kevin mcgrath

My Feedback: (1)

Anyone here familar with the laser incidence meter
originally sold by Great Planes and since discontinued I think?
Color me stupid but I cant understand the instructions for settting this tool up to take readings of the stab and wing incidence.

Gray Beard

My Feedback: (-1)

They work prety well but I always had to zero it first on a level counter top in my shop then without touching the little laser I would attach it to the wing or stab and look at the reading. Every time you touch the on off switch it moves the little light so it needs to be reset every time. I hope that's what you don't understand. I gave mine away to one of my old students after I got a couple of the Dubros.

Crash Campbell

Hi Kevin,

I still use mine, it works fine if you do as Gene said and make certain your thrust line is level. In my case a [i]"little scientific instrument tap"[] two or three times to make certain the readings were the same was required as sometimes it would stick in position after adjusting the incidence.

Cheers,

Colin

vicman

My Feedback: (10)

I always wanted one of those. So far the Robart is doing the job.

Gray Beard

My Feedback: (-1)

Still need to know what Kevin doesn't understand, I have been known to mount my meters backasswards and had to scratch my head for a little while.

dingo9882

I am not going to lie to you. My experiance with it was not positive. Just like somebody else said I was constantly resetting it and in many cases was off a little. I went and bought the hanger 9 meter that is digital and I have had great success with it. I also hear very good things about the Robart one.

Zor

Hi Colin,

Are you really measuring incidence using the thrust line as reference ?

Zor

Crash Campbell

Hi Zor,

Depending on the plans and kits that I have built from yes. The norm has been the datum line and the thrust line have been the same in most cases in models with symmetrical wings, albeit there is usually right thrust.

Cheers,

Colin

Zor

Hello Colin,

I hope I did not disturb you with my question. I was more concerned with the beginner or uninitiated that might have considered the thrust line was always the reference which of course you and I know it is not.

Cetainly if someone changes the down or up thrust angle he is not changing the incidence.

If the thrust line happens to be coincident with any selected reference (data) line then there is no difference but that rarely occur.

I nearly always select a data line coincident with the chord of the stabilizer or parallel to it. I am hoping the reason is obvius even to the beginner since the stab and elevator always try to line up in the flight trajectory.

The only reasons why they would not are due to the CG location or the decalage angle.

Zor

JollyPopper

My Feedback: (6)

I would like to ask a question here. Airplanes want to fly straight and level with the horizontal stabilizer at zero degrees incidence, right? If that is correct, why can't we lift the tail of the plane until the horizontal stab is at zero degrees (dead level), support it with a block or something in that position, and then check the incidence of the wing? If that works, we don't need to worry about datum lines or thrust lines or anything else. Level the tail and check the wing.

vicman

My Feedback: (10)

Close to what I do there JP. Most of my planes are 0-0-0 so I mount up the firewall to an angle plate and set everything off it. These are all Q500 models mind you.

Crash Campbell

Hi Jolly Popper,

What you say is generally correct in my experience, however there are some models that have the horizontal stabiliser set other than at zero degrees. One that springs to mind is the CG Ultimate that has the horizontal stabiliser set at +1.25degrees and both wings and thrust at zero degrees. I have built a few others that also varied from the norm you describe but as usual I "disremember" like I do most things these days.

Cheers.


Colin

Gray Beard

My Feedback: (-1)

Still no idea what it is that gave Kevin his confusion about the GP meter. Guess it could have been how to level the plane first. Most plans or kits show a datum line to be set from, some others have a line marked on the fire wall and one marked at the back of the fuse for datum. ARFS can be a problem but most have the stab set at zero and if the stab is already mounted you can set from that.
As Colin pointed out though, I have run into a number of planes with some odd incidence for the stab too, even built a couple with adjustable stabs.
Until Kevin says what has him confused though????????? Just hope he doesn't pull a Gene and mount the meter backwards then wonder how he is so far off.
Colin, funny you should mention the Ultimate, I may be working on a trade to get another of the 50cc sized?? I don't want any more Bipes but it's hard for me to turn down an Ultimate!!! An offer was made at the field today though!!
Gene

Zor

Hi jollyPopper,

Hope this help without going though all the 5 forces acting in flight.

Zor

MTK

Depending on the level of precision you are after, the Robart or Dubro will get you within maybe 1/4 degree if you are lucky. For sport planes this is okay.

If you need higher precison than that, and I do, a height gauge is the right tool for the job. Harbon freight has these for about 25$. The scribe is made of carbide and will get very precise measurement down to 0.001". The scribe marks of the wing and sta LEs and TEs are about 10 thou wide so the carbide scribe can split the marks easily. Getting within 5 thou on these measurements will produce a superior flier, hands down

A height gauge is one of the few indispensible tools during set-up; checking wings, stabs and control surfaces for flatness is a cinch. Requires a flat table to work as your refernce.

vicman

My Feedback: (10)

Matt,
I've used the hight gage method plenty but after I got the robart I use it just to verify. The robart makes quick work of the job and by my experience is much closer than your results.

Gray Beard

My Feedback: (-1)

I just noticed I used the word Dubro, my bad, I use Robarts. Brain Fart!!!!![&o] I have never known anyone using a height gauge to set incidence but I will check them out my next trip to HF.
Gene

MTK

To determine angle you need some very basic trig knowledge. A height gauge gives distance measurement and you may want to convert that to an angle.

If you want to measure plug-in panels with precision, this is one simple way.

The very best way I've ever used is employ a transit to measure. Every angle needed is measured from a side view. The great thing of transit use is that the model does not get touched; one measures stab, wing and down thrust directly from the side view... On second thought, this is probably too advanced for many and possibly does not belong in this forum.

vicman

My Feedback: (10)

A transit??? Dude, that is just silly!

I don't know what kind of planes you build but the ones I build are pretty high performance most of the time. If someone started to scrutinize them with a transit I would be likely to punch them in the neck.

kevin mcgrath

My Feedback: (1)

First thanks for all the responses.Second ,sorry to take so long answering ,but I didnt know the input would be so quick.
To clarify......I am an experienced kit and plans builder.I have both the Robart (old one) and the great planes meters and have had the same problems as you with the laser unit.
The model involved is a flying boat named Miss Grandin ex RCM plans I have had kicking around for years and it uses a pylon mounted engine (Just bought a new four stroke OS 1.10 for it)
We have a new waterfront field here in the Soo,and we fly land and water at the same time so I just had to have something for the water.
There is a datum line on the plans but it has been a while since I used the incidence meters and I let the sparse instructions that come with the Great Planes unit confuse me.
I ended up setting the meter to 0 on my level workbench,then scribing the datum line on the model hull,and using a level set the model level with the bench .
I then took the readings on the wing which turned out to be spot on,and the stab which needed a little saddle trim to get it to 0.
I checked the readings several times and got within 1/2 degree each time so called it quits. Then I checked all again using the Robarts unit with the same results.
BTW the angles on this design are 0-0-0.
Thanks for the help.........Kevin

Gray Beard

My Feedback: (-1)

If you want the instructions I can get them to you this week.
Gene

kevin mcgrath

My Feedback: (1)

For the laser unit? Ialready have them thanks.......they just werent much help

Zor

Kevin, thread participants and readers,

I find this thread very interesting ___
Why ? ___because it has up to now brought up some topic concerning incidence itself as compared to setting up the laser meter.

It also brought up the degree of precision in measurements and thus indirectly any requirements for such precision. It is mentioned that some people need a better accuracy than 1/4 degree. I did not see any explanation for such accuracy to be needed.

My understanding is that incidence angles are set in the construction of the airplane and that it is so for full size as well as model airplanes.

The reference line (also sometime referred to as the "data" line) is usually taken as the chord of the stabilizer or a line parrallel to it.

Occasionally the reference line is taken in relation to the fuselage profile as a convenience in the construction. In such cases it may result in specifying some incidence angle for the stabilizer.

The whole purpose of specifying incidence angles is to establish a relationship between the main wing(s) and the tail surfaces referred to as the "decalage angle". The decalage angle is the difference between the main wing(s) incidence and the tail incidence.

The effect of the decalage angle is to control the pitch stability of the airplane.
That stability can be positive, negative or neutral.

Positive stability tends to bring the airplane back to a straight line trajectory as seen in a vertical plane.
That straight trajectory can be climbing, level or descending depending on the available thrust.

As the tail tends to set itself in the air stream with zero angle of attack or its equivalent if some trim has been set, that establish an angle of attack of the main wing(s) . So if the decalage angle is too small there will be a main wing(s) lift less than the weight so the CG will travel in a descending trajectory such that the angle of attack of the main wings will produce a lift force equal to the weight at a steady descending speed. Some phugoid oscillations will take place and gradually decrease in amplitude.

If the decalage angle is too large resulting in larger lift from the main wing(s) we may have level flight at a specific speed.However it mayalso result in negative stabilitywhich is not desired. Some aerobatic pilots do use negative stability which makes it easier for them to do the aerobatic patterns but they seldom fly their airplanes in other conditions than doing aerobatics.

Aerobatic airplanes prefer neutral stability or slightly positive. It results with the comment from many fliers that say "my airplane goes to where I point it" . The resulting flight trajectory will depend on rhe thrust available which of course also has to do with the power setting of the engine.

Along with the decalage angle being optimized, the CG location also plays an important factor.

So how accurately is the incidence angles necessary and how accurately is an incidence meter necessary and that is "if a flier is really concerned with it".

I think that most fliers are not concerned. Buy an ARF or RTF, go fly it and try to avoid crashing.
For those who are concerned, the degree of measurements precision will vary with the purpose of the individual.

That is part of the attractiveness in this hobby. We can all do our own thing and enjoy the hobby our own way.

Personally I never owned an incidence meter. I have been satisfied with taking measurements as accurately as I could wih a level surface as verified with a good quality level instrument and measuring to 1/64 of an inch to specific points (specific locations).

This brings the matter of "where is the wing chord crossing the leading edge curvature of the airfoil" ?
Some little tricks during measurements are usefull to obain better accuracy. That also depends on the operator taking the measurements.

A bit of calculations reveals the angles.

Any specified or desired wing(s) washout should also be checked.

In the case of a flat stabilizer the chord is obviously half way through its thickness but using the top or bottom surface changes nothing as the angle is the same.

Since the incidence angles usually seen on drawings rarely show any fractional values, it does not appear to be a strict high accuracy requirement.

Have fun playing with incidence, decalage and perhaps thrust angles if that is something you enjoy.

and have a great time in the hobby.

Zor

Edited by Zor 07 Nov. 2010 at 19:26 EST to correct some spelling and add a couple of details.

Edited by Zor a second time to change second paragraph from "It also bought up the degree of precision " to "It also brought up the . . . . ." (correcting typing errors that change the meaning).