Help with determining flying behavior
 

Hi, everyone. I had been away from the hobby for 25 years and decided to get back into flying with a new GP PT-40. For the first flight I had an old KB .45 on the plane and had a good flight. The plane behaved well from flying characteristics perspective except for the fact that I had to use full down elevator trim to get the plane from continuously climbing at full throttle.

I had some issues with the motor so decide to upgrade to a new OS .46 AX II. Obviously a big difference in power output. The weight of the two motors was close but not exactly the same so I checked the CG. Originally with the KB the plane was very nose heavy, had to put the battery towards the very back of the electronics compartment and with an empty fule tank the plane was still nose heavy. The OS seems to be a bit lighter so the balance came out better, not quite as nose heavy. I flew the plane today and again had to supply full down trim to keep the thing from trying to continuously climb.

About 1 minute into the flight, doing the second circular track over the flying field I allowed the plane to go into a moderate dive still under power. I started to apply a modest amount of up elevator to pull the plane out of the dive and the nose snapped up so fast I can't believe the wing did not fold up! From there anytime I allowed to the plane to dive, when I applied even a small amount of up elevator the nose to pitched up so fast and ended up be pointing between 60-80 deg vertical. I decided there was something not right and brought it in for a landing. As the plane slowed down it's flying characteristics calmed down and I landed.

I checked the CG on the bench based on the markings I have on the wing (4 1/8" from LE) and the plane still showed nose heavy.

At this point I'm trying to figure out if the plane was behaving like this due to a CG issue, or if I simply was flying the plane faster than it should be going with this powerful motor on it. To a degree I would expect the plane to behave the way it did if it is tail heavy, which I don't believe it is, or if I was using a lot of control surface throw, which I'm not, I'm using the recommended beginning throw from the manual.

Would like to know what other people may think the issue is...hopefully I've overlooked something.

Thanks,
Matt

Ok let me the first to welcome you to the forum Matt:cool:

Now lets talk about CG. For any trainer such as yours it will fly best and needs to be at 25% mac. In other words quarter chord, this is also where the spar of most (not all) trainers is located. With a straight chord untappered wing such as yours even without using the spar as a reference you can simply divide the distance from the leading to trailing edges by four and that will give you the CG you need to target.

Now to the engine. Let me congratulate you on your choice of engine, The AX line is my all time favorite. First I cannot remember if the PT 40 came with down thrust or not but if it does not for what ever reason it sure would not hurt to add a little down with some shims under the top side of the engine mount. Also from reading your post it sounds like you are flying this airplane at full throttle. Please forgive me if not but I must address this. And that is no trainer is going to fly well or at least as intended at full throttle all the time. Most will fly perfectly after takeoff and climb at around a quarter throttle.

One last thought also it sounded like you may have been recovering even from a slight dive at one point and the leading edge lifted off the saddle. This will cause the behavior you seem to have experienced at one point. Its cause is far to few rubber bands on the wing.


John

Unless your wing chord is 161/4 inchs I think your 41/8 figure for CG is to far to the rear.

I agree with John that the wing must have lifted which increased the incidence to effect the sudden pop up of the airplane. Same thing happen to me over 40 years ago with a Falcon 56.

I have never been a fan of the PT-40 trainer. It was designed to fly with absolutely no pilot input, and about anything the pilot does just upsets it's free flight tendencies. The amount of decalage and it's balance point means it will fly level at just one speed. Any change of power will cause it to climb or lose altitude much more than what is typical of most trainers. Much of this can be taken out of the design with changes to the wing incidence and engine thrust. Some variations of the PT-40 also have excessive dihedral, though later versions of the kit showed a lower angle to be used.

The original design would have been well suited to fly with just rudder and throttle control, never touching the elevator or ailerons.

Do you have any down thrust in the motor? I am guessing you don't. A high lift trainer wing will really want to go up with no down thrust in the motor.. if the motor has a little upthrust it will REALLY climb.. Also next time you fly it take some cardboard and shim the trailing edge of the wing up a bit, basically add some down/negative incidence in the wing..That also might help...

When you trim it out for high speed flight does the plane glide or DIVE when you pull back to idle? If it dives I think thats another sign you have up thrust in the motor..

Thank you all for your input, very good information and I appreciate everyone that responded.

I will add some down thrust as this is not included in the default build - when trimmed and cut the throttle the plane does not dive.
I built the version of the wing that has the lesser amount of dihedral, I was pretty happy with how it flew the first time.

I will add more rubber bands, I must not have used enough today, I do recall using more the first time I flew it compared with today's flight.

Regarding the CG, the wing cord is 12", that should equate to a 3" CG then. Any idea why such a large discrepancy between this calculation and what GP calls for? Just trying to understand why this would be.

Matt

I believe on many wings 25 percent is considered the super safe starting point. It will give you a positive nose heavy place to start... when you aren't sure where the cg is.

Then you start moving the cg back to where it makes the plane slow down and land best .. without becoming tail heavy... If you go to far it can make for a white knuckle flight ! ;)

A friend of mine had the same trainer you do, it flew great.. the fuse is so blocky and strong we called it "THE FLYING 4 by 4".

Don't trim it for full powered flight. Trim it for a speed/throttle setting 15-20% faster than landing/take-off speed. Should be around 1/2 to 2/3 throttle - closer to the center of a typical flight envelope.

Kurt

Thanks for the replies, I'll make sure the CG is at 3" and adjust reward as necessary.
I'll trim for less than full powered flight, both John and Kurt have indicated this plane was not really intended to be flown at WOT.

Matt

[QUOTE=mpieklik;11802821]Thanks for the replies, I'll make sure the CG is at 3" and adjust reward as necessary.

Matt[/QUOTE

You are welcome Matt and you will find that moving the CG and calming down on the throttle most of the time your airplane will turn into a puppy dog. As noted by foodstick CG is always a compromise with many varibles depending most on the type of flying intended and there is not a generic conventional trainer out there does not respond well indeed to the 'safe' 25%. Many will adjust in practice aft to as far as 28% but 33% as practiced by some 3D type of airplanes is very inappropriate for a trainer and that is where those instructions are suggesting. GP is usually pretty good with that type of thing butt you will never see me balancing a trainer at that point.

It is very common for some instructions that comes with the unknown arfs to be grossly wrong in both cg and throws. At least that is my opinion.



John

Once trimmed at 1/2 throttle I don't touch trim again. I agree that it sounds like you don't have enough rubber bands on the wings. Lastly you are checking CG with an empty fuel tank correct?
Your aircraft is going to climb with power on but not at the attitude you described

What prop are you using? You have a ton of power. I would prop it for climb, not for speed. You'll see a lot more benefits to that. You'll have better acceleration that can get you out of trouble faster. Your speed range will be more limited. That means that the plane won't be as wildly out of trim when you hit the throttle because top speed won't be as much higher than cruise as it used to be.

I would only make one change at a time.

Definitely +1

I certainly do heartily concur with Vertical Grimmace's suggestion and one step at a time is always what should be applied not only airframe changes but also this needs to be applied to learning to fly. Airmanship is always and for all of us a climb up an airmanship ladder and omit basic steps (rungs) then that will always come back to bite you in the butt latter. This is why most here and virtually all experienced flyers will suggest getting help from a mentor (instructor).

Sorry back to the airframe. In my opinion two things need to be accomplished before the next flight, You might call that step one:

Move that CG forward AND at least double your wing band count. Step 2 adding down thrust later only as needed.

John

When I built my pt 40 I eliminated the rubber bands completely and used the screw down method to hold the wing on, works great and looks better too........

I am wondering how many rubber bands, and size did you use? The "Zooming" you are describing was most certainly caused by the wing lifting. If I remember (been so long since I used rubber bands), You need at least 10, #64 on a plane that size. It was standard to do 4-5 on each side, straight back, then cross 2 more to help keep them on.

try putting a couple of popsicle sticks under the trailing edge to raise the back of the wing up around a 1/4 inch. that hould help a little on the trim at full throttle, not a lot but a little.keep adding a popsicle stick until you like the way it flys.

I'm using a 11x7 prop.

Yes, I definitely think it was due to lack of rubber bands. I realized this after John mentioned it. First time I flew I used 12, this time for some reason I used 8...will not do that again.

Matt

Yes, I agree. Already planning to move CG forward and use 14 rubber bands. I'll see how it flies at half throttle and introduce down thrust from there if needed.

Again, appreciate everyone's help. I've found people at the club and online are generally willing to help. Just starting to wonder why I waited 25 years to get back into this hobby!

Matt

Certainly

I might adjust the CG, but I don't know that I'd go from 4 7/64" to 3" all at one time. The Great Planes web page and the PT-40 manual indeed indicates 4 7/64 from the LE. You move it straight to 3" and you might end up with that white knuckle ride someone mentioned further up. :eek:

Fantastic matt I am sure you will do well now and agine welcome back. Yes that "why did I wait so long" emotion and response is what all here and person to person mentors, are looking for and is the payback, indeed.


As far as the CG (once agine) goes, that white knuckle ride will occur if the CG is to far back and it seems to me that is exactly what you had on your first flight. There are no white knckles with a cg to far forward only an airplane that lands a little to fast.

Now the the manual for your airplane suggest the four inch an change figure as well as suggesting a half inch margin both ways. The 25% is a safe starting point and as a practical matter I am willing to bet 'Two burned out Glow Plugs aginst a stale Glazed Donut that the final resting place for that CG where you just love it is around 28%. Hey 28% is 3 3/8's or thereabouts.

Indeed Matt do not be afraid to experiment with CG in small increments and Hey Ya'll come back now and let know if I won my Stale Gazed Donut:cool:

John

John, Do you set those CG points with fuel in the tank or with it empty?

When you get that plane trimmed and balanced you will have a fun plane to fly. I bought one built but not covered. I reglued all the joints I could reach, it was totaly built with CA and every time you bumped it something made a pop noise. I took out most of the dihedral and made the wing bolt on.

This plane was so stable that if the winds were dead calm I would fly it in a circle in front of me getting lower and lower till I could drag the wing tip on the ground. That impressed every one. They were equaly impressed when I finaly cartwheeled it. Mine just had a TT GP 42 for power. That was all that was really needed.

If it works here is a picture of my plane. And like you I am returning to RC. For the third time.http://www.rcuniverse.com/forum/atta...mentid=1994810