Trimming CofG and thrust lines

[Konrad]

I recently visited my flying club on a day I normally don't. I'm retired and as such I like to leave the weekends for the the working class 😏.

 

I was shocked at how many folks don't use the CofG or thrust line to help with the trimming of their ships.  I was flying (maiden)my new Freewing Avanti S V2. After the second pass I knew that the Avanti S was way too nose heavy at the factory setting. I could tell this as both my left and right high G turns resulted in the nose dropping (tuck). Also in knife edge she puller real hard to the canopy. And then there was the inordinate amount of speed need to keep the nose up on landing. 

 

If I didn't know better, Freewing has the CofG so far forward to help with parts sales. That nose tuck in the turns was actually scary. And that high speed landing was just asking for the Avanti S to bounce and generally loose control after the wheels first make contact with the ground. 

 

The second set of batteries saw me move them 30mm aft to place the CofG 7 mm aft of the factory marks. Took off re-trimed the elevator and found that this change alone made the Avanti S a real nice sport jet that I could land real slow with full flaps. I'm now needing less than 2% trim mixing for the neutral knife edge.

 

What surprised me was that there where 3 other Freewing Avanti S (both V1 and V2). They all flew less than ideally and all landed like a brick! To my shock these were all high time airframes and that the pilots just thought this was the nature of the ship. I asked how they were set up and they all said per the manual. I verified that the CG looked close to the marks. All but one said his throws were also per the manual. 

 

The Avanti V1 pilots thought my landings were as a result of the new ground stance of the V2. No my landing were as a result of the use of 90° flaps and proper placement of the CofG. Still needs to go aft but the placement of the mains is limiting this. The flights show that I can go even further aft but then there would be little or no weight on the nose wheel. 

 

I talked one of the guys into cut down his throws so that high rate was a bit less than the manual's low rate and that his new low rate was about 80% of this new high rate. We shoved his battery aft so that his CG was about 6mm aft of the factory marks. In less than 2 minutes the guy had a grim on his face from ear to ear. He said that in 3 years of flying his Avanti S it never flew so well. I said wait until you land her.

 

I had him set up his landing like he normally would (less than 45° flaps). I had him come in a bit lower than he normally would and told him to hold 25% to 30% throttle. I had him try to maintain this altitude about knee high above the runway by slowly adding up as the model slowed down. Much to his surprise the nose came up and held on as the model passed by.  He cut power and added a bit of flair for a landing that was actually smoother than mine. He is changing his battery straps to allow for a much more aft CofG! I hope this will cost Freewing the loss of some nose gear sales.

 

I was also maidening my 90mm Freewing F-16 v1.  Again per the manual CofG she tucked hard in the 90° banked turns, and landing where far too fast. The F-16 was giving me fits as I kept moving the CofG aft to control the tuck in the turns I was getting all sorts of issues with the rest of the flight profile. For example I needed 3/4 forward stick while inverted. But on landing I would get  into a high alpha lock.  The key to the problem was found by checking the thrust line. I'd trim the model for the best 90° banking turn. Then trim the elevator for level flight at full power and full speed. I'd make a level pass and as the model passed me I'd slam the throttle shut. Much to my horror as the throttle came to idle the model's nose rose up quickly (jumped up). This was the clue that the elevators were carrying a lot of up trim to compensate for the down thrust in the the thrust line. This accounts for the model going into an alpha lock on landing as I cut the throttle to land. Unfortunately changing the thrust line in an EDF model is a bit involved. 

 

Here you can see that I cut off the tail pipe and raised the tail cone to effect some up thrust. The F-16 is now flying better and landing much better!

 

So the take a way is don't lock yourself into thinking that the manual specs are the best for your trim, they aren't! They are a starting point and need to be adjusted as the model's flight performance dictates. Unfortunately this isn't easy to see as there is a lot of cross talk between the trimming variables. Like I've said before It often takes me 10 or more flight to get a model anywhere close to being set up.

 

 

[Steve Colman]

I can relate to this post Konrad; it's exactly the same at my club in every detail. I am no expert by all means when setting up a model, but I do put in the time and effort via many trimming flights to get each model flying as best as I can.

 

The weekend warriors, many with years of flying experience, turn up time after time with models that are very poorly set up it beggars belief. Then, they bemoan the flying characteristics of the model and wonder why theirs doesn't fly as well as my example. Some listen and make adjustments to their models while others just aren't interested and don't want to take the time and effort required to make improvements and continue to blame the model - especially so when they ultimately crash it!

 

As the phrase goes, you can take a horse to water but you can't force it to drink.

 

I also wonder why some buy a nice scale model and proceed to fly it like some demented 3D model. We have one guy who recently bought an E-Flite Beechcraft D 18 and he flies it in the same manner as his sport aerobatic models with no thought to the nature of the real aeroplane. He is also one of the group that takes little heed to what anyone else says, often flies with disrespect to others and seems to care little about basic safety. He is, of course, also the guy that has the highest crash rate of models in the club by a big margin!

[Paul De Tourtoulon]

At our field there are a lot of arf models Seagull etc, I found that most of the thrust lines for the engines didn't need adjusting, just the C og G.

[Engine Doctor]

The few seagull models ive encountered have all been built/ made in factory uneccessarily heavy . They all seem to use heavy gauge wire pushrods .... why ? Some simple mods and the models fly a lot better not habing to carry all the heavy metal arround. Ive alsofound never to trust C of G and throw setting that manufacturers give , they are usually miles out .

[Brian Cooper]

With the advent of ARTF models, it seems that some of the basic skills of setting up an aeroplane to be trimmed properly have been lost (assuming they were learnt in the first place). 

 

Many now seem to regard trimming as a "beep beep beep" exercise on the stick trims. . . Oh dear. 

 

The subject of setting up and trimming can have enough chapters to fill a book.  But it's a subject worth mastering. 

It's all part of the game we call "model flying". 

[Christopher Wolfe]

As a Free Flight diehard I can well agree with Brian's comment about proper trimming of a model aircraft.

 

Remember, your ARTF might have been designed by an aeromodeller but then it was assembled in a factory by people who were possibly not aeromodellers and just trying to earn a wage 🙃

 

* Chris *

[Adrian Smith 1]

I echo ED's comment. In most of my ARTFs the CoG settings and throw distances (where provided) are rarely reliable in the final analysis. It also depends on your style of flying too. I tend to view these manufacturer suggestions as only a starting point. And of course trimming is just not a one-off job - it's an ongoing process. 

[Konrad]

8 hours ago, Steve Colman said:

...

 

I also wonder why some buy a nice scale model and proceed to fly it like some demented 3D model. We have one guy who recently bought an E-Flite Beechcraft D 18 and he flies it in the same manner as his sport aerobatic models with no thought to the nature of the real aeroplane. ...

To a large extent I might be that guy. That is unless the model is an engineering project for a full size aircraft I want my model, scale or sport, to fly as well as I can get it.  This is to take off the pilot work load. Rarely do my scale models have airfoils, twist or CofG that look anything like the full size. I also don't need or want the safety margins of the full size in my models. But like you once I get the scale model trimmed I try, at times, to make the flight look like the full size.

5 hours ago, Engine Doctor said:

The few seagull models ive encountered have all been built/ made in factory unnecessarily heavy . They all seem to use heavy gauge wire pushrods .... why ? Some simple mods and the models fly a lot better not having to carry all the heavy metal around. Ive also found never to trust C of G and throw setting that manufacturers give , they are usually miles out .

Wow, you must be more attuned to your models than I. I can't feel the wing loading difference of a 600^2 inch model with 2-56 vs 4-40 hardware.  I might notice the CG shift if the model has a long tail boom. After a while I've learned to expect certain OEMs/designers to set up model in ways that are vastly different than the way I'd set up a model, such as Freewing. With Freewing I set the servos so that my controls are about 60% of the manual's call out. I also set up my battery bay to allow for a the CofG to be set 10% or more aft. I will maiden at the recommended CofG but usually I'm placing the CofG way aft on the second flight.

7 hours ago, Paul De Tourtoulon said:

At our field there are a lot of ARF models Seagull etc, I found that most of the thrust lines for the engines didn't need adjusting, just the C of G.

My experience is different. I find that most ARF models need the thrust line adjusted. For example the ROC/FMS 980mm P-39 needs the so much of the right thrust removed that the spinner is out of position by 12mm or more. I find that most designs have far too much nose down thrust to compensate for being far too nose heavy. Once I get the CofG aft I often have to remove much of the down thrust.

 

4 hours ago, Christopher Wolfe said:

As a Free Flight diehard I can well agree with Brian's comment about proper trimming of a model aircraft.

 

Remember, your ARTF might have been designed by an aeromodeller but then it was assembled in a factory by people who were possibly not aeromodellers and just trying to earn a wage 🙃

 

* Chris *

Rarely are they designed by folks that actually fly models. But to your point there are often build/design errors in the models. See my thread on the Flyfans 64mm Su-27.

https://forums.modelflying.co.uk/index.php?/topic/60012-su-27-flyfans-fatal-flaws/&tab=comments#comment-1024187

Errors like these were designed into the product and really have little to do with the non-aeromodeller assembling  the model.

Edited November 11 by Konrad
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[Paul De Tourtoulon]

20 minutes ago, Konrad said:

My experience is different. I find that most ARF models need the thrust line adjusted. For example the ROC/FMS 980mm P-39 needs the so much of the right thrust removed that the spinner is out of position by 12mm or more. I find that most designs have far too much nose down thrust to compensate for being far too nose heavy. Once I get the CofG aft I often have to remove much of the down thrust.

I only do IC models, none of that electricity stuff,,,

[Konrad]

4 minutes ago, Paul De Tourtoulon said:

I only do IC models, none of that electricity stuff,,,

Doesn't the IC model respond to physics the same as any other model?🤔

 

Given time I'm sure you will see the light,,,

Edited November 11 by Konrad

[Paul De Tourtoulon]

4 minutes ago, Konrad said:

Doesn't the IC model respond to physics the same as any other model?🤔

 

Given time I'm sure you will see the light,,,

Of course, it does, but just to name a few that I have that are IC and haven't needed correcting, Seafire, P 47, P 40 Pitts Ju87, Zero,PC 9, Hurrican,ME 109,

Piper cub, Gambler,SE5a, Tiger moth, Dago Red, all in my garage, c of G sorted and of course the servo throws, and appropriate props, no down thrust

or side thrust needing to be altered.

 

Ps all mine,,,,

[Konrad]

10 minutes ago, Paul De Tourtoulon said:

Of course, it does, but just to name a few that I have that are IC and haven't needed correcting, Seafire, P 47, P 40 Pitts Ju87, Zero,PC 9, Hurrican,ME 109,

Piper cub, Gambler,SE5a, Tiger moth, Dago Red, all in my garage, c of G sorted and of course the servo throws, and appropriate props, no down thrust

or side thrust needing to be altered.

 

Ps all mine,,,,

Good to know. I assume this usually means that the spinner and cowl are in alignment when you find the proper CofG. I assume that at the manual CofG (nose heavy) the model is pitch sensitive with the change in power settings (not enough down thrust). 

[Jon H]

I am in full agreement when it comes to c/g. Too many think its set in stone and the manufacturers recommendation is an immovable value. In reality, most models are massively nose heavy and come with excessive rates to compensate. Most of my models are massively 'tail heavy' vs the plans. 

 

When it comes to thrust lines i have gravitated towards just mounting everything dead straight as it saves a great deal of hassle and makes no difference to the handling of the model beyond a need for a little more rudder use. As i use the rudder all the time anyway its of no consequence to me. 

[Konrad]

32 minutes ago, Jon H said:

I am in full agreement when it comes to c/g. Too many think its set in stone and the manufacturers recommendation is an immovable value. In reality, most models are massively nose heavy and come with excessive rates to compensate. Most of my models are massively 'tail heavy' vs the plans. 

 

When it comes to thrust lines i have gravitated towards just mounting everything dead straight as it saves a great deal of hassle and makes no difference to the handling of the model beyond a need for a little more rudder use. As i use the rudder all the time anyway its of no consequence to me. 

Not trying to sound like part of a mutual admiration society, but I agree!

 

Far too many designs have too much thrust off sets. I initially set up my models with some down and right thrust if for no other reason that I don't want up of left thrust. Side thrust idsa problem if one spends much of the flight inverted. 

 

Getting folks to wake up the left thumb in another issue. But, yes folks need to learn to fly the model not just guide it around the patch.

Edited November 11 by Konrad

[Jon H]

I mostly fly scale stuff, WWII fighters and the like. They never fly straight anyway and a 5 inch spinner looks ridiculous with side thrust on it so i just fly around the problem. The only model that has given me any trouble is my 1/4 stampe as it will only stall turn to the left with the engine at 0-0. I cant coax it into going right as i run out of rudder before the model stops moving. Still, its a small inconvenience and it stall turns nicely to the left so that works for me!