Bonified Wingnut

I have a Pheonix Models Laser(picture) I have only flown this plane once that ended up in a deadstick. The plane seemed to be flying good while I was trimming it and the engine quit. I was quite high so I decided to make a nice wide circle back to the runway. Within about 2 seconds the plane seemed to lose all airspeed, tipstalled and almost fell out of the air. It was all I could do to get it upright and set it down in the grass. I have repaired the damage and figured out the engine issue but I don't want to go through that again. The plane has a wing loading of 22-23 oz sq ft. I question where the recommended CG is since it is almost %50 cord.

My question is: Is it possible the plane is tail heavy and with the engine running it was keeping it stable? Or is it the wing design? Should I move the CG forward before I dare to fly it again? It would have been nice to have had a video to replay. I might have been able to figure it out.

carrellh

I know that tapered wings are different than straight ones.
I think you need to know the MAC (Mean Aerodynamic Chord) of the wing, and set the CG based on that.
http://www.airfieldmodels.com/inform...amic_chord.htm

JohnBuckner

My Feedback: (1)

I don.t know who in their right mind would reccomend a cg at 50% mac for the vast majority of airplanes (I am not talking about old timers here). At any rate any who did should never be listened to agine as thats absurd.

This is certainly the cause of your crash and a safe point to start out is around 28% mac.

John

JohnBuckner

My Feedback: (1)

For what its worth those percentages of the chord are always expressed as a percentage of the MAC or mean aerodynamic chord.

That easy on a straight chord wing like some of your others but is also easy on a: leading edge taper, trailing edge taper, eliptical or a double tapered wing for these purposes:

Simply measure the chord half way between the fuselage and the wing tip (not at the wing root) and this is the MAC that you measure from the leading edge and project that to the fuselage. This is your target CG.

Its quick, easy, no computer needed plenty of accuracy and the only common sense required is for you to decide what percentage of that MAC you want the airplane to ballance at. Its pretty darn easy to develop that common sense on your own.

MinnFlyer

My Feedback: (4)

He never said the CG was 50% of MAC, he only said 50% of the chord - I would assume that is 50% of the chord at the fuse.

This is a swept wing which would push the CG back at the root. It's easy to see how 30% at MAC can be 50% at the root.

That said, it could be that the CG is back farther than normal for 3-D flying. If this is the case, you can either move it forward to suit your flying style, or prepare to keep the nose down in the event of a dead stick to keep up your flying speed.

Lnewqban

Hey Mike, aren't you flying to the Toledo show?

The Tower website states for the Pheonix Models Laser:
Center of Gravity: 5-1/2 " (140mm) Back from the wing's leading edge

http://www3.towerhobbies.com/cgi-bin...&I=LXNZZ7&P=RF

For a wingspan of 50", the root chord should be about 11", placing the CG around middle of the root's chord.

It seems to me that the model lost speed due to too high AOA.
It just take a couple of seconds to raise the nose and make the thing stop in midair when it is powerless.

Dead stick = Nose pointing down all the time just until flare = High airspeed

(Advice from a dead stick expert: me!)[sm=bananahead.gif]

JohnBuckner

My Feedback: (1)

And of course there lies the problem careless recommendations of a percentage without specifying the MAC reference.

I maintain that this problem was caused by an excessive aft CG.

John

Bonified Wingnut

This will help.
I understand how to get a good CG starting point, I have scratch built a couple of wings and deltas. The thing that I don't get is what part the long tail moment of this plane could play in the balance. The plane has a 50" wing and a 54" length. Here is a picture of the wing and the CG I have drawn on the image. The camera has a fisheye effect to it so it is hard to get a straight picture.-BW

Bonified Wingnut

I have had quite a few myself, and when I started flying I used to kill the engine as soon as I was lined up with the runway. I flew alot here at home and there was no room for any rollout. It did seem a little touchy when the engine was running but when it quit all I could do is keep it flat If I tried to turn or nose down to get some airspeed it seemed to get worse.-BW

Bonified Wingnut

As a matter of fact here is one the same day with a Sonic with an Identical wing loading.-BW

MinnFlyer

My Feedback: (4)

As you can see, even though this is a rough drawing, the CG is not out of line by any means - That said, if it's flying nose-up without power, you either have up-trim, or the CG could move forward a bit

PS, Juan, I leave tonignt. Geoff and I are driving to Toledo this year.

Bonified Wingnut

Thanks everyone. I think I will add a few ounces of lead to the nose for the next flight just to be sure. Hopefully this time the engine will run long enough to practice stalls in it to get used to it.-Thanks again.-BW

Lnewqban

I don't question your skills by any mean; just remarked that it is very easy to panic when approaching the ground in dead stick and to pull up, reducing the airspeed too soon and too high.

That behavior is weird!
Could you explain "All I could do is keep it flat" and "it seemed to get worst" a little more for me?

Lnewqban

Yes, the long and powerful tails have an effect in the balance.
The theory is that all the surfaces can lift, including the fuselage.
The imaginary point where all the lifting forces of all those surfaces can be considered as concentrated in is named Center of Lift or Center of Pressure.
The pitch stability depends on the distance between that point and the CG.
Things get ugly if the CG moves aft the COL or COP, since the model becomes unstable (trying by itself to relocate the COP behind the CG to regain stability).

For more exact location of the COL or COP of the whole model, following the above theory, this on-line calculator can be used:
http://www.geistware.com/rcmodeling/cg_super_calc.htm

Practical calculations disregard the lifting effect of the tail and fuse, considering only the main wing, which COL is always located at 25% of the MAC.
This practice is safe, because we locate the COL and CG forward from its real position; hence we are farer from any unstable condition.

I also suggest you to read thru this old thread:
http://www.rcuniverse.com/forum/m_8947745/tm.htm

Best luck!

Bonified Wingnut

Yeah killing the engine is wierd but where I was flying there was no go-around so it didn't matter, and the sooner the plane stopped rolling the better off I was.. And as far as "keep it flat"-once I regained cotrol of it if I tried to nose down or turn or anything it would fall off to one side, so I kept the plane level and kind of pancaked it into the grass. Kinda like a Harrier with no forward movement. Thats what gave me the impression of tailheavy.-BW

Oh thanks for the links.

ARUP

Tail heavy airplanes are very 'twitchy to uncontrollable' depending on just how 'tail heavy' they are so the engine running or not would not make much difference. Maybe the incidences of main and tail planes are offset by engine thrust line. Measure everything, c.g., incidences, lateral balance, moment arm, control throws, etc. to see if something is grossly out of alignment. Previous posts about MAC ~28% are spot on and a 50% is improper. Good luck!

Bonified Wingnut

The %50 percent I was talking about was the root cord not the mean cord, which actually works out to the same place. I think I will still add nose weight for the next flight I can take it off if I have to later. I am beginning to think that maybe the wind direction changed after the engine quit and when I turned toward the runway it was with the wind instead of into it. So essetially the airplane was flying backwards with no thrust.-BW

cfircav8r

My Feedback: (1)

How windy was it? Higher winds can make it very difficult to judge airspeed due to high ground speed down wind. If it stalled on the down wind turn than you could have just slowed down too much. I'm sure you know this , but it can be difficult to recognize in the heat of the moment. Just one more possibility.

Bonified Wingnut

It's possible because I think I remembered having to glide away from the runway to maintain control. and I think the wind was changing direction alot that day.

cfircav8r

My Feedback: (1)

If the wind was changing alot then you would not even have to be that close to stall to get hit with a sudden tail wind. The wings stop flying at that point, and control inputs can make it worse without knowing the actual airspeed. What seems appropriate normally can just make it worse. the problem is the best thing to do is power out of it (I know, Dead stick) or drop the nose way downto get airspeed. Hard to do when low.

BillyGoat

To address the main topic that has developed… For me 'recommended CG' is just a starting point for the maiden flight. I adjust CG to give the right balance (no pun intended) of flight characteristics I'm looking for over several flights. As suggested already you can't forget other factors such as engine thrust, incidence, ect, when trimming for the right flight attributes. CG is only part of the equation.

Not enough information is available to say the tendency for your plane to stall is/was amplified by an aft CG. And quite honestly CG is a few items down the list I’d be looking at if the plane truly did become a different animal when power was lost.

Bonified Wingnut

I thought I had just about gotten it trimmed out when the engine quit. But it only ran for about two min. It was a little touchy but it seemed to be flying alright. I really can't change the incidence and the thrust line seemed ok. I didn't have a lot of pitch change with throttle position.
But like you said when the engine quit it was a different plane altogether. Maybe a downdraft or something. When it quit it was almost like a parachute came out of the back of the plane. I used to make a habbit of video recording maiden flights...of course I didn't this one.-BW

BillyGoat

It is possible to trim a plane for straight and level flight that is grossly out of alignment. But that’s all you will get is a plane that fly’s good straight and level. You say the thrust line "seemed" ok. To have a good flying plane these things need to be measured with a good incidence meter. Without checking EVERYTHING to make sure it's right to the designers specs, you will just end up going in circles making changes. For instance, at this point moving the CG forward MAY solve one problem but can cause two more.

With all the baselines in check and set right on the money, thats when you can start to make adjustments based on what the plane does when it’s flown through various configurations.

Bonified Wingnut

It is really difficult to check the incidence on an ARF that I have no idea what the designers specs are. I also have flown quite a few free flight planes in my life so I know about alignment.
I've had some planes that were hell to trim and this plane showed none of that. I really think this one without much to go on is going to end up being trial and error. Until I understand why it stalled so quickly I will have to keep the speed up.-BW

jaka

Hi!
You definitely know if the plane is tail heavy! If so it gets very sensitive on the elevator!
Have you checked the wing for straightness?? It sounds as if the you have got wash-in in wingtips..! A twisted wing in the wrong direction will show the same flight manners you described.