I've recently completed the build of a Phil Ramsey Glouster Gladiator Bipe, we had a problem on its initial test flight in that the engine had to much down thrust, this has now been removed, however in inverted flight the bipe just wants to climb, and up elevator needs to be kept in to stop it climbing.

Now before you even start to mention engine thrust, and C of G, the expert flyer's at our site have carried out all the checks they can think of, and they says it not the engine thrust line, or C of G, this I think leaves wing incidence, or tail plane, if this is the case has anyone an idea of checks that can be done, unfortunately all these incidences are built into the model, so adjusting may be a problem.

The plane flys fine while the right way up, landing, taking off no problem, but rolls, and loops a nightmare.

If anyone has an idea where the problem may be I will be grateful to hear your comments, before I go and get myself an incidence meter.

Thanks

what you related says this to me:
the plane is tailheavy
the upright trim makes all acceptable bu rolled inverted the trim which was keeping it level - takes over
incidences in models by the way are NEVER to be even close to full scale
a degree or so is about it
especially between wings
a degree in one foot is about 1/4"

Unless you have some very odd incidence angles at play here I can't think of anything that would cause such an odd case other than agreeing with Dick.

What happens if you trim for level hands off flight at mid throttle and then push the plane into a 45 degree dive and release the sticks? Does it nose up to level, carry on in the dive or try to steepen the dive? If it does either of the second two then it's tail heavy.

The only other thing may be that you have sticky hinges and you're not returning to a consistent neutral with the elevator?

Incidence angles on their own should not cuase such a phenomenon as the angles, engine thrust and tail angle all blend together to form an overall trim setting that reacts with the CG of the model to make it fly the way it wants to.

Well I'm pretty convinced that the C of G is correct, although the plane was tail heavy before I used a 26cc petrol up front, this sorted it out and the C of G appeared correct to me, though as this is measured on the top wing it is only possible to use your fingers as a balance point which is not the best way, I will double check.

Also I will try the dive and see what happens, as it is it trims out in level flight, with the elevator in a nuetral position, no obvious up or down trim. One of the guys at the field did a similar test, took the plane real high, then a vertical dive, which from what I remember, it just went vertical down, with no deflection.

I'll let you know what happens.

that says ---tailheavy
why?
as speed increases the correct incidence would have started a nice pull out to level
hate to bust a bubble but hands off upright and inverted is simply impossible

That's why I suggest the 45. From straight down it has nowhere to tuck to. But Dick is right, A scale model should at least show signs of trying to pull back to level from a dive like that. You're either tail heavy or right on the dangerously neutral point.

The point that Dick and I have been trying to make is that it is still possible to trim a tail heavy plane for level flight at one airspeed. But it's what happens when things change that determine if you have a positive or neutral or negative pitch stability. Trimming for level flight at a cruise speed and pushing it into a dive and releasing the stick is a great way to see what it does with the extra speed. A properly positive stability trim will use the extra speed to pull the nose up and return to level flight. A neutra model will continue at the same dive angle while the speed continues to increase. A tail heavy model will tuck the nose because it required negative decalage between the wing(s) and tail in order to balance out the rearward CG. In this last case with the extra speed the tail lifts TOO much and it pushes the nose down.

I was about to say that I don't follow your logic here: How would it indicate tailheaviness if a plane doesn't pull itself out of a vertical dive; but I think I understand after pondering for a minute ... you've already trimmed for level flight on the tailheavy plane by introducing down elevator, so now, when you put the plane in the dive, the elevator works to hold it in the dive. Whereas, on a correctly balanced and trimmed plane, the horizontal stab normally adds a downward vector at the rear to counter the CG in front of the center of lift. So, in a vertical dive, where the CG vector lines up with the thrust vector, all you have left are the lift and stab vectors, it will begin to pull itself out of the vertical dive, at least until there is a balancing component of the CG vector to counteract the rotation. Howzzatt?

Phil

yes.

The forces needed to work against gravity will work differently-relative to direction/attitude of flight
up/down/inverted

.... and with speed too.

Speed acts on the wing to stab trim to increase the lift on both surfaces. That's why you do the dive test. It indicates what it does with an increase in speed. Stable models will pull up the nose, neutral will continue being neutral, tail heavy unstable models will try to tuck further.

Other than that your logic about the process is spot on.

I have a small model Cub that has the same behavior. Inverted, it wants to climb, not severely, I don't need to change the trim much but it is not like the more aerobatic planes that I have with symetrical airfoils which need for me to push the stick forward when upside down.
The most plausible explanation that I can think of is the fact that cambered airfoils have a pitching moment and the elevator trim has to cancel out the wing's pitching moment as well as counter the nose weight. While inverted, the wing's pitching moment is positive and tries to lift the nose of the plane. Or maybe, the wing's downwash misses the horizontal stab while flying upright and hits it while flying inverted.

A change in wing incidence alters the way the fusilage sits during level flight. Give the wing a few degrees of positive incidence and the plane flies with the tail high, like a Cub does. Some sailplanes reflex their ailerons for high speed flight. This has the effect of lowering the effective wing incidence and keeps the fusilage aligned with the airstream for minimum drag.

The pitching moment is one factor. But the model would need to be very close to neutral stability for this to be a factor. Or, in some cases, the layout of the model is extreme.

The Slopoke is one such model. If set up with minimal positive pitch stability at it's normal speeds it has a wide enough chord and short enough tail moment that it can tuck just due to the poor aerodynamics of the design. I have not flown one but I would not be a bit surprised if the Clancy Lazy Bee is another model with this sort of poor behaviour if set up with the CG in a rearward but still stable, for the most part, manner.

Well folks, check the C of G again, this time I measured off the plans and inserted panel pins where it should be, I had a pen mark before, which was very difficult to tell if you had your fingers in the correct place. Well as you all said it was still tail heavy, had to add an extra 1kg lead to the front, to give a slight nose down, beats the hell out of me how it managed to fly in the first place, mind now I got the extra weight to contend with, will check it out this weekend, and if its OK, try and lose some weight from the rear, change push rods etc, to carbon fibre etc.

Well test flight today with the extra lead proved you all correct, just baffled the experts at our site, as the plan flew great before the lead, but obviously this was the problem, may just need to remove a bit, as it now needs about a third down elevator when inverted, and I would like to try to reduce the weight if possible.

Thanks for all you advice.

I second this motion.

If you're having to hold about 1/3 down stick now then that suggests that you went too far forward with the CG. Or at least more forward than you required. Try moving the CG back a little at a time until you only need a slight push of down for inverted flight.

In one of my earlier posts to this thread I described how you can do some dive testing to check for just how pitch stable the model is. Go back and try this as you move the CG back and retrim for level flight. It's a great way to tell just where the model REALLY wants to be balanced and not where someone thinks it should be.