Inverted V-tail and CG calculation

[TugBoat]

I am in the final stages of completing a MS-6 Kristal. This is a 140" twin boom with inverted V tail. (See photo - note ruddervators missing.)

I normally do a quick CG check against the plans to makes sure that the indicated CG is reasonable, expecially with something that is a bit unusual like this.

The problem is that all the calculations that I have done suggest that the CG as indicated on the plans doesn't provide much static margin. However, I have been using the "V-tail" calculations, and I can't find any references to CG calculations for inverted V-tails.

In a few discussions I have had it has been suggested that the inverted V-tail will produce more lift than a normal V-tail - and that perhaps this is the reason that the plans show a "rearward" CG. Although, everone elses CG calculation also suggests that it should be more forward.

On the other hand, the TLAR system suggests that the CG as indicated on the plans is about right...

So:

- Is there some effect of the inverted V-tail on the CG calculation? If so what?
- The spreadsheet that I normally use requires the 'Stabilizer Efficiency", what sort of value would apply to an inverted V-tail?
- Does anyone have any comments on the CG for this aircraft?

I have attached a Excel "CG calculator" with the airframe data entered, the inverted V-tail is in the "V-tail" section of the calculator.

Thanks,
Tim

Main wing:

Span: 140"

Airfoil: Goe497 (modified at the tip)

First 12.75" of the wing:
Chord: 10.25"
Remainded of wing, at 4 degrees dihedral:
Root Chord: 10.25"
Tip Chord: 7.25"
Tip sweep back: 0.875"

Inverted V-tail (including control surfaces):

Airfoil: Symetrical, something like NACA 0010
Chord at aircraft boom: 10.5:
Chord at tip join: 7.5"
Leading edge sweep back: 6.5"

V-tail angle at the tip join is about 108 degrees.

Leading edge of V-tail is 27" aft of the trailing edge of the main wing.

Suggested CG is 3.375" aft of the leading edge.

[vintage1]

As far as I know the CG is calculated identically but with the horizontal projection of the tail surfaces used instead of the actual area.

[JetPlaneFlyer]

I put the same data in the calculator I use (which ias normally very accurate and includes factors not considered in the Suter speadsheet)

I get a 8.4% static margin with the CG at 3.375".. Which sounds about exactly where it should be.

Steve

PS.. inverted V or normal makes no difference to CG.

[CloudyIFR]

Tugboat,

I'm glad you find the spreadsheet useful. There have been siginificant updates with the spreadsheet and the latest can be found at www.TailwindGliders.com I've attached the newest version with the data from your model already inserted. She sure is beautiful, I'd really like a flight report afterwards!

JetPlaneFlyer,

You're very well respected in my opinion and I'd be interested in what's missing in my spreadsheet?

Thanks to all.
Curtis
Montana

[JetPlaneFlyer]

Hi Curtis,
The speadsheet CG calculator been using is by a guy called John barker, I can take no credit for it whatsoever. It’s nowhere near as user friendly as yours or as advanced in it’s user interface, nor is it as wide ranging in it's application.. However it does consider lift slopes of the wing and tail and wing downwash effects, which not all other calculators take into account. It also has a slightly different basis for it’s calculations which you would have to discuss with John himself.

In my experience it produces very accurate results which are generally a little less conservative than other calculators. It’s results tie in very well with empirical data.

John passed me a new version of the calculator recently that also estimates required tail incidence angle, though this functionality is still under test. I've attached a copy of the spreadsheet, hopefully John won’t mind; this is the old version without the tail incidence function.

If you wanted to discuss the functionality with John I'm sure he would be delighted to hear from you, he's a very knowledgeable guy. PM me if you do want to get in touch.

Steve

[TugBoat]

Many thanks to everyone for the useful information. It is always nice to learn something - even if I can't always remember it later!

I was fairly certain that a V and inverted-V would be the same - but one thing I have learnt in this hobby is that not everything is as obvious as it looks.

I will post a link to the flight report once I get to that stage.

Curtis,

Thank you for your response. The spreadsheet (now spreadsheets I notice) is easy to use and has always served me well. My apologies for not acknowledging where my original spreadsheet came from. I must admit that I have no idea where I got my copy from... I think someone just said "you need this"...


Tim

[CloudyIFR]

Good Morning,

This forum sure is a great place to hang out! What great folks who share their knowledge and experience!

Tim I can't wait to see photo's/video and a flight report, I bet you'll have a grin from ear to ear one you get that beautiful model flying!

All the best to everyone.
Curtis

[HerkS]

Tim, There are a couple of differences between a "conventional" V-tail and the inverted arrangement that you are using. These are small influences and for most models the effect is mostly theoretical.

With the conventional V-tail arrangement, rudder control has an anti-roll effect. That is the control deflection to give (for example) right rudder, has an aileron effect giving left roll moments. So, besides providing a right rudder yawing moment, the control deflection acts like a small set of ailerons rolling the model to the left. Your inverted V-tail works the opposite of this providing a rolling moment in the same direction as the rudder movement. This is a small effect, but it can be noticeable on some models.

Also, a conventional V-tail adds a small amount of positive dihedral effect to the overall model. The inverted V-tail adds the effect of a small amount of anhedral or negative dihedral. This also is a small effect so is not noticeable unless the tail is unusually large.

[ghoti]

I believe that CG position is a function of the wing not the tail. Am I missing something? bill

[JetPlaneFlyer]

I believe that CG position is a function of the wing not the tail. Am I missing something? bill

Bill,
Yes you are missing something.. CG needs to be calculated considering the the tail and tail moment arm (sometime refered to a tail volume).. Other factors come into play too but the tail is the largest influencing factor.

Steve

[CloudyIFR]

ghoti,

Lets think about this for a moment. Please realize I'm not an aerodynamiscist.

Why do we have a tail? When you have an airfoiled wing the airflow goes faster over the top then the bottom, this creates a pitching moment. What that means is the leading edge of the airfoil is trying to pitch nose down if you will when air is passed over it. This pitching moment happens to occur at the 25% location, oh perhaps +/- 2%. This location is the same for all airfoils but the amount of pitching or the force is different depending on the airfoil.

Something needs to counteract this pitching moment and put our plane by into equillibrium. That's the tail. The tail's airfoil is actually upside down compared to the main wing and produces a down force. So to balance our plane this force needs to equal the amount of negative pitching moment the wing has. So it's back to the ole playground see-saw. If you have a short tail tail you'll need a lot, if you have a long tail you'll need less. Sames goes with the size of the tail, a longer tail can have a smaller horizontal area than a shorter tail, as the force is greater.

Hope this rambling helps, but I've only had a half a cup of coffee this morning! :)

All the best.
Curtis
Montana
www.TailwindGliders.com

[TugBoat]

The CG does appear to be correct. My initial flights were with it 5mm forward of the plan position - and I am certain that it will come back a little with no problems:

Report: http://www.rcgroups.com/forums/showpost.php?p=12352569&postcount=153
Photos: http://www.rcgroups.com/forums/showpost.php?p=12352692&postcount=154

Thanks,
Tim

[BMatthews]

Cloudy, you're on the right track with your explanation but missed one factor.

The tail does not need to lift downwards in all cases. with more rearward CG locations that are still considered as "normal", whatever that is, they do lift upwards. But in all cases the wing must lift up more than the tail. As long as that situation occurs you're going to see the proper positive pitching stability in the model.

The extreme examples of this occur with a lot of contest free flight models where the tail moment arm length is super long and the tail area is a big portion of the wing. Even with the 50 to 100% CG location (yes, that's right. A CG located at the trailing edge of the wing. It was rare but there's more than one that was designed to work with this sort of extreme.) the stabilizer still provided a positive pitch stability or the model would have crashed. The key is that the tail has to always lift less than the wing so there's a positive pitch stability factor maintained.

Strange but true.... :D

[vintage1]

Its even worse. If some other way is used to ensure pitch stability - like a very high wing and a very low CG, you can even run on the same angle of incidence of the main wing and tail.


I don't recommend it though. Those pylon winged FF models were very tricky to trim, and did occasionally get into irrecoverable attitudes.

[CloudyIFR]

I've never been a free flight modeler, but I do read a lot of what they do and I think those folks are the experts in balance. Yep, I've read about those way far aft CG's. Thanks for the insight, it was interesting.

Curtis

[JetPlaneFlyer]

As a general rule of thumb, for planes using typical RC sailplane type airfoils..

If the model ballances around 25% mean chord or further forward then the tail will generate downforce
If the CG is about 33% mean chord then the tail will carry no significant load
CG aft of about 40% mean chord the tail will be lifting


Here's an illustration from a Martin Simons article that illustrates this....