ESM Focke Wulf center of gravity?

[Artto Ilmanen]

Gents,

 

I'm finishing a build of an ESM Focke Wulf 71" with a Laser 150 on the nose.

 

As this is my first ESM warbird I'm struggling with how to measure the center of gravity in a correct way. The manual says 140mm but where am I supposed to measure it? (See picture)

 

If I measure the COG at the centerline of the fuse where the wing meets the fuse I need to add 1150g of ballast 😳😳 inside the cowling to balance the model at the 140mm COG!! This with 2 x 150g RX batteries mounted as far in the front of the fuse as possible, just behind the firewall. I can't help but think that something is is not right, even though the Laser 150 is not a very heavy engine.

 

Hence, I wonder if the way I measure and mark the 140mm on the fuse is not quite right? I appreciate any comments

 

 

 

 

 

 

 

 

 

 

[Chris Freeman 3]

140mm from the very front of the wing, the cg must not be further back than 25% of the wing cord and you also will not need much elevator movement so rates and expo are important. Nose weight is one of the aspects of building warbirds as it is needed on almost all the designs as the real aircraft had very heavy motors that resulted in short nose moments. If in doubt add lead as a nose heavy aircraft might fly badly but a tail heavy aircraft may never fly again.  

[Ron Gray]

@Artto Ilmanenwhen using the Vanessa balance rig is your tailplane level? It’s difficult to judge from the photos but it looks a bit tail down.

[Artto Ilmanen]

Chris and Ron

 

Thanks! So the very front of the wing so the way I measured it should be correct. Unfortunately.

 

Ron: the tail is a bit down in one of the pictures. But the model was level with the 1150g simulated weight on the nose. I needed that much weight to reach the COG at 140mm.  For comparison, I checked your Bearcat Laser 240V thread and it seems you didn’t need much extra weight. So I’m a confused now. I expected something like 500-700g be needed but 1150g is much!

 

 

 

Edited January 4 by Artto Ilmanen

[Jon H]

Your balance rig isnt doing to work the way you have it set up. An easy way to balance is to install a hardpoint on the c/g (140mm in your case) and install a small screw in hook. A loop of rope/string and its job done as just pick it up. You can do lateral balance while you are at it using this method. The alternative is to put a small screw on the c/g point just behind each retract unit. You can then add weights, and then just pick it up on your fingers using those screws to locate yourself. As long as its pretty close it will be fine. Remember that you will use up around 200g of nose weight (fuel) during the flight and it makes naff all difference to the model in real terms. Its not as sensitive as many people think. I used the screws/fingers method on my ugly mustang and have made no adjustments so far. 

 

Using the rope method i balanced my DB Hurricane on the c/g noted in the instructions. As we talked about before, it was mega nose heavy and i have removed nearly 1lb since. I also noted half way down my takeoff roll with the Hurricane that the elevator was over sensitive so kicked it to low rate before i had even left the ground. Once in the air it was still too sensitive on the low rate, but high rate was needed for landing, so this was adjusted after the flight. It then all changed once i removed the lead and finalised my elevator/flap mix.

 

Again as discussed yesterday, a minimum of elevator travel will mean that even if the c/g is a little on the lively side its not going to matter. Excess rates are usually a far bigger problem than a c/g that is slightly out. 

 

In my experience the ESM/YT c/g placements have been fine, but their elevator rates excessive. My Sea Fury also has an incidence error built into the tail so it climbed like crazy on its first lift off from the runway. Trimmed back down it performs perfectly. Since then however i deliberately trim all my models nose down before their maiden flight to prevent them leaping into the air. 

[Ron Gray]

I must admit that I tend to have the model upright when using the Vanessa rig but have used the loop round wing or loop round fuselage as you have done without any issues, so not really sure what Jon is referring to?

 

My Bearcat has got noseweight added but nowhere near the amount you are suggesting, but the 240v is a bit heavier than the  150 and I would say that the fuselage is also shorter than the FW.

[Artto Ilmanen]

Thanks Jon and Ron,

 

I can do what you suggest as a benchmark to my Vanessa Rig arrangement. I must confess that this is the first time I'm using it. I wonder what I'm doing wrong?

 

Anyways, my retract wheel wells to not allow mounting a small screw on the c/g point just behind each retract unit as you suggest - but I could install a small screw eye hook ( if this is what you mean) and lift the model with a loop of rope. Or I could just add small marks with a parking pen on top of the wing just beside the fuselage at C/G point and lift the model with fingers. Please see pictures

 

 

[Jon H]

2 hours ago, Ron Gray said:

I must admit that I tend to have the model upright when using the Vanessa rig but have used the loop round wing or loop round fuselage as you have done without any issues, so not really sure what Jon is referring to?

 

I have to admit the venessa rig is a new one to me and i thought he was doing something else. But having looked it up and educated myself on its operation i come back to my original assessment. The whole thing is a solution looking for a problem. Its over complicated and its operation is too reliant on the quality of the assembly of the rig. I am also not entirely convinced it will do what they say it will anyway.  I will test it though just for my own information and to see if im just being grumpy. 

 

2 hours ago, Artto Ilmanen said:

but I could install a small screw eye hook ( if this is what you mean) and lift the model with a loop of rope

 

yes that would work. You can use a luggage scale if you want to see how heavy it is. I rarely weigh my models as i just dont care. They weigh what they weigh, i cant do much about it now its finished so why worry about it?

 

 

1 hour ago, Artto Ilmanen said:

my retract wheel wells to not allow mounting a small screw on the c/g point just behind each retract unit as you suggest

 

Put them behind the retract units themselves. Makes the model more stable when you lift it. 

 

In fairness the screws are an addition to my efforts with the mustang to make it easier for you to find the points. I just went the width of a finger behind the retract mount screws and lifted it up. The balance was a bit nose heavy thanks to the inline, so i moved the battery behind the pilot which helped, but i added some lead i had kicking around to the tail and that was better so went with that setup. It flew fine, probably needs some adjustment with more in the tail, but its only flown 3 times as the retracts need attention, tanks need changing etc, so i will make changes after more stick time and in the final configuration. In any event, setting the c/g before the maiden took all of 10 minutes to decide what was going where. I just dont worry that much about it as i can guarantee i will change it after the first few flights anyway. As long as its not a mile out, it will be fine. All you need is for the balance to be close enough for the model to be flyable during the test flight. With the low rates already discussed, you make this window of controllability much wider. Im not saying you should ignore setting the c/g, im just saying dont get too bogged down in mm and g of adjustment. Broad strokes, get it in the ball park and just go fly. Anything 130mm to 150mm will likely be perfectly ok. Once flown you can make adjustments to suit the model, your own tastes, the feel of the model, etc. You cant set the perfect c/g on the ground, it is not a fixed entity set in stone. All you are aiming for is one 5 minute flight. Get up, trimmed, stall test, down. The model then returns gets stripped down, inspected, fuel use checked and timer adjusted, changes made to rates c/g etc. if its all good i will do one more flight, then its back to the hangar for a more detailed inspection and further changes if needed. 

[Artto Ilmanen]

Ok thanks for all valuable comments

 

Upon rechecking the CG as Jon suggested It tuns out I need 835g lead on the nose to start with. This means about 7300g or 16lbs total weight excluding fuel. Once I'm done with the first flights I can probably take off some lead on the nose: I tested already that if I remove the small tail wheel servo (22g)  and replace it with a pushroad I can save a 100g on the nose. I can also change the batteries to lighter ones and save another 100g. So total weight of 15,5lbs give or take  should be easily achieved. And maybe another 200-250g of lead can be removed from the nose when more experience on how the model behaves.

 

Anyways, I think no more guesswork is needed with this initial CG adjustment. But a funny learning experience on the warbards and how much weight they sometimes need on the nose (due to our models mimicing their full size counterparts with very heavy engines, of course)

 

Ron: the your Bearcat is about 10cm shorter than the Focke explaining somewhat why I seem to need more lead on the nose. 

 

 

[Graham Davies 3]

On 04/01/2024 at 08:46, Jon - Laser Engines said:

Again as discussed yesterday, a minimum of elevator travel will mean that even if the c/g is a little on the lively side its not going to matter. Excess rates are usually a far bigger problem than a c/g that is slightly out. 

Hi Jon,

 

Completely agree about low rates being the way to go. Quick question though; what do you start with? I'm coming to the end of a scratchbuild 1/6 scale spitfire. My first time at this scale. For a 74" span spit, what elevator and aileron rates would you start with on low and higH?

 

Cheers!

 

Graham

[Ron Gray]

On 04/01/2024 at 12:28, Jon - Laser Engines said:

I have to admit the venessa rig is a new one to me and i thought he was doing something else. But having looked it up and educated myself on its operation i come back to my original assessment. The whole thing is a solution looking for a problem. Its over complicated and its operation is too reliant on the quality of the assembly of the rig. I am also not entirely convinced it will do what they say it will anyway.  I will test it though just for my own information and to see if im just being grumpy. 

Not so Jon, sure you need a place to hang it and you need that to be over a table or whatever supports the model but once those are in place it really is an easy bit of kit to use, especially when the models get larger and heavier! 

 

PS You are being grumpy!

[Jon H]

1 hour ago, Graham Davies 3 said:

Hi Jon,

 

Completely agree about low rates being the way to go. Quick question though; what do you start with? I'm coming to the end of a scratchbuild 1/6 scale spitfire. My first time at this scale. For a 74" span spit, what elevator and aileron rates would you start with on low and higH?

 

Cheers!

 

Graham

 

I have to admit, i tend to just guess. I have flown so many warbirds now i just take a gander at it and wiggle the rates around until it looks good. I have no idea what the actual measurements are. Again, my goal is not to set up the model for life at the outset, only to get it through the maiden. I adopted this policy years ago as i found most models have really excessive rates. Its frustrating as i have sufficient experience to make an informed judgement and ignore the book of words but I feel bad for those less experienced struggling with a model, or indeed loosing a model due to incorrect rates from the factory. A friend at my old club had a H9 60 size Spitfire that he simply hated flying because it was set up according to the instructions and impossible to land. He tried adding expo to help, but to no avail. I deleted the 60% expo, took the rates down from 75% to 20% and the thing flew like a trainer, sailing in as stable as a super 60. 

 

I recently helped a club mate with a DB 80 inch Spit, and while i cannot remember the settings we used in mm i know the 'maiden' rate was probably half that of the recommended low rate in the instructions, and i think i lowered it further afterwards. We had a 3 position rate switch with kit high, kit low, and my 'maiden low' option dialed in. I think i took off on kit low and found it excessive, went to maiden low right away. After landing changes were made to a bunch of stuff. I think Kit high was binned for good, kit low modified a bit (i forget which way) for landing/takeoff and we set up an elevator flap mix for landing too. 

 

With a gun to my head i would guess...8mm deflection for low? maybe 12 for high? Something along those lines. Its hard to say as a Spitfire has a wide elevator but Artto's 190 has a narrow one. 8mm on both would give very different deflection angles. I would say 8 and 12 are a half decent start point. 

 

On the elevator flap mix i consider these essential as most warbirds want to dive to their deaths with gear and flaps down so need large amounts of up elevator mixed with flap deployment. This is contrary to the expectation of most. In any case i set it all up on the ground so that lowering the flap deflects the elevator up. I set the value to 1% or something just to make sure it works then leave the menu open on the tx  for the maiden flight. Once up and trimmed, i stall the model clean to get a clue about its behaviour, then drop the gear and flaps and keep adding up elevator to the mix until i get to a near stalled condition. If beeping the buttons on the tx while flying is a bit intimidating, employ a passer by to beep your buttons. I have done it so many times i can do it by feel so never take my eye of the model. With all that done you can do your entire approach without much (if any) elevator input as you can control the pitch of the model with power. More power to climb, less to descend. As its trimmed for this slightly over stall air speed it is completely impossible to stall the model unless you start tugging on the elevator. Just fly it in using power instead then a tiny tug of elevator to flare. It takes some getting used to, and if you are a little hasty and just chop throttle and dump gear and flap at the same time the model will probably loop all on its own due to the excess speed. Another club member i was helping with large warbirds discovered this the hard way as he kept rushing his approaches. Once he got used to a longer and more procedural style of landing approaches he has no trouble at all guiding his 30cc H9 Spitfire back down to earth. 

 

All of this applies to what i would consider a 'normal' warbird. Wood, fibreglass, maybe foam wing, ic/electric and of a weight appropriate for its span. So a 60/65 inch warbird is 8-12lbs, 70 inch 11-14, 75 is 14-16lbs, 80 inch 17-20, 19-22 for 88 inch, etc as ballpark numbers. 

 

For ailerons and rudder just max them out (unless that gives 3d levels of deflection), then set an aileron rate at say 75%. Get it in the air and adjust them down later. Again, i have a flying rate for ailerons on my warbirds set to give a roll rate slightly faster than scale at full stick deflection. Whatever that rate is, i then add about 15% and set that as a high rate to give me more authority while trying to land in a choppy wind and as a cover against an aileron servo/linkage/cable failing. 

 

Hope that all helps and makes some sort of sense and with the low/less sensitive elevator rate you can shift your cg aft without fear of nasty things happening. The ultimate goal is to keep the c/g as far aft as possible as this will be a great help in preventing your warbird spending its days sniffing the dirt. You can then taxi out, turn and takeoff like a right smug so n so with your propeller tips completely intact. Scale wheels will help with this too. I think 4.5 inch is right for a 6th Spitfire

 

 

 

48 minutes ago, Ron Gray said:

You are being grumpy!

 

bah. 

 

9 hours ago, Artto Ilmanen said:

I tested already that if I remove the small tail wheel servo (22g)  and replace it with a pushroad I can save a 100g on the nose

 

Sounds about right. 22g servo, maybe 5:1 ratio tail:nose? 22x5 = 110g ballast needed. Personally i dont bother with steerable tail wheels any more. Mine all free castor and i only have issues if its a howling crosswind, which i dont fly warbirds in anyway. Also with a free castoring wheel you can do neat pinwheel turns which look really authentic. 

[Graham Davies 3]

Thanks Jon, that's really helpful.

 

The trouble with scratch building is EVERYTHING is a guess. So far, I'm usually not far off, but I am very aware of the balance of CG/ elevator sensitivity/ noseovers!

 

My Spit is a Mk1, and at 74" will weigh around 8lb, so very light. That does make a lot of things easier, but can also lead to undesirables, such as the temptation to get off the ground too soon. All should be well if I remain patient, and stick to the processes.

 

Good advice about the flaps. I'm not familiar enough with my TX to modify the mix during flight, but I will set up a couple of mix options on a switch to keep me out of trouble. Again, the light weight may be my friend here. I will check them at  height, and if it points to the ground I can recover and try again with an alternative mix. I do have a couple of flap settings already, so I should be able to keep it manageable.

 

I have a fixed tailwheel. Taxiing is all but impossible on our strip, so I usually try to make the take off a bit easier...

 

Graham

[Artto Ilmanen]

I'm pretty sure the Vanessa rig works - it's just that I'm not used to it. That's why I wanted to check things in accordance with Jon's method. Anyways, time will tell if I need a larger engine or if the 150 has the power to perform loops, etc that the real thing could do. I initially thought I could easily achieve the 15lbs total weight but now I'm at around 16lbs with the almost 2lbs lead on the nose and if I'm lucky I can achieve the 15lbs as I get used to the model and can put it on a diet. If needed, then a Laser FT200 or a 200V. By the way, is the power output about the same with a 200V and a FT200? Or is the latter more powerful?

 

Jon, I will test the free caster tailwheel to save max weight on the nose, thanks for the hint!

 

Edited January 5 by Artto Ilmanen

[Chris Walby]

I put my FT200 in a Eurobat and then a Balsa USA cub and on both occasions I had to add lead to achieve C of G (like nearly a Kg), so what? The FT is a light engine compared to others, but as the models were already built not much I could do. The point is the Cub just ended up the same weight as it was originally with the Super Tigre in it.

 

You think you are adding lead, but its only because you want the C of G in the right place. If you put an inline or 2 stroke with all the additional kit that comes with it then you would not have to add lead to compensate for a light engine!

 

My Acrowot has a Laser 70 and no lead as its heavy compared with a 2 stoke, but then you would have to add lead to get the C of G.

The Dual Ace has 2 Laser 70's in it, spot on target weight and no lead, but the 2 stroke versions have a pile of lead in them to get C of G.

 

The answer is to build a light tail end of a model, then a light engine will balance things out with no lead required.

 

As said before its far better to fly a model with the C of G in the right place than one with a very reward C of G so it is what it is or lighten the tail and mover everything else forward, but my guess is that it will fly just nice the weight it is and save a lot of work!

 

PS as for looping I have seen someone loop a Hurricane that only had enough power to fly and very gentle climb...just very good at energy management! 

[Artto Ilmanen]

Hi Chris,

 

Very true, the weight required on the nose is what it is. It's just that my other models are aerobatic or semi-scale civilian airplanes with longer nose so I have been able to get the CG in correct place without much lead needed. So I'm good now - I just felt puzzled at first by the amount of extra ballast needed. I do happen to have a heavier engine (a Laser 240V ) without a model so I could use that and be good with dead ballast. However, I feel the 240V may be too big an engine (I'm not into the American style where you put a crazy engine and race through the sky no matter what the model is) so I think I will try the Laser 150 first. Jon says that even at 15lbs I should be  good - I may be a half lbs heavier so we'll see.

 

By the way, I have watched your youtube videos on the FT200 / Eurobat many times - thank you! The FT200 seems to be a great engine, among the other V- twins.

[Frank Skilbeck]

Don't feel too bad my friends 28% scale 30kg 190 has 2.5 kg moulded into the front of the cowl, and that has a Moki 215cc radial.😁

[Jon H]

I do not think you will be wheezing round and unable to loop. As Chris says its mostly an energy management sort of deal and it will loop for sure. Plenty of people have flown the 70 inch ESM warbirds using our 150/155 and they are normally around the 15lb mark. My pica p40 was this span and weight too, it was fine. 

 

You could use the 240v, then fit a bigger prop to tame it a little. But the 240's i have in service currently are delivering great performance in models of up to 23lbs so at 16, it would be speedy!

[Artto Ilmanen]

38 minutes ago, Frank Skilbeck said:

Don't feel too bad my friends 28% scale 30kg 190 has 2.5 kg moulded into the front of the cowl, and that has a Moki 215cc radial.😁

  oh boy! 🤣

[Artto Ilmanen]

15 minutes ago, Jon - Laser Engines said:

I do not think you will be wheezing round and unable to loop. As Chris says its mostly an energy management sort of deal and it will loop for sure. Plenty of people have flown the 70 inch ESM warbirds using our 150/155 and they are normally around the 15lb mark. My pica p40 was this span and weight too, it was fine. 

 

You could use the 240v, then fit a bigger prop to tame it a little. But the 240's i have in service currently are delivering great performance in models of up to 23lbs so at 16, it would be speedy!

 

As spoken earlier with you, I aim to put the 240V in another model, such as Hurricane. So I will start with the 150 in Focke and then if needed fit a FT200. I don't like overpowered models very much and thus, if I'm not mistaken, the FT200 is a better choice than a 240V. 

 

Jon: How about the silencer position when the engine is side mounted? Is it ok if it's pointing down or will the hot silencer  being pretty closed to the carburetor cause trouble with respect to the carb getting cold air? This is the best orientation with respect to the cowling. I could, of course,  rotate the silencer to point more towards the center but that's not as good looking plus I aim to install a secondary engine mount for the lead which is somewhat preventing me to rotate the silencer to point to the center / bottom of the cowl. (by the way, I actually asked this in email to you but maybe this is something that others like to know, too)

 

 

[Ron Gray]

That’s a very similar setup to my Laser 80 powered LA-7 and it hasn’t been a problem in that.

[Artto Ilmanen]

49 minutes ago, Ron Gray said:

That’s a very similar setup to my Laser 80 powered LA-7 and it hasn’t been a problem in that.

Yes, I thought it wouldn't be an issue as there should be plenty of airflow by the carb to cool things out. I do remember with some of your models (one of the inlines? the mustang / Inline 160?) you had to do some modification to get cool air to the carbs. I don't recall the cause, though.

 

Also, I had planned to mount the lead in the fuse centerline below the engine and therefore I was thinking I can't rotate the silencer towards the centerline. However, with respect to the lateral balance I realized that it might be a good idea to mount the lead on the opposite side of the centerline to counterbalance the side mounted engine cylinder weight. 

 

[Jon H]

5 hours ago, Artto Ilmanen said:

How about the silencer position when the engine is side mounted

 

As Ron says, it will be fine

 

1 hour ago, Artto Ilmanen said:

However, with respect to the lateral balance I realized that it might be a good idea to mount the lead on the opposite side of the centerline to counterbalance the side mounted engine cylinder weight. 

 

I wouldnt worry. Just mount it under the engine

[Artto Ilmanen]

Thanks Ron and Jon,

 

All clear. I will also remove the tail wheel servo and I change the batteries to lighter ones. Let’s see if I can get to nearer 15,5lbs to start with

[Ron Gray]

4 hours ago, Artto Ilmanen said:

I do remember with some of your models (one of the inlines? the mustang / Inline 160?) you had to do some modification to get cool air to the carbs. I don't recall the cause, though.

It was my Giant Super Sportster powered by a 200 in-line. The problem wasn’t cooling air it was air turbulence around the carbs which meant that it wasn’t running as cleanly as it could. I could have redesigned the baffles to prevent it happening but decided the extend the air intakes with silicone tubing into the fuselage, this fixed the problem and also made the engine even quieter!