Andrewmc

Flying my 4* at the weekend and the first time for a while that I did not have to adjust the engine for good running or fight the wind and was just able to fly. Not a breath of wind. Managed to improve the trim settings of the plane control surfaces to remove a bit of crabbing which I hadn't picked up in the windy conditions.

I noticed two things however:

1. when flying a 45 deg inverted up line the climb angle flattens out - not dramatically but gradually more like an upside down expo curve if you know what I mean. Generally this would indicate a need to move the cg slightly rearward. What increment should be used for this 1/4 " at a time? How does a full fuel tank affect these cg changes? Generally after filling up I need 1 or 2 clicks of up elevator which I take out as fuel burns during the flight.

2. On landing when I have the approach angle set up as I fully close off the throttle the nose of the plane lifts, again not balooning but enough to upset the landing approach. Should I take out some down thrust? If so how much at a time? The engine is side mounted on a Dave Brown mount so putting washers behind the bolts is not possible as it would create an eccentric offset. I was thinking abount making some shims from 1/64" ply and slipping them under the engine mount?

Thanks in advance for advice.

mvallyman

My Feedback: (32)

For engine thrust up/down. fly the plane level at 3/4 to full throttle then shut down the throttle. If the plane continues to fly level its good, if it noses up then it has too much down thrust, or noses down, too much up thrust. Homemade wood shims will work fine for this.

Adjust the CG so that you have to give it a little down elevator to stay level when inverted, from that point adjust it so that it fly's best for the kind of manuvers and flying you do. There is no set place on the CG range, its more about how it works for you personally. A rearward CG will work better for some manuvers but can also cause poor tracking, its all a trade off.

David Bathe

Your engine-off ballooning can be cured by removing some down thrust. Aim at about 1deg/neg. Look at the plans to establish the approximate amount built in and adjust accordingly.
CG sounds good. Go back further if you want to experiemnt but as mentioned above, tracking really bleeds off. Just a note: Everybody believes that aerobatic planes have their CG set towards the rear, so much so that there isn't any drop when inverted. Watching the smooth style flying at say a F3A event just seems to confirm this. But having been in that environment, I'll tell you that just about everyone has their GC set a great deal further forward than you'd believe! You've just got to get used to adding a down elevator during the inverted sections until you reach the point that it becomes real smooth. Experiment (we've all done it) and see... but you'll eventually comeforward again.

Andrewmc

Great thanks alot - I will leave the CG alone for the time being, it really does not need much down elevator to maintain inverted flight.

The down thrust was originally 2° negative and I will shim that up in 0.5 deg increments (or however much 1/64 ply shims make)

David Bathe

I'd guess you'll be needing about a 1mm shim to remove 1deg on the average .46 sized engine.
You should aim to set the CG so you need a definate press on the elevator stick... not just a little push.
You'll need to find the correct balance between CG and elevator throw to achieve this.
Again, in F3A aerobatics there is surprizingly little controll surface movements.
One tends to set up an aeroplane in such away that it normally flown using relitively small stick movements. This is something that becomes normal.
For smooth aerobatics the opposite tends to be true. People tend to use diffined stick inputs. It's a little strange to get used to in the beginning, using the full travel of the sticks instead of half... but it seriously helps define your finger movements and smooth out the flight.
Start using of the dual rate/flight condition switches to get a feel.
Enjoy.

Ed Smith

Before you do anything do this.

If it is not already there place the tank equally about the balance point. Then the trim will not alter as the tank empties. The basis for further in flight trimming is now set.

Ed S.

Jetdesign

My Feedback: (8)

If you are going to change your CG, do that first. The method you use to check you CG is the proper one - roll to inverted 45* up line - the plane should gently fall off the 45*.

I actually had the same exact issue a few days ago - plane fell off 45 too abruptly, and power on was level but power off caused a nose up attitude. I added 1/2oz of lead to the tail of my plane (120 size/69" wing) and a washer under the bottom engine mount bolt for a slight amount of up thrust. Worked very well.

Andrewmc

Moving the tank to the balance point is not practical but would be what I would normally aspire to for the very reasons you mention.

gaRCfield, the fall out from the inverted 45deg upline is not a plummet but it does not keep the upline. I guess a 1/4" aft shift in CG is a good place to start. Do I do the adjustment in downthrust at the same time - normally I would only make one change at a time so the cause and effect is definitive, but if experience indicates that both adjustments will be required I will do both simultaneously.

jetmech05

if you are always adjusting trim...everything you have mentioned could be a trim problem......Most folks I know set trim at 1/2 throttle and leave it there....I rarely adjust trim.....It's worth a try

anuthabubba

My Feedback: (102)

What are the incedences of the wing and stab? I'd guess you may have too much positive in the wing or too much down thrust.

Terry in LP

Jetdesign

My Feedback: (8)

Yeah, I moved both of mine at the same time, too.

Don't worry at this point about the distance you move the CG (1/4" or 1/2" or whatever) - just move the battery back a little and try it, move something else and try it. You are close enough that you won't be making any major changes. The recommended CG point is just a reference point. I added 1/4oz of lead to the tail and tried it, then added one more and it was perfect. Plane just very gently falls off the 45 line.

mvallyman

My Feedback: (32)

Just a "heads up" about trim changes.
The stock 4 star's plastic control rods for the elevator and rudder will shrink and expand quite a bit more than other plastic/nylon control rods as the temperature changes. I experienced as much as 4 clicks difference between morning and late afternoon. So if something doesnt seem right about a CG change you made, it may be temperature related to the control rods.

Jetdesign

My Feedback: (8)

Sheesh, I guess I need to be a little more careful...does the inverted 45 trick work for setting up a CG on a plane like the 4-Star? I learned all my trimming tricks after flying planes with symmetrical airfoils. Not sure how it all would change with a semi-sym. wing.

Rudolph Hart

You must have huge temp changes even for plastic.If it worries you go carbon fibre pushrods and links.Prop off set as a rule of thumb usually ends up a propshaft width sideways and down on most models.

Andrewmc

I experienced the trim changes you mentioned with Sullivan Gold''n'Rods where from morning (cool) to afternoon (baking hot) I would need almost all the down trim available to fly straight. So I replaced them with 4/40 steel rods inside the red outer sheath, much better since - possibly carbon rods would be even better.

I would be interested in knowing if the 45 deg inverted upline is an appropriate test for a semi-symmetrical wing as well.

Incidence is HS 0° and wing +0.5° (measured as best I can with the strip ailerons)

min$2crash

I have used the 45 inverted trick on semi-sym airfoils. Yes it works for them, too. But a semi-sym is going to fight inverted more, and need more pitch to get lift, so I would just shoot for falling off the 45 line inverted after 2-3 seconds. If you get it too tailheavy, it will go UP inverted.
BTW, I forget where, but I picked up a great idea for the steel pushrod replacement to avoid buckling, or at least use the red sheath to reduce it:
Put some short sections of Golden Rod on the wire to act as little bearings before making the Zbend. Say, a 1/8 to 1/4" long piece every 3-4" along the rod. You may want to add heat from your heat gun to help this assembly process. Then the rod will be bearing on the tube with less slop, so you don't buckle the rod when yanking up a high speed because the red tube that is secured at each bulkhead can stop the buckling earlier. I did this with 2-56 rod and the "standard" Golden Rod sized for 2-56 ends. If you are using the bigger stuff, it should go over 4-40. If you are using the standard stuff, switch to 2-56 with bearings and you will save some weight.

Andrewmc

Since the last post I had not done very much and was just flying the plane as it was. we had had some very still mornings and the ballooning on landing was more noticeable, also the tail would drop a little, so I stripped the engine from the airframe and added shims to take out some downthrust and to add a little right thrust.

This alone made a huge difference to how the plane flies, no more ballooning and no fighting the rudder on take-off.

I re-checked the cg and it seemed tail heavy - I guess it shifted when I added the steel pushrods. I added 15g nose weight and then recentred all the control surfaces and trims. Now all it needed was 2 clicks of up elevator for trim at half throttle, no rudder trim and no aileron trim - and the flight characteristics are now superb. The plane settles into a predictable glide for landing and flares easily without being twitchy.

This is how the plane was meant to fly - I guess previously with my inexperience at flight trimming a plane I had inadvertently added cross trim from the rudder and aileron, oh and 45 deg inverted uplines now hardly falls out at all, only a slight (but definite) down elevator required to maintain level inverted flight.

Fantastic. Thanks to all for your input it has all helped.