Decalage & porpoising

[Vince inTX.]

My spirit elite porpoises when I try to fly straight and level. The porpoising is connected to speed i.e. speed increases nose starts to rise and plane slows down nose pitches down and speed increases. The CG is with in specs . At this point I presume it must be a decalage problem. I have the ability to adjust the horizontal stab to some extent (raise either leading or trailing edge ( I kit bashed a bit)) and of corse I could adjust the wing as well. In my thinking I should raise the leading edge of the stab or lower the leading edge of the wing.

What is the opinion of the group. I have never played with this important adjustment before. If I adjust the stab what incriments should I use between flights.

Please tell me it there is a can of worms in here that should not be opened.

As always, any help is appreciated.

Vince

[Sparky Paul]

"c.g. within specs"... is it forward or at the aft limit? A tail heavy plane will pitch down with increasing speed.
Yours does the opposite..

[schrederman]

Paul's correct on this one. I would begin by assembling the model and leveling the wing so it is propped up on a flat level surface. Move everything around until the center of the wing leading edge and the trailing edge are equal distance from the work surface. Measure from the leading edge of the stab, and from the trailing edge of the stab...not the elevator. The aft edge should be only about 1/16" to 3/32" higher. If it's more, it will take more nose weight to offset the decalage. I set my models up this way and start with the CG at the aft limit as published.....and go back from there, moving as close to 0 decalage as I can..... However, rumors to the effect that I balance my models at 30 percent of the stab chord are not true.....

Jack Womack

[Ollie]

See:
http://www.rcgroups.com/forums/showthread.php?s=&threadid=209020
post number 6.

Porpoising is caused by too much stability in conjunction with gusts or abrupt control return to neutral. Forget the CG specifications. Gradually move the CG aft by removing 1/4 ounce, or less, of nose weight between test flights. Stop removing nose weight when elevator control becomes too sensitive. Reduce elevator travel, use dual rates or use exponential to tame the elevator control sensitivity. Adjust the elevator neutral trim or the decalage to set the desired trimmed flight speed. As you move the CG aft, the porpoising becomes slower. The slower the porpoising the easier it will be to kill it with pilot applied elevator. At neutral stability the period of the porpoising will become so long that the porpoising will disapear all together but the plane will go where it is pointed and will no longer have a trimmed flight speed to return to. Flying a plane with neutral stability requires much concentration to keep the glide airspeed constant and under full control. Use as little stability as your concentration, reflexes and piloting skill will permit while flying smoothly. The benefits are reduced porpoising, easier control at high speed when penetrating back to the field after chasing a thermal downwind and better signaling of lift by the plane.

There is a trade off. The less pitch stability the less porpoising but, also, the less relaxing the plane will be to fly.

When picking a design, pick one with a long tail moment arm compared to the average wing chord. The reason for this is to have more pitch damping. The greater the pitch damping the quicker any pitch oscillations will die out and the more stability that you can tolerate. The more stability you can tolerate the more relaxing the plane will be to fly because the pilot work load will be less. This is because pitch damping increases as the square of the tail moment arm.

[averen]

Vince,
When are you going to come to Austin and fly with us!?

I too have a Spirit Elite. After initially balancing the model to specs I too noticed this problem. I initially started with .75oz in the nose...this balanced the model to where the plans stated was the CG. Initially the plane required some up elevator to fly straight and level. After a couple of months flying the plane like this I decided to get brave and lost all of the nose weight...

Now the plane really comes alive. When put into a dive (yeah, I know this is controversial...) the plane has a very slight recovery after a good long dive. The plane REALLY shows lift and sink! The biggest thing that I have noticed is that the plane is extremely responsive...I barely even have to touch the controls and the plane is turning etc. It is now twitchy...which I love, it really keeps you on your tows but the benefits of the flight are WELL worth it.

Start dropping that useless weight :) you'll enjoy the results!

There are 2 of us in my club that fly the Elites. They preform exceptionally well for their price once you get the bugs worked out :)

I'm not sure exactly where I have the CG...I haven't put it back on the CG machine since I changed it...I would expect that it's around 40% chord or in that general area.

Averen

[BMatthews]

Averen, why do you think the dive testing is controversial? It's always been an accepted method for setting the balance point AFAIK. What you did is no more or less than what Ollie is suggesting but you did it in one big step if I read you rightly. No big deal as long as it worked out well for you which it obviously did.

Vince, you could do a lot worse than to follow in Averen's footsteps but perhaps follow Ollie's advice to creep up on the best trim rather than doing it in one big leap.

[Ollie]

For a description of the dive test and what it means see:
http://www.polecataero.com/
go to Articles and select the article by Dr. Mark Drela on CG location.

The dive test has the virtue of testing the stability of the model over a wide range of angles of attack from very low up to the stalling angle of attack. This is important because, unlike full scale, the flow over model wings often involves laminar separation bubbles that effectively vary the shape of the airfoil with changes in angle of attack because the size and location of the laminar separation bubble changes with angle of attack.. The effect is to move the aerodynamic center some as the angle of attack changes. As a result of the aerodynamic center shift, the stability also changes with changes in angle of attack. Without dive testing it is possible to trim a marginally stable plane to a stable angle of attack and yet be unstable at other angles of attack.

You can get a practical measure of stability by timing how long it takes to pull out of a 30 degree dive from the time the controls are neutralized until the plane stalls. About 3 seconds for brginners, 5 seconds for intermediate and 6 or more seconds for highly skilled pilots seems appropriate.

[averen]

BMatthews - I've heard a lot of arguements on weather the dive test is a true show of the CG or not on RCSE...but then again everyone has a different opinion on that board :)

And yes, it probably is a better idea to slowly drop weight.

Averen

[BMatthews]

Averen, I guess some folks like to argue about anything... :D

To me it makes perfect sense. Sure you can math a design to death but in the end our simple forumulas DO NOT take into account all the effects of design elements. For example a poly wing has a much higher center of drag than most aileron wings. That higher drag center has a stabilizing effect on the flight of the model just as surely as the balance point does. But none of our basic tail volume or other stability equations has a spot for including that in the analysis. But the dive test takes that and more into account in a whole system sort of way. If anyone needs a buzz word for it then call it a Gestalt Trimming method... :D


Side note-
This wing drag thing showed up in spades in the indoor free flight world. For those models with their post mounted wings it is the accepted method to use the tail volume method to calculate the neutral or 0% margin of stability and then actually put the balance pont either right on that location or even to put the balance a few % to the REAR of the neutral point. The idea being that we relied on the very high drag center of the wing for stability instead of the actual aerodynamic factor. The models STILL showed signs of high reserves of stability even with the CG behind the neutral point. If you tried this with an outdoor RC glider you'd have matchsticks in a moment

[Ollie]

BMatthews,
That is a very good point about "pendulum" stability. The paraglider is an extreme example of "pendulum" stability. The para glider has anhedral yet is very laterally stable. It has no tail and no reflex in the airfoil yet it is very stable in pitch

[BMatthews]

Oh yeah, the pendulum effect. I'd forgotten about that with the high wing bit. I was hung up just on the drag moment. But yeah that also plays a big part with extreme planforms like a hang glider or indoor model with it's "heavy" rubber motor and motor stick located so low.

Neither of these is taken into account in our classic calculations for stability but they certainly do play a part.

[Vince inTX.]

Wow guys thanks for all of the info. I will start with the "remove weight/fly....remove weight/fly" method first.

Ollie...as always your replys are informative and and somewhat over my head but it forces me to learn more about the technicle aspects of this great hobby. Thank You!

Jack, Is that something that could be accomplished at the field under the devine supervision of the "Head Bo Chi Chi"? If not, I would gladly surrender the model to your capable hands for an unspecified period of time...... if you were so inclined. I hope to be at the field on the 20th and I hope to see you there.

Averen, I would love to come up there and slope the dam with you guys however my wife (36 years old) suffered a stroke in october so travel is restricted for a while. It would be great if you guys could come down here for the Deep South or the All Wood Round Up. I would really like to see how others are flying their Elites. I am the only member of the Houston Hawks that has an Elite at this point in time. Lots of woody RES and moldie full house stuff but no other wood full house but mine.

So once again guys thanks I will proceed as recomended. I'll start pulling the lead boogers (5oz worth) out of the nose next weekend.

Vince

Vince

[Ollie]

Five ounces of lead in the nose! No wonder it porpoises.

Replace the chunks of lead with lead shot mixed with just enough modeling clay to hold the lead shot together. The mixture can be tamped into the nose and will stay in place until you dig out a little between flights to shift the CG back incrementally. Use elevator trim at the field to readjust the hands-off airspeed. When you are finished flight testing and return home, that is the time to reduce the decalage so that the off center neutral trim of the elevator can be reduced to near zero.

[Vince inTX.]

Ollie, I agree that is a lot of lead. But I did modify the tail so that the stab is removable. I have the C.G. is exactly on the spot as the plans prescribe. I presumed ( incorrectly ) that , that would produce acceptable performance. So it is time to start moving it back.

To all, I have been "gently" corrected as to the existance of another Elite within the Hawks organization also atleast one full house woody. Unfortunately I dont get to the field as much as I would like.. I would know this stuff.....I think ;-)

Vince

[BMatthews]

Vince, don't feel bad about following the directions... :D

They set the balance so that beginners can have a chance of flying successfully. No one said it was intended to be the optimum balance point for performance. That part is up to YOU and your little dwarve helpers... :)

[averen]

Vince,
OMG! I agree that 5oz is pretty excessive! I think the most I ever had in mine was about 1.5oz...now I have none :)

I know a couple of the guys are going to head down there for Deep South. I haven't had enough stick time this year to be competative so I probably won't make it.

We're planning a DLG contest for early May. If DLG is your thing it should be a pretty good contest :)

Sorry to hear about your wife. It's odd that someone her age has a stroke. Best of luck to you and her.

Averen

[schrederman]

Vince,

That's an easy process and we can do it on the tailgate of my truck. The alternative is using an incidence meter, which I don't have. We'll take a close look at it next time we're at the field.

Jack

[Vince inTX.]

Thanks Jack!

I hope to be there this comming saturday.

Once again, thanks to all!

Vince