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From the Lab December 1997

Steve provides a thorough examination of v-tail mixing, and presents his tools and source of the month..

Flying in the Winter

Yes, readers, here in New England (northeast USA) winter is fast approaching. We have already had our first real snowstorm, and most nights the temperature drops below freezing. It remains as difficult to go flying as it has most of this year - most days, it is just too windy for me. Fortunately, it is usually calm very early in the morning, so that is the only time I get to do much flying.

But calm or not, it is cold. There has been some recent discussion on the eflight listserv about flying in the winter. I have written about this before (see the December 96 'From the Lab' ), but I will say it again: the best thing you can get to make it comfortable to fly in the winter is the Radio Glove, from B.C Inc of Payson UT.

tray4.jpg (9273 bytes)

The Radio Glove completely encloses your transmitter, with shaped openings on either side for your hands, and a clear plastic faceplate (actually a window) on the front so you can see the meter, display, and trims. The Glove closes with velcro, and includes an elastic strap to hold the transmitter in place inside. The overall design is pretty sophisticated, with stiffeners where required. The material used on the outside has enough 'body' (what the fabric folks call 'hand') that the hand openings retain their shape even after you remove your hand from the glove. Cost is US$39.95 including shipping in the US.

I use my Radio Gloves (that's right, gloves, I have one of each of the two sizes, for different size transmitters) in a Falcon Expert tray from JV Trading in Norway. See the JV Trading (Norway) Falcon Trays web page [http://www.multinet.no/~janvinje/index.html] for information on a whole line of models, and the May 97 'From the Lab' for more information on trays in general. JV does not currently have a US distributor, but you can order directly from Norway by paying with a credit card, and the prices are quoted in US$. The Falcon Expert and Plain trays will fit any of the Japanese brand transmitters sold in the US and UK.

If you use a transmitter tray, you really need longer control sticks. JR does sell a set of 'extended length gimbals', but they are not really long enough (I speak from experience, I have a set in my radio parts drawer). I use stick extensions from Guidari R/C International, which are available to fit most types of transmitters. These stick tips are actually plastic, so they do not get cold!

Addresses:

JV Trading,  O.Solbakken 43, 7550 Hommelvik, NORWAY  Check web page for prices and shipping.

Radio Glove, 1077 So. 930 W, Payson Utah 84651 USA  801-465-4228     US$34.95 plus US$5 shipping. R. Brent Billings

Guidari R/C International 170 University Ave W, Suite 12-103, Waterloo, Ontario N2L 3E9 CANADA   US$17.95  plus US$5 shipping

 


OK, What Happened to the Fashion Review?

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Last month's column included a photo of my almost complete Simprop Fashion, which was supposed to be reviewed in this month's E-Zone. OK, where is the review? Did I destroy the model already?

The Fashion has been flown successfully a number of times, and is currently sitting, completely intact and flyable, in my workshop. The review is not finished because I had problems with a few things about setting up the plane and the radio, and I wanted to provide an explanation and a resolution to the problems as part of the review. If you have read any of my reviews, you know I try to be thorough, and I share everything I have learned about the model, rather than just saying 'It is a plane, you can build it and fly it', like certain print magazines do.

The problems are really not specific to the Fashion, but rather are general enough to discuss in a column, so I will start in on them here.

The Fashion has a V Tail

V tails are cool. The look cool, they may be lighter, they may produce less drag, and hey, they look cool. They look really, really cool.

But V tails, especially V tails on a plane without ailerons, are anything but trivial to set up. A V tail has two control surfaces, called ruddervators, that must work in unison to provide both yaw (rudder) and pitch (elevator) control. I am not even going to try to explain the dynamics of the control surfaces here. My discussion will be limited to how to use them, rather than how they work.

The instructions for the Fashion trivialize the setup by listing required direction of surface motions for each control input, and giving suggested surface throws, as follows:

Control Action Left Surface Right Surface Control Throw, HLG Control Throw, Electric
Left Rudder

down

up

18 mm 13 mm
Right Rudder

up

down

18 mm 13 mm
Up Elevator

up

up

15 mm 10 mm
Down Elevator

down

down

15 mm 10 mm

That's all there is - there is no additional discussion of setting up a transmitter or how to fly the plane. I don't know if other kits do a better job of describing how to setup V tails, because this is the first one I have ever built - although I will find out soon enough, as another plane with a V tail will arrive this week.

The problem here, as it turned out, is that a V tail is just not that simple. Take a look at what the control movements actually look like, and consider the implications of each - and keep in mind that these are simple inputs - mixed rudder and elevator are even more complex:

neutral.jpg (12678 bytes)
neutral v-tail deflection

right.jpg (12864 bytes)
right rudder v-tail deflection

left.jpg (12384 bytes)
left rudder v-tail deflection

up.jpg (12204 bytes)
up elevator v-tail deflection

down.jpg (11225 bytes)
down elevator v-tail deflection

 

There have been numerous mechanical mixing solutions for accomplishing the control mix published, and a number of products manufactured, but I am not even going to touch on them. There is simply no room in the Fashion for a mechanical mixer - or even an airborne electronic mixer. To fly this plane, you need a programmable transmitter, and that is really all there is to it.

One problem with a V tail is that for each rudder movement, one surface is down - and one is up. The up surface causes a 'phantom' up elevator effect. This might be good if you did not already know how to fly a glider - but I do, and some control inputs have just become instinctive. One is that a turn requires rudder control and some up elevator to keep the nose about level. I am used to flying this way, and do not even have to think about adding elevator control when turning. The combination of the phantom up elevator and the up elevator added by the pilot results in too much up elevator - possibly to the point of a stall. When an attempt to turn the plane becomes a stall, it is very hard to control the direction of flight, which is what it felt like on my first flight. The solution - as discussed below - is to add differential to the ruddervator control, so that the movement is about 60% down and 40% up. I really wish that the setup information for the Fashion mentioned this, but I found out about it in a long conversation with Sal DeFrancesco at Northeast Sailplane Products (NSP) [http://www.nesail.com]. Sal has flown a many V tailed planes, and when I described the problem, he immediately knew what was happening.

Another problem is that everything interacts with everything - and the computer radio makes this worse. I do not have experience with a wide range of computer radios, so this discussion will be limited to my recently purchased JR XP783.

Yet another problem is that computer radios are apparently all set up with the assumption that a V tail plane will have ailerons, so the V tail controls are rudder (left stick) and elevator (right stick).

Setting up a Computer Radio for a V Tail

Setting up the Fashion on the XP783 was one of my first real efforts at setting up a computer transmitter. My previous efforts consisted mostly of setting up a subset of the required controls for the Multiplex Graffiti on the XF622, which could not handle the required camber control. I was sure the XP783 could handle everything I needed, but I was not sure how. The JR manual is mandatory for figuring out how to get at some of the functions, but is otherwise pretty much useless, because it only explains what functions are available, which is very different from explaining how to use them!

I set the model type to Glider, which ought to make sense, but I am no longer sure. I set the wing type to V-Tail, which also ought to make sense - except that it may have been simpler to use Airplane (Acro) mode and select the Delta/Elevon wing type. I have not tried this approach yet, but I will do so soon. Unfortunately, this is not simply a matter of reprogramming the transmitter - I will have to take the entire plane apart and change the servo wiring. In Glider -V Tail mode, the right ruddervator is controlled by the 'rudder' servo (rudder = right!) and the left ruddervator is controlled by 'elevator' servo. In Acro-Delta/Elevon mode, the right ruddervator is controlled by the 'elevator' servo and the left ruddervator is controlled by 'aileron' servo. Who designs these things, anyway?

The V tail control is driven by the rudder and elevator inputs (sticks), but I wanted the input to be aileron and elevator, so I used a 'free' mixer to mix 100% of ailerons into the rudder. This type of mix is an 'input' mix, and it affects the transmitter inputs, rather than the servo outputs. The down side to this mix is that the rudder stick is active - if you bump it, the plane responds.

This much was simple. The next step was to set the overall control throw, and then modify it with differential. The mechanical setup of the Fashion provides far too much throw - a clevis at the servo prevents using the inner hole of the servo arm, and the length of the control horns is limited by height of the fuselage and the need to prevent them from hitting each other. So the overall throw had to be greatly reduced at the transmitter. This is is accomplished by changing the dual rate settings - I used 50% for 'full throw' and 20% for 'reduced throw'. Really! Dual rate is also an 'input' control, so it affects the gain of the control sticks before any mixing. Reducing the elevator throw by changing the dual rate setting affects the throw of both the right and left ruddervator servos, and the same is true of the rudder throw. The downside of this is that with very little servo output throw, even a tiny bit of linkage slop or pushrod flex can be very noticeable. The V tail makes this even worse, because even a little bit of slop affect both controls, so the effect can be pretty confusing in the air.

The 'aileron differential' function cannot be used here, because it assumes that (of course!) you want more up than down - and there is no way to change this (Acro - Delta/Elevon mode is no different, either). So differential must be added by using 'travel adjust', which is an 'output' function. Output functions affect only one servo, so to set equal differential for the two servos, both channels must have the same settings. Except that servos are generally not identical, and may not move the same amount for a given input signal change. This must be the case with my servos, because I had to use different settings for the 'elevator' (left ruddervator) and 'rudder' (right ruddervator) servos to get the same control surface movement. JR allows different settings for the 'travel adjust' for each direction, so this is how differential is accomplished. I used settings of 50% for one direction and 100% for the other (and 55% and 110% on the other channel!) to get about 8mm of throw and 10mm of throw for rudder input. This seemed to be what I was looking for, but it always looks like there is not enough up elevator. Elevator travel can be adjusted by going back to the dual rate setting - but no matter what you do, you always get differential at the outputs, so I have more down than up. I have yet to find a way to prevent this with the XP783.

Now if the V tail gets bumped out of alignment, or the pushrods change length with temperature, or the radio mounting plate moves, or there is any slop or flex in the linkage, both rudder and elevator are affected. This makes it difficult to keep everything setup properly. Sal at NSP said that a plane with a conventional tail can be setup in an afternoon of flying - but a V tail can take a month. At this point I have to agree! Each flying session has felt like I had a different plane, because something was changed.

OK, now why do the servos move slightly (giving down elevator!) when I change the setting of the rudder dual rate switch - which is set to 100% for both positions? There is no such movement when the aileron or elevator switches are changed, and they are set for different values at each setting!

OK, What Now?

Yeah, what now? Well, the review will be published in the January E-Zone. I promise! I want to get in enough flying that I can say that this is what I did, and it worked - more than once. So far I have had one truly great flying session with the Fashion, when I was doing tight thermal turns, and even flying it inverted! I have also had several sessions with mixed results. I know from that one session that the plane can be made to fly wonderfully, but I am not sure how to keep it that way. Consistently good flying is the key to making a plane a regular flyer, so I will try to figure out what I need to do to make this a consistent flyer. One simple change may be to modify the clevises so that they can be used on the inner holes of the servo arms - allowing more servo output throw, and reducing the effects of linkage slop and flex (there is not much - but I think it has an effect).

V Tail Resources

DJ Aerotech web site[ http://www.bright.net/~djwerks/askdj/ ] seems to be the mother lode of information on V tails for models, especially planes without ailerons. DJ Aerotech manufactures the Monarch and Chrysalis HLG and Monarch E Speed 400 planes, all of which have V tails, so they are certainly very familiar with them. The web site contains several documents under the 'Ask J & D' section about setting up V tail differential. This is the only written discussion I have found on this topic!

'Plane Geometry' Aircraft Measurement and Design Programs by Blain Beron-Rawdon is a set of Excel spreadsheets for designing airplanes, or analyzing the design of existing airplanes. Separate spreadsheets for conventional and V tail models are included. Approximately US$22. Contact evd(at)netcom.com

Guide to Computer Radio Control Systems by Don Edberg is the only book available that explains anything useful about using programmable transmitters. Information is provided for most units available in the US. This is a book about how to use the functionality of the transmitters, rather than the long list of instructions on how to access individual functions provided by the manufacturer's manuals. There are lots of diagrams and tables. This book does mention V tails on planes without ailerons. A quick look through a JR manual, and a quick look through this book, and it is pretty obvious why Futaba is letting Edberg write their manuals. Approximately US$23. Contact dynamic3(at)flash.net

Model Aircraft Aerodynamics by Martin Simons has about one paragraph on V tails, noting that control interactions my limit total control available. Simons notes that control limitations may even prevent exiting a spin.

The Illustrated Guide to Aerodynamics by H.C. Smith contains about two paragraphs on V tails, noting that there is no improvement in skin friction drag or weight over a conventional tail (for the same stability and control). Smith claims that V tails have very good spin recovery characteristics. Go figure.

Model Aviation July 96 'V-Tails for Models' by W.F. McCombs mostly discusses how to determine the proper size for a V tail, but does include some additional discussion. Unlike the author's most recent contribution to MA, this one does not require the reader to send money to the him just to get a copy of something that explains what all the variables in the equations actually are.

A Note about Who is Right and Who is Wrong

I found some contradictory information in the above sources, and I am not in a position to comment on who is right and who is wrong .. yet. Give some thought to what you are reading, and whether the writer just throws ideas out, or backs them up with an explanation.

 


Source of the Month

Micro Mark publishes a catalog of small tools specifically for modelers. It is not specifically for airplane modelers - there are a lot of tools for model railways, carving, dollhouses, and the like. But there are a lot of model airplane specific tools, and a lot of generally useful tools. I have not bought anything from them...yet. They have a web site at http://www.micromark.com or you can call them at 800-225-1066. They publish a catalog several times a year, and advertise regularly in Model Aviation. Worth a look!

If you would like to recommend a 'Source of the Month', please feel free to contact me at skranish(at)ezonemag.com

 


Tool of the Month

tweez.jpg (14462 bytes)

This month's tool is tweezers. No particular brand, no particular source, no particular type. I must have about 10 tweezers in my tool chest. They are available in a huge variety of sizes and shapes. My absolutely favorite pair (that seem to be misplaced at the moment) are Swiss made stainless steel tweezers that come to a fairly sharp point, and are serrated (a series of tiny notches) inside the point. These tweezers are great for holding almost everything - pulling up covering, pulling it down tight, removing or inserting screws, holding anything with glue on it. I use them all the time.

Tweezers may or may not be available at your hobby shop. Any good hardware store should have some - the General brand shows up in local stores here. Don't be afraid to spend US$5 to US$10 for a really good tweezer - the one really good pair will get far more use than all the cheap ones that sit in your toolchest. Micro Mark (see Source of the Month, above) sells a variety of tweezers.

If you would like to suggest a 'Tool of the Month', please feel free to contact me at skranish(at)ezonemag.com

 


COPYRIGHT

This document is copyrighted (c) 1997 by Steven Kranish, and may not be copied or used in other forms of publication (electronic or paper) without written permission from the author. I will probably grant permission, but I would like to know about it, so go ahead and ask.

 


CONTACTS

If you have any questions, please feel free to contact me at skranish(at)ezonemag.com

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