I am building a VQ Models Pilatus PC-6. It is a high wing plane with a straight wing. This model has a flat bottom wing. Other modelers have reported a great tendency for this plane to climb when speed is increased. Some of tried to change the main wing incidence (raised the trailing edge) but had little effect. Others have added down thrust to the motor and had good results. The horizontal stabilizer is set at zero incidence.

My questions are:
1. Is this tendency due to the flat bottom wing (my guess).
2. Can this be properly compensated for without adverse effects on slow, medium and high speed flight?
3. Will a change in the horizontal stabilizer incidence help?
4. What is the best method to correct this problem?

Thanks,
Vic

Great looking pilatus!
The "problem" is simply a characteristic of of ANY setup whre the thrust is not inline with the center of drag.
So the thrust can be pointed to reduce the issue or you can simply learn to change trim at different power/speed settings
That's what you have to do on the full size versions.
To answer each qiestion
1
1- flat bottom Clark Y whatever - all the shapes which are NOT symmetrical , simply have a built in angle of attack- you can't fix that - just adjust angle to suit .
2- Not really
3- no- speed will always produce more lift no matter how you angle stuff.
4- no

You have to accept that all airfoils and wing/engine placements are compromises. As are CG placements.

To get good visibility - the wing is above the pilot
To get more power - a long turbine engine made the prop location - way out that.
The big ol clunky looking wing has lots of lift over a broad speed range
The result is a great utilary aircraft

A plane which looks like an EDGE 540 is far easier to trim for minimum variances with speed.
But then - you wouldn't have a Pilatus
would you?

Vic,
A wing generates lift in proportion to the square of it's velocity. So, when all other things are held constant the plane climbs when prop thrust and airspeed increase. Notice I said "proportional" not equal. The wing also generates lift in direct proportion to the angle of attack of the wing. Why am I bothering to mention this? We can use one to compensate for the other. Here's a common example. When landing everyone knows you have to steadily increase up elevator as airspeed falls in order to maintain a constant descent rate to touchdown. What you are doing is increasing lift with angle of attack of the wing to compensate for the decrease in lift due to falling velocity.

Designers use the same technique to automatically maintain altitude with an airspeed increase. Think of your Pilatus as a yardstick attached to the ceiling by a string. The string is attached to the yardstick at it's center of gravity so it is balanced level. If we tape a quarter to one end of the yardstick that end sinks slightly and the yardstick is no longer level. Downthrust in an engine is just like adding weight to the end of the yardstick. A prop with 0 degrees of downthrust directs 100% of the force horizontally. But if the engine is angled down at 2 degrees, about 0.1% of that thrust is straight down just like adding the quarter to the end of the yardstick. Even better is the fact that the downforce increases in perfect proportion as velocity increases. In a nutshell, we put some downthrust in our prop so that as lift increases in proportion to the square of the plane's velocity, it also decreases due to downthrust pulling the nose down and decreasing the angle of attack of the wing reducing lift. When everything is designed correctly there is no climb when speed is increased.

All full size a/c will climb with power, and descend without power. Your Pilatus will fly level at one power setting. That is governed by the angular set up between the wing and tailplane, and the weight of the model. So far there is not much you can do about that, you can play around with downthrust, but the real one doesn't have strange thrust angles, the pilot just uses power to climb, trim to fly straight, at some lower power 'cruise' setting, and descends at idle. Perhaps, instead of trying to fly it like an aerobatic 'pattern' airplane, which is a pretty artificially designed 'one task' model, you should fly it like the real thing. Take an 'intoductory' at you local full size field, and see how the pilots juggle trim, power and airspeed, then try it with the Pilatus, much more satisfying to fly it properly, than just 'driving' it around the sky.
Evan, WB #12.

This tendency to climb is not related to the wing section or the incidence angles. If you alter the wing or stabilizer incidence without doing anything else you will merely change the static trim for the model. In flight you'll end up compensating for that by altering the elevator trim until you're right back where you started.

So what is making the model climb? It's the CG position and the resulting trim the model requires to fly level in cruise. If you move the CG back you'll need less decalage (the angle between the wing and stabilizer). And less decalage means a weaker tendency for the model to want to climb with a change in airspeed.

First off a airplane that wants to climb when speed is added is entirely natural and desireable in all but fancy aerobatic stuff. However what happens when the tendency is too strong is we end up fighting the model all the time.

Now obviously you don't want the model to react like a totally dead neutral "point and shoot" 3D aerobatic model. A Porter just isn't that sort of flyer. So a compromise needs to be found. You want to move the CG back in small amounts that are equal to about 1% of the chord until you find that it flies more to your liking. Obviously moving the CG back will require some down trim be added to where the elevator is at the moment. Test this with the engine at a steady moderate to low power setting by diving the model and then letting go of the stick to see how strongly it wants to lift the nose as the speed builds. Also you can test this by seeing how much down stick you need to hold the dive. When it seems to be flying with a slight but easily managed tendency to lift the nose in high speed dives and maneuvers NOW play with adding a little downthrust. However don't try to add enough downthrust to totally kill any tendency to climb. Climbing with added power is totally natural and desireable for this type of model. You just want it to get to where it doesn't nose up so strongly that it's unmanagable. By balancing the trimming between finding the best CG position and adding a little downthrust you'll end up with a model that flies well at cruise, goes into a nicer glide rather than a strong dive when the throttle is cut and climbs in a calm and natural looking manner when full throttle is used. If you go too far with moving the CG back you'll find that the model will get to where it doesn't want to pull itself out of a dive on it's own. It won't be UNstable but you'll find that you need to be on it more for watching out for pitch changes due to your flying and from encountering turbulence. What's happened is that you're setting it too close to the neutral point. That's fine for aerobatic models but not nice for a scale'ish style of flying unless you're a hot dogging aerobatic type and don't mind the feel of this and having to be on top of your model all the time. For most casual sport flyers though a moderate setting where the model still has a postive tendency to lift the nose in a dive but not do so TOO strongly is the best setting.

So why not just leave the CG where it is and add some downthrust? Because of the transition to an idle glide and how the model will behave when you go into a dive. If you don't optimize the CG position FIRST there will always be a strong tendency to nose over into a rather steep and fast glide when you cut the throttle. Also the downthrust only holds the nose down when you're using the engine for speed. If you dive the model out of a maneuver or a scale like steep spiral dive there will still be a super strong tendency to want to nose up strongly from the added airspeed. It'll have you complaining that the model wants to balloon all the time when you least expect it and that the airfoil is to blame because EVERYONE knows that flat bottom airfoils balloon, right? Nonsense, ANY airfoil will balloon from a high speed if the model trim is set up with a strongly forward CG location. So don't get sucked into that old wive's tale. Just because some is good this isn't a case where "too much is just right".

So how do you get the model to fly fast and level? There's this little control on our transmitters called an elevator trim. Learn to use it as a main flight control if you want to fly fast for a while. Then when you want to slow down use it again to add a bit of up trim. Real pilots do this and it soon becomes entirely natural. When I fly sailplanes the elevator trim is my "throttle" and I use the trim lever almost as much as I use the two sticks. If you just want to fly fast for a few seconds then learn to just ease the stick forward to hold the nose down when flying fast then ease off the pressure as the model slows down.

So to summarize. CG too far forward- strong tendency to lift nose during a dive or when throttle is applied and drop the nose strongly when speed is lost or throttle is cut. CG set just right- model still retains positive pitching stability but tendency to drop and lift the nose with speed changes is moderate and manageable with no surprises when something different is done. CG too far back- model is not quite pitch stable in all cases but has almost no tendency to climb or drop the nose with speed change but pilot must be on top of the model all the time. A "point and shoot" setting. Downthrust can help but it is a bandaid that can also cover up a bad trim setup.

Now in the end you're likely going to find that your elevator trim is quite strongly down. This will be the result of the rearward CG. Likely the elevator response will also sharpen up. If you don't like the quicker response then by all means reduce the amount of throw. As for the down angle just live with it. If it REALLY bugs you then measure the angle of the line between the leading and trailing edge of the stabilizer and elevator and slice into the model to move the stabilizer down to this new line. The model will then fly at your new trim with the elevator in line with the stabilizer. The ONLY advantage to this change will be how it looks on the ground. The model won't care when it's flying because the trim is set in concert with the CG position.

Dang BM, that was good stuff. I like.

WOW!! I knew you guys would come through with great explanations and solutions. I will use all of the solutions when I get this bird in the air. I did not know that the CG have such great effect on this attribute of how airplanes fly. thanks to you all.
Regards,
Vic

Very nice write-up BM...very thorough. Two follow-ups on this:

1. so, this is not related to the center of thrust and drag line?
2. Regarding your summary on CG. When using the various trim charts that are out there, why is method 2 for CG to roll inverted and cut power. Why inverted and not just staight, level and upright with a power cut?

Again, nice job with this!

Tom

Never read a trim chart on rolling inverted and cutting power. On a flat bottom airfoil, even with power, it would take fair amount of down to hold level flight even with a more aft CG. I have used the rolling inverted to check CG with a symetrical wing but even then I don't cut power.

I must say that whenever BMathews replies to my questions, he is always thorough, accurate, and most importantly - understandable. Even though he replies in engineering terms, he breaks it down to where us normal modelers can understand it. Thanks BMathews for your great reply to this and other questions we have asked.

Trim charts
too bad they rere not on softer paper

1) Not specifically but by doing the trimming of the plane as noted it also takes into account the thrust line and center of drag. A high wing model has it's center of drag much higher. This produces a positive pitch force that works in concert with the pitch response due to the decalage angle while a low wing model will have the drag couple from the low placed center of drag fighting the decalage trim. So if I took two identical top view models but from the side one was high winged and the other low winged and set them up in the dive testing so they both nose up at the same rate then the high wing model would end up with a slightly more rearward CG location than the low wing model. This isn't something that shows up in any of the online CG calculators. It's not a huge effect in any case so it is buried in the range of suggestions for % of stability. Along with this center of drag axis we also have such things as fuselage effects and likely a couple of other things that also don't show up in the typical stability calculations. The beauty of doing direct in flight testing is that all these factors are automatically taken into account as they add or cancel with the decalage effects and CG location.

2) I've only ever seen the inverted testing being suggested for aerobatic models. But since I don't fly pattern I've never had occasion to try it. It may or may not work with a cambered section but then such models don't make it a habit to spend huge amounts of time upside down or running nice stable vertical lines.



LOL Right to the "heart" of the matter as always Dick.

Not the first time Dick has cracked me up....won't be the last.

Sorry Jezmo...you are right. I was going from memory. The power off comment was wrong as I just looked it up.

I found myself refelecting on the posts and got twisted around. Please don't tell my wife cause she marks down all my mistakes.

BMathews,
How did you conclude the CG of the model in question was set too far forward? The original poster doesn't have the plane in the air yet. He was referring to the model's reputation "in general". The plane's reputation might be suffering from any one of a host of maladies: poorly matched wing lift/weight, excessive decalage setup, insufficient downthrust.
Another point: Adding downthrust actually helps reduce pitch-up caused by thrust moment in high-wings. Further, chopping the throttle of a plane with downthrust will cause it to momentarily pitch UP, not down.

Regarding constant climbing planes, fiddling with elevator trim etc. Is this some new form of macho? "only real men fight their airplanes"(LOL). I prefer to control mine with dynamic feedback. Regards.

Pretty hard to trim a plan via the internet.
Adding guesswork to theory and dividing by unknowns - makes it all a SWAG at best.
Anyway - The ideal trim -for someone who actually uses throttle management will be different than that searched for by someone who flys full on or off only
I just converted my trusty old Senioria to electric ( Istuck in a setup which is pulling 300-500watts - depending on batery setup.)
Te REQUIRED thrust is a guess - and 300 seems like enough for the 4 lb /800 sq in setup.
prior to the change - the power was a 20 K&B Sportster on stock silent mufler and a 10x5 prop - turning 8000+
power was good enough
750 watts is about one HP so I think the 1/3-12 hp will do the job

On the Pilatus - the correct trims will also be based on power applied
guessing on correct amount of downthrust is a bit futile - till one decides on how it is to be flown
I love those models - everything purpose built -
You can't beat a big wing and lots of power.

I really do appreciate all of the responses to my initial post. As I stated, all of the reports from other models have commented on this climb with power attribute which we all agree will happen especially with a high wing plane with and flat bottom wing that is trimmed for straight and level flight at medium cruise speed. I had this issue with my trainer and many other trainers that I instructed students to fly with. When I wanted to fly faster than the normal cruise wpeed, I just held in some down elevator OR put in a couple of clicks of down trim on the transmitter.

I consider myself an intermediate flyer with good skills. I will probably do a little bit of the suggested solutions to get the plane to fly well in a range of speeds around the medium cruise range (CG location and motor thrust adjustments). For speeds above and below that, I will just use the elevator for short periods and use the transmitter trim for longer durations at high speeds.

Thanks to all for your help. Great on-line community here!

rmh, I got this plane becoause of the odd paint scheme AND for a relaxing, tool around the sky flyer. I have a few 3D capable planes that I attempt areobatics with. This will just be a fun flyer that I will do an occassional loop and roll with. I will be using a BIG motor (HXT 50-52b capable of 1000+ watts). With my intended 4S battery, motor, and prop combination, I plan on getting about 600-800 watts input to this 6 pound plane.

CC, you read a lot more into my posts than I put there.

It doesn't matter if he had flown model or not. He related about others in trying to correct their own experience with the design by altering the wing or tail incidence without realizing what the actual cause of the behaviour. So I presented what I have found to be a better way to deal with the issue.

Adding excessive downthrust to a model that really needs a CG shift to a lesser positive position will do exactly what you said. The nose will tend to jump up when the throttle is chopped abruptly. That's why I suggested setting the CG before playing with adding a final seasoning of downthrust. And it's not only a high wing model that'll do that. A low wing model with an excessively forward CG and resultant decalage angle will do the same thing.

I suppose having to fight the model constantly COULD be seen as a form of machoism. But I'm not a "wife beater wearing macho dude"..... I'm generally lazy and don't like arguments with my models. That's why I use my trim lever as a main flight control for my models that benifit from doing so. It's not an issue with a well set up aerobatic model since those are set so close to neutral that they fly almost the same from power off to full power. Sailplanes, on the other hand, benifit from using the trim almost constantly since it's hard to hold just the right amount of trim for more than a couple of seconds. And general fly around type designs can often benifit from a couple of trim adjustments at various times as well. Using the trim lever a lot on my sailplanes makes it easier for me to do well. With my poly ships I've often set the thermal turn using the rudder and elevator trims and then just stood there watching with my thumbs off the sticks for the next 5 minutes while the model flies in the thermal. Set this way it does better at centering itself than I can manage.

In fact I use my elevator trim so much that I installed 3/4 inch long 2-56 screws into the elevator trim levers on my old style transmitters. That way I could easily feel where the trim was located by the angle of the extension. I detest the new digital click trim levers since I no longer have a way of checking the trim position easily on one Tx and have to look at the LCD screen of the other to see where the trim is set.

Your secret is safe.

Somewhre -in my piles of photos -is a 8.5 x11 of a tx I designed -and published pics in the mid 1980's -in Model Builder
The tray type tx had three trim KNOBS.
I could fly a plane using just the knobs.
Once it all seemed correct- Iusedthe sticks ( ProLine open gimble.)
I detest digital motor trims
My 9303 has a mechanical trim on the speed control
In reading various comments on trims -,it appears many flyers add switches for various power settings of cruise - landing/shut down etc.. - To each his own - I find that awkward
To me it would be like trying to play guitar using chord keys on the fingerboard.
(like playing an autoharp )