notice any difference in a model Pitts (nice model BTW, I had a quick glance at your build thread).

Try flying your model at a low cruise speed and trim out the elevator. You will notice it's no longer 4° down, it might even be aligned with the stab now, or it might even be up a small amount. It depends how slow a speed you trimmed for. So the trim is speed dependant, the faster you go the more down trim is required. This effect is more noticeable with a forward balance point. Do you have a lot of power? If so you may be cruising faster than the designer designed for. Again more down trim would be required.

If it really worries you then the best fix would be to cut out the horizontal stab and shim up the leading edge a degree or two, you might be lucky enough to guess it just right so that the elevator now fairs with the stab. Now the whole elevator and stab are making the same lift as the 4° deflected elevator was before but with maybe very slightly less drag. Seems like a lot of work to end up with the same trim but elevator faired.

You could move the balance point forward, which would require less down trim, but the aeroplane would fly a lot worse. More elevator deflection required for the same pitch response, more drag, more likely to nose over on landing, heavier model unless you can move stuff forward, more down elevator required when inverted etc.

Some down thrust would help, and might be a good idea if you are getting a lot of pitch up as speed increases due to forward balance point. (moving balance point aft might be better in this case though).

Reducing the wing rigging angle would also require less down trim. This affects the thrust angle as well though. Let's assume for arguments sake that your wings fly at 1° nose up at your normal cruise speed this means that your fuselage and engine are 1/2° nose down (wings are rigged 1 1/2° nose up right?). If you reduce the rigging angle to zero you will need up (less down) elevator to get the wings back to the flying angle of 1° nose up. The fuselage and thrust angle will now also be 1° nose up unless you increase down thrust by the same 1 1/2°.

If It was me I would concentrate on adjusting the balance point and control throws, mixes, expo, and trim to get the model flying the way I like it. Then, after parking it, give full down elevator before turning the receiver off. That way the elevators would look scale hanging down like they do on a parked Pitts and no one would know what the trim setting was.

Hope this helps,

Dave H

Robert, I would see if the wings can be adjusted to zero. Of the two aerobatic bipes I have flown ( CG Ultimate Nd Dave Patrick Ultimate ) both of them had the wings at zero with the stab at 2 degrees positive. I am currently building a Reed Falcon and as per the plans it has both wings and stab at zero but engine with 2 degrees up thrust. The common point on all these bipes is the wings are at zero. Before doing any mods I would get comfortable with it and do some CG tests. Pull to a 45 degree up line, roll inverted and see what it does with sticks at nuetral. What your looking for is a gradual arc back towards the ground. If that checks out the see how she tracks on an up line. If she tucks towards the gear and CG is good then yes you would need to reduce the positive incedence in the wings.

Work with the balance and thrust angle to get
it to fly the way you want first. (knife edge,
inverted, vertical up and vertical down, high
speed, low speed, stalls and spins) THEN
start messing with the stabilizer angle.

Jenny

Just spent four hours at a friends garage going over everything. The wings are 0°, stab 1-1/2°, and the engine is just a tad down maybe 1/4°. The only thing I can figure is it is either tail heavy or needs more down thrust on the engine. The balance will get checked again tomorrow. Down thrust is not the direction I want to go if it can be helped, way to much disassembly to get to the engine mount bolt nuts. I am considering adding a lb to the nose and see what that does. I will figure something out and with the suggestions here I have plenty to think about.

Measuring the rigging angles and the balance tells you what they are; it doesn't tell you what they need to be. Flying the model will tell you what they need to be.

If your elevator isn't lined up it means that either the designer didn't bother changing his design after test flying (more common than you'd think) or your model is different than the original. Heavier, lighter, faster, different balance point, thrust line, etc, etc, or even that the rigging is different.

You are proposing to take a (presumably) nice flying model and adjust the balance point to a worse position just to make the elevator line up. IOW you want to make it fly worse so that it will look pretty.

You don't adjust balance and down thrust to get the elevator to a given angle, that's putting the cart before the horse. Instead you adjust the balance and down thrust so the model flies the way you want, then trim the elevator as needed.

You have a choice;

1. You could accept the designers balance point, rigging angles, and control throws as being correct.

2. Adjust some or all of those things to have the model fly more the way you like. Speed and Jennifer have mentioned a couple of points and you can google more extensive trimming methods (triangulation trimming is one example and there are others).

At the end of the day the elevator will be where the elevator needs to be. And that's the correct position for it. If it doesn't line up with the stab that's totally irrelevant.

The designer may have had the elevator close to lined up with the stab but if you've changed something like installing more power for example, or changing the balance point, then you should not expect the elevator to line up any more.

Dave H

Well Dave, you touch on a few points that I did not mention as there are many changes to the original design. First on the agenda the wings were moved from an S1-S position to an S1-T position, on the full scale this was to accommodate a much larger engine, and such is my position as I went with a much larger engine. Now as this has been done the math to correct the balance point was calculated as well as all other points. As for positioning of incidences these were set to a full scale's position as is most everything. I typically like to build scale and expect the flying characteristics to be the same, being such there should be no down elevator trimming required as a Pitts of this nature is typically a bit nose heavy. Thus my concerns about the down elevator, and it was the first I have had do this in 35+ years of modeling. Now I will say I may have jumped the gun coming on here and asking for opinions as I had not done much investigating into the issue. I guess I was just thrown for a loop with such as I thought everything was perfect by the math and such.

I will follow up with the issue next post.

I posted just a bit ago on my build thread.

Woo Hoo! I have figured it out, you know, why the ele is trimmed down. I have been racking my brain over this as it just did not make sense to be with it not being tail heavy. Well, turns out, it is tail heavy. Here is what happened. I was trying to figure out how to send the spreadsheet to a friend where he could open it, turns out it is his computer and not the spreadsheet. Anyway, while I was working with it I decided to run the numbers one more time. Yea, I'm anal enough to have wrote down the figures. And the first time I looked at it and thought "Yep, it says I am nose heavy". And just as I cleared it something went "Whoaa dum azz". So I punched them in again and it turns out I was reading the results wrong, the spreadsheet was telling me what the tailwheel should weigh and for some reason I was reading it as to what it does weigh. I have no idea why, I mean come on, I just typed in the actual weight, but somehow. Duhhhhh!!! It is a tad bit tail heavy which for this plane is very tail heavy as it really needs to be nose heavy. So, next week I will move as much possible forward but until then I will add some nose weight and see how it goes.

Argggg, I'm an idiot.

I really do appreciate every response to my question. And, as it may be, it was a loaded question for I was wrong with my calculations.

Well Robert, well done on the redesign, establishing and measuring a safe balance point. I still think you are doing things backwards now though.

Did the model show signs of being badly tail heavy when you flew it (I'm not talking about the elevator position here)? If it flies ok why not try the cg tests Speed, Jennifer and I are recommending. That will tell you if the balance point is right or not. If it passes the cg test but is too twitchy reduce the elevator throw or add expo.

If you arbitrarily move the balance point forward it should still be safe enough to fly (as long as you don't go to far) but why would you if it involves modifying and extra weight that may turn out to be unneccesary.

I understand the full size S1-T may still be on the nose heavy side, but you can be sure it flies inside its safe cg envelope. That's a legal requirement. It seems odd to try to replicate what most would see as a fault for scale reasons but each to their own.

By the way that elevator aligned with the stab trim setting hardly ever occurs on full size aircraft. It can only be right for one particular balance, weight, and speed. As soon as you change speed or burn off some fuel the required trim will change. Even the S1-T will have the elevator deflected down if you fly fast enough

Here's some good tests for cg;

1. Trim for hands off straight and level at a medium to high cruise speed, push forward into a 45° down line and release the controls.

If it pitches nose up quickly that's nose heavy.

If it pitches up slowly that slightly nose heavy.

If it maintains 45° that's neutral.

If it pitches nose down it's tail heavy.

2. Trim as above and then roll inverted hands off.

If the nose drops (models nose up) strongly, nose heavy.

Weakly, slightly nose heavy.

Stays level, neutral.

Nose rises (down relative to the model), tail heavy.

3. Trim as above, pull 45° up then roll inverted.

same symptoms as 2. above.

Most pilots prefer slightly nose heavy.

Another down side of too nose heavy is that the elevator trim changes strongly with throttle changes. IOW when you close the throttle the model goes into a steep dive, and when you open it the model climbs strongly. Down thrust helps with this problem as does an elevator-throttle mix but you can't get it dead right. This in addition to the other things I mentioned in my first post.

At the end of the day you do what you are happy with though, I'm sure you will be fine either way.

Best of luck,

Dave H

Dave, I am not changing or moving anything until I get a few good flights on it. That maiden was not enough to know any habits of the aircraft. The up-line, down-line, inverted, fast and slow speed passes, will indeed tell me what it may or may not need.

Thanks my friend.

Well I learned a few things here Robert from your ? so it is all good. Since full scale AC have trim tabs I would assume it is not uncommon to have to trim out some at different weight, speed, etc variables. Congrats on a successful maiden a long time coming and a stellar looking AC.

G'day Robert,

Just caught the last couple of pages of your build thread and saw the maiden video.

Very impressive model, and having the owner of the original posting as well. Very impressive, what a great project.

Dave H

Robert, sounds like you are on the right track. Everything that Dave states as far as CG tests are good points. Rapture. Knowing that full scale airplanes need to have their pitch trim adjusted constantly is where most model pilots get hung up. They see this condition with their models as being normal. IMO getting the CG in the " sweet spot " will allow the model to keep the same pitch trim from 1/3 to full power without any reduction in tracking stability. The end result of this and about a dozen other trimming tidbits gets you a model that not only performs aerobatics well but is easier to fly across the entire envelope.

Thanks Speed, all I knew was the down elevator is an indication something was amiss. Now I can fix it and then worry about all the other flight parameters of trim.

I agree with Dave H. Once you've done a successful maiden (congrats!), flight testing is the way to decide the balance point. The down trim is not an indication something is amiss.

By 1917 many aircraft had adjustable stabs, and after the war it became the norm. But most of our models have fixed stabs, so the elevator trim must suffice. Trying to fit the rest of the trim to a stab that cannot be adjusted is going about things backwards.

Jim

Our planes are designed to have neutral control surfaces, or at least mine are, so if it is not inline with the opposing structure there is something amiss. All, and I mean all, of my airframes have this arrangement of neutral control surfaces.

Mine too Robert. I'm going to catch some flack for this I'm sure but the guys who accept different pitch trim as airspeed changes just because it is standard full scale practice are doing so blindly through a lack of knowledge of how CG and proper trimming tecniques effect our models. If more would listen with an open mind they would have skills to make their airplanes fly better and easier.

LOL! I know, I get it all the time by the club KIA's. When I built my Cub and Waco I made a removable stab adjuster for setting up in flight. Once set, a dab of epoxy and remove useless components, and then fly tail appropriate for that airframe. The Pitts had the right numbers, as noted from several previous builds, hence I knew something was wrong. I have had several good pilots fly my models and they all say the same thing once done, "Man, that thing flies so effortlessly". I must do something right!!!

Robert aka acerc
Naaaa that's just 30+ years of learning and building .


Cheers Bob T

Had the same problem on a BH pitts. I too thought it was an incidence angle.