I understand why one needs to and how to set up incidence etc, the notion of decalage and all. How does one determine the FRL? If its a plane from plans I guess the plans would have it but on an ARF how do you determine the FRL? Without the FRL it will be next to impossible to set incidence right? Can anyone please enlighten me? Thanks!

You can use the cord line of the stab, the engine thrust angle/firewall or cord line of the wing in relation to the stated design incidence/thrust angles of the particular design. Or make one up.

Terry in LP

Terry ... I may be reading you wrong but if I take the line off the H stab then that is on the assumption that the stab is set at 0* incidence. That again can give me as a referrence to check decalage same as taking from the wing chord line etc. But I am thinking if I want to ensure that both the H stab and wing incidences are at a certain amount then I will need the FRL as the referrence as the incidence is taken in relations to the FRL but decalage is only the difference between the stab and the wing. Am I correct or am I just not getting it right?

If you have no plans, you have to start somewhere. There are 3 things you're trying to line up - correct? That would be thrust, wing incidence and stab. incidence. What you really want to know is how these 3 things relate to each other not necessarily where they relate to some line on the fuselage. OK, I know, if you DO have a reference line you can align everything to that but I'm assuming in this case we don't. So what do you do? Simple. Pick one of the 3 and measure the 2 other items in reference to that. Make sense? If you have a non-adjustable stab. you can use the stab. as the datum. First, put your incidence meter on the stab. and zero it out. Now put it on the wing. Record the number for future reference. You have just measured the wing - stab. decalage. Finally, put the meter on the thrust washer and measure the thrust angle. This should tell you all you need to know. Even if you have an adjustable stab. this method works. you can paly with the stab. to get your thrust line whwere you want it and THEN adjust the wing to get the wing incidence where you want it.
In some cases, I have started with the thrust angle. I do this when I know exactly what the thrust angle is supposed to be. I set my incidence meter to read the correct (known) thrust angle on the thrust washer, then proceed to the wings and stab. Like I said, what you're concerned about is how these 3 things relate to one another.
Hope this helps, or at least gets you going in the right direction.

John Pavlick
Team Black Magic, Tech-Aero Designs

One should always start with the thrust line. Thrust is THE dominant vector at partial and full revs which is the condition most flown. About the only time other vectors dominate is when engine is at near idle or slightly above.

I set the backplate vertical and shoot a lazer beam down the fuse to the tail; the beam centered on the backplate.

Then mark the reference line at the wing station and stab station and work off these marks. I prefer the stab either on the marks or slightly below. BUT if you have an ARF, the cutout is probably already made, so you may have to bisect the stab opening with the lazer and adjust the thrust backplate to be orthogonal to the stab.

My designs have the Wing below about 1/2"-3/4". Dihedral always has worked out to be minimal (top of wing flat) when wing is positioned on the fuse this way. Wing about +1 degree to thrust line; stab at zero to thrust line. If you have an ARF, you may need to make adjustments as needed to fit these parameters. That is, if you want a model that trims easily with minimal electronic mix

MattK

Okay guys ... I get it now. ARF will mean you got no FRL to work from. Hence, you only want to get the relative incidence between the H stab, wing and the engine thrust line etc. I understand now that the engine thrust line is important as that will roughly be the centre of the plane. Thanks a lot guys!

John ... you said "In some cases, I have started with the thrust angle. I do this when I know exactly what the thrust angle is supposed to be. I set my incidence meter to read the correct (known) thrust angle on the thrust washer, then proceed to the wings and stab." Does that mean your firewall is set as zero and then you dial in the necessary engine thrust and that becomes the base for your referrence? So the referrence line is actually the engine thrust line? Am I correct to say that?

Matt ... so the laser beam is used to set your referrence line huh? Interesting ...

Really appreciate your input guys ... appreciate it a lot.

Hi John

I understand about the 3 things relating to each other ie thrust, wing, and stab incidence.

Can you explain further on how to get the fuselage level accurately from nose to tail (pitch axis) and wing tip to wing tip (roll axis) before one start to work on these 3 component mention? Tips and approach?

Having said that if you know the FRL and the engine thrust (down and right), I presume the main focus now is the decalage of stab vs wing and the wing incidence. Thats it?

OK some explanation is in order. First, what Matt said is absolutely corect. He's been doing this longer than most of us! LOL There are many ways to get the job done, and a few of them are correct. Pick your poison. I ran into this problem when I was building a World Models Groovy 90 ARF. I had no reference line of any kind. No plans either. I called up the distributor and they told me that the Groovy 90 had 2 degrees of downthrust built in. Don't hold me to that number - it was a while ago. Whatever it was, I used the method I described for "if you know exactly what the thrust angle is supposed to be". The firewall was angled so that when you installed the engine, it gave you 2 degrees of downthrust relative to the "invisible" fuselage datum line that they forgot to show in the instructions (probably on the Master plan somehwere in China). All I did was use the "known" parameter to establish my reference point. This is one method and it works. Like I said, You're working primarily with 3 things: thrust angle, wing incidence and stab. incidence. When you have plans thst show a datum line on the fuse, you can just align everything relative to that. Works great. If you don't have that important piece of information, you can find it as long as you know one of the other key pieces. If you don't you can get fairly close by making some observations. On ariplanes that don't have adjusable stab's, the stab is a good reference point. What I'll usually do is zero my meter on the stab then measure the thrust angle. If that looks reasonable I'll set the wings realtive to the stab - about .5 positive. This gets you started. Remember, you need to fly it to dial everything in. The bench setup is just to get you started.

It's important to make sure the plane doesn't move when you're taking all of these measurements. If you put it on a cradle, make sure it's strapped in really well. You don't want anything to move while you're making adjustments (other than the part you're trying to adjust) or else your measurements will be inconsitent.

John Pavlick
Team Black Magic, Tech-Aero Designs

John ... many thanks for assisting us newbies here. Explanation is clear for sure. Does confirm what some of us were guessing only. Thanks a lot guys!

Cool, I'm glad I was able to help. As a side note, what the datum line does is give us a common reference point for all the measurements. I think we all understand this now but here's something to keep in mind. One of the things you're trying to do when you trim the plane for straight and level flight is establish a good presentation of the fuselage. Ideally, the "datum line" will be at 0 degrees relative to the horizon. You CAN trim the plane to fly straight and level with the nose high or even with the nose low. In that case, the datum line will NOT be at 0 degrees relative to the horizon. The datum line depicts the "desired" attitude of the fuselage in straight and level flight. That's what Matt was talking about. He likes to set his planes up so that the stab. is effectively flying at 0 degrees in straight and level flight with no up/ down thrust. You will however see some planes with deliberate downthrust (i.e stab. is set at 0 degrees, thrust is at 1-2 degrees down referenced to the datum line). In this case, the designer has flown the airplane and determined all of the recommended settings based on flight testing. This makes your setup task much easier than it is when starting from scratch with an undocumented ARF. Keep in mind - different airplanes react to trim adjustments in slightly different ways. You can't set 2 totally different airplanes up with the same thrust angle, wing and stab. incidence and expect them to behave EXACTLY the same way. Like I said, the final adjustments are made when you fly and trim the plane. That's another story...

John Pavlick
Team Black Magic, Tech-Aero Designs

Actually John and Tianci,

My stab will fly at slightly negative incidence because the wing AOA will dominate and seek it's own equilibrium. The fuse will follow where the wing equilibrates. Thrust is the dominant force under all conditions past 1/4 throttle or so, therefore you don't want a whole bunch of negative in the engine. Remember that the powerplants of today's pattern models push back tons of air not just a few pounds. That's the reason why I don't use any more delta than about 1 1/2 degrees total between the main vectors in question. Right thrust is another matter and it complicates things dramatically. I try to minimize right thrust also. I use the minimum possible to get the desired flight character....it usually works out to around 2 degrees right for a conventionally designed pattern model.

(My new design is not conventional and will use about 1/4 degree of right and zero down. At least my experiments on a smaller model worked this way. I'm not certain that it will work as well on the full 2 metter size)

My final settings always are done after flying, of course. On occasion I've had to increase dowthrust slightly to improve the powered vertical upline without compromise to the downline (increased by -1/2 degree at most). This wasn't always required and depended to some degree on fuse shape and most importantly, fuse length. It's the spiralling air stream hitting the stab again and all the strange stuff that causes

Keep in mind that most pattern models are shaped very similar nowadays so these settings will hold pretty much across the board (for pattern models). If you are working with some other type of model, then other settings may be required.

regards

MattK

EXCELLENT THREAD!!! – Many thanks.

I've learnt a lot with this thread but only one thing that is most important ... when I get a nice pattern plane I shall ship it to Matt or John to set it up ...

Come on, you can do it yourself! It's not that difficult, it just requires a methodical approach and patience. Make only one adjustment at a time so that you can see the effects. Don't change wing AND stab. incidence at the same time for example. After a while you'll be able to fly someone elses plane and tell them what adjustments to make. The more planes you fly and trim, the more experience you'll get. I'm actually kind of new at this (Matt is a seasoned expert) but I am an Engineer so I guess that helps. Even with all the "science" there still is a little bit of a "black art" aspect though. Just don't be afraid of that. You'll do fine.

Matt made a good point. Most modern Pattern planes do behave in very similar ways. Aside from inherent coupling issues, the trimming process is pretty much the same. I normally set my planes up with .5 - 1 degree of downthrust, .5 degrees positive in the iwng and 0 on the stab. Fly and adjust from there. Usually it only takes a little adjustment on the wing incidence and sometimes a minor C.G. re-location. Mixing is the last thing you do and most of the time you don't need to do too much of that if you've done everything else correctly. If you find that you have a lot of mixes, you should either disable them all and try to trim the plane mechanically or find another plane. Modern Pattern planes require very little mixing. Scale and 3D planes can present more of a challenge. That's what I meant by different planes requiring different setup and trimming details.

John Pavlick
Team Black Magic, Tech-Aero Designs

John ... I agree its not that hard to do all that, just that the ARF fever has really made hobbyists lazy and into sloths. Thanks for all the info.