Conference vmszoo::rc

    
    	Dan, I really think that a .20 sized engine is to small.
    If your going with a four stroker then I wouldn't go with
    anything less than a .40/4.  Don't just look at this plane 
    alone. Look at what else the engine could be utilized in.
    I myself would use a 46 enya or the 49 OS. 
    
    						Tom

    A friend of mine just purchased a home-made plane from a friend.
    The wing span is 6' and the fuse is about 50".  I don't think it
    weighs more then 3lbs.  He wants to know what size engine to buy
    for it.  I'm not really sure.  What does the world of rc think?
    
    Ken

    	Need more data. Ie,
    
    			Glider type plane?
    			Wing foil type?
    			Construction?
    			Fuse frontal area
    
    			etc,etc,etc
    
    							Tom

    I'll do my best to describe this monster!
    
    It is a high wing (wing is flat bottom with very little dihedral),
    the fuse resembles the Eaglet 63 and is about 50" long.  Its made
    up of balsa plywood *(the plane is solid)*, and the engine compartment
    is only enclosed by the sides(typical setup).  Its been flown before
    but he doesn't know what size engine was in it.  Again, the wing
    is 6'long and measures about 18" across.  Weight is approx. 3 lbs.
    
    Does this help? (I'm not that up on my terms so bear with me)
    
    Ken
    

    What you are describing does'nt sound all that different from
    my Kadet Sr. It flys real well with an OS .40 four stroke.
    Wayne

    	That 3lbs without any engine, right?
    
    	What's the aspect ratio of the wing?

    
    re .0
    
    The following table is from "Radio Control Scale Aircraft" by Gordon
    Whitehead.  It's an English book, so the loadings may vary from
    USA practice.  Note the power loadings are for 2 strokes.  I leave
    it to you to extrapolate to 4 strokes.  Note the book was written
    in 1980, so it is reasonably current.
    
    TYPE OF MODEL	WING LOADING	POWER LOADING		EXAMPLES
    
    	Vintage		12-14 oz/sq ft	260-300 oz/cu in	Antionette
    								Avro
    
    	Homebuilt	14-16		230-260			Jodel
   	WW1 bomber						DH9
    	Biplane trainer						Tiger_Moth
    	1920's light						Moths
    
   	WW1 fighter	15-20		230-260			Camel
    	Lightplane						Cessnas
    	Trainer	(light monoplane)				Chipmunk
    	1920s/1930s fighter					CurtisHawks
    	
    	Aerobatic 	20-25		215-260			Jungmeister
    	Racer							Cosmic_Wind
    	Advanced trainer
    	WW11 fighter						Tomahawk
    
    	Jet - prop driven 25-28		215-250			F86D
    	    - ducted fan  25-28		200-230			F16
    
    Some readers may not agree with the exact numbers, but this is a
    good starting point.
    
    Good luck.
    Tony.

    
    What do you mean by "aspect ratio"?

    
    
    Hmmmmm.....
    
    The wingspan is 6'.
    The chord (if this is from the leading edge of the wing to the trailing
    edge of the wing) is approx. 18".  What's the ratio???  4 to 1 ???

    
    I told you this thing was big!  Again, it is home made, no plans
    or anything.  So what size engine would be suitable for a monster
    like that?
    

    I have read thru this note but cannot find the answer to my question
    : How do you guys size up an engine for a scratch build airplane.
    
    Jacques
    

"You were going to add nose weight anyway so why not make it useful" 8^)

One method is to look at similar style planes in catalogs and see what they 
recommend for engines for something with similar wing area and weight. If they 
give a range, choose the largest 8^)

    I can hear the groans as I try to answer this question. I'm known for 
    usually doubling the size of any engine but for you I'll be good.
    
    As always the question promotes a question. "What performance do you
    want/need?".
    
    (I always need a pure vertical climb. so I use a weight of the 
     model vs the thrust rating of the engine of greater than 1-1).
    
    Back to the question:- 
    
    The easy answer is to copy. There are so many models out there that it
    is relatively easy to find one like the one you intend to build. 
    
    Most of these models will have a range of engine sizes.
    
    When I design a plane I calculate the wing area and decide what wing
    loading I want. I usually chose 24 oz per sq ft for a pattern plane and
    as little as 11 oz per sq ft for a fun-fly plane. The trick is to get a
    as low a wing loadin for as big an engine as possible without making a
     plane that has too much drag.
    
    As usual not a definitive answer but a clue or two, I hope..
    
    E.

    
    OK, OK
    My intent is to design scale model of twin engine aircrafts. The first
    one I would like to attempt is the Fokker D-23, which is a pull/push
    twin. I would like to make it 1/5 scale which would bring the wing
    span to about 91" the cord being about 16". As for weight, I have no
    idea. I will try making the shape totally in foam, reenforce with
    Fiberglass. For engine I have a choice between single engine with gear
    and shaft going to the back for the push prop or twin engine. I have
    no idea of what kind of horsepower/rpm I need. Performance, I'd like
    the plane to at least be able to perform like the real one does.
    
    Jacques
    

Designing twins eh?  I've done a lot of research on the subject. I've got a pair
of 25FPs that I was looking for a home for. I took the RCM plans catalog and
searched out every twin in the book and wrote down the wing loading and span
for each design. It's amazing how much they varied. The biggest wingspan was 
72 inches and the highest loading was in the 32 oz range. 

Somewhere back in RCM one of the guru's published a rule of thumb type formula
relating engine displacement to plane weight I think. I plugged the info I had
gathered into the formula and it seemed to make sence. You have to figure a twin
as a bit larger than the equivalent single. As an example, Two .25's really add
up to about a single .60 vs a .50. That's due to the fact that two .25's swing 
more effective prop area  than a single .50 can.

I ended up deciding to build the P-38 that was published in RCM a few years back.
However, since it was designed for .15's I decided to enlarge the plans a bit
to match the fuselage to the engines better. I followed Eric's suggestion of 
copying by taking a close look at the Wing Manufacturing short kit for the P-38.
They specify an engine size of .25 to .45 for their P-38. The test kit in RCM
was flown with a pair of .25FSR's and had plenty of power. I blew up my plans
to just a bit under the specs for the Wing kit. That should be pretty close I 
think with a comfortable margin for error. 

>    no idea of what kind of horsepower/rpm I need. Performance, I'd like
>    the plane to at least be able to perform like the real one does.


Something else to keep in mind. Somewhere in this file Al Casey made the comment 
that most people have unrealistic expectations for the performance capabilities
of full size planes, especially warbirds. We usually give them far better 
performance than a full size could ever achieve. With that in mind I'd try for
something that would give me realistic performance at 1/2 throttle and extra
performance at full throttle for getting you out of tricky situations. You can
always throttle back.

Model Aviation had a German WWII vintage push-pull plan about three years ago. 
It might give you some ideas. 

    Found two things.
    
    1) In the June 1991 issue of RCModeler on page 4 there is an add for
    a Beach D-18 twin engine which has 114" span and use two Q35 or G38.
    So it look like what I need is two of ither one in mine. I could make
    mine 1/4 scale and get 113.5" span and use the same engines.
    
    2) In the "Standard Handbook for Mechanical Engineer" they do tie in
    power requirements with weight, wing area and drag and offer a serie
    of equations to calculate the power needed for a particular design.
    After I have mine on paper I will try to calculate what power I should 
    need according to these equations and see if they can be applied to
    Scale Planes.
    
    
    Jacques
    

    
    I just tought of another solution to find the power requirements for a
    scale aircraft. Let me know what you guys think of it. Theorically
    when you reduce the size of an vehicul and want to keep the same
    proportional performance you should reduce the power by the same level
    as you reduce the volume of the vehicul. So if I do 1/4 scale, surfaces 
    are reduce by 1/16 and volume by 1/64 therefore I need 1/64 of the
    original power of the airplane. For example, a Beech Bonanza B36TC has
    300hp engine which means that a 1/4 scale of the airplane would need
    4.7hp. Is this reasonable for a 113" span scale aircraft.
    
    Maybe we can add a correction factor because we don't need a comperable
    usefull load.
    
    
    Jacques
    

I was looking thru the last year's RCMs for something else and I noticed that 
Col. Art Johnson had a discussion or two about choosing the right engine size 
in his "Scale views" column. If you don't have them I'd be glad to copy them 
and send them to you.

    Thanks, which RCM contain these article I don't have the ful year
    
    Jacques
    

RE: -.1

I'll take a loo tonight and let you know.

    
    A friend of mine built a push pull WWII airplane with around 90"
    wingspan. He put in A Super Tigre 2500 in Front and a Webra 60 (I
    think) in the rear end. Sufficient performance on the Super Tigre,
    better performance with both. plane weights around 24 pounds.
    
    Bernd

    I'm scratch building the big Ziroli Taube to fly at Rhinebeck this
    year.  I'm trying to decide on the size engine to use.  I want to go
    4-stroke, and have narrowed it down to the OS 70 or OS 90 Surpass. 
    Here are some parameters:
    
    Wingspan: 88"
    
    Wing Area: can't remember off the top, but I'd estimate 1200 -1300 sq in
    
    Airfoil: flat-bottom
    
    Weight: between 8 - 10 lbs, but I build heavy, so lets say 10 lbs
    
    Wing Loading: about 17-20 oz/sq-ft
    
    Flight Performance: it doesn't have to be aerobatic or have vertical
    performance.  Takeoffs should be fairly quick, ie 50 feet, slow flight
    is very important for bomb drops, balloon breaks, and spot landings. 
    It should have enough power to get out of trouble quickly, but it
    doesn't have to fly fast.
    
    Question: will the OS70 Surpass be enough engine?  I'd prefer to go
    this route if possible since I don't want to overpower this plane.  But
    if it will be _very_ marginal, I need to know this now and go for the
    larger engine.
    
    OS 90 weighs only 2 oz more than the OS 70, so there isn't much of a
    weight penalty for more power, but there sure is a price penalty!
    
    Any comments?
    
    Joe, who-is-ready-to-lay-out-hard-cash-for-another-engine (groan!)

    The 91 would be a better route. I found mine to be an excellent
    performer. Charlie Nelson used one in his previous Top-Gun winning
    Waco. The engine has a great idle and very good throttle transition.
    
    I have observed the 70 Surpass on several occasions and though it runs 
    well it does not seem to be as proportionally powerful. 
    
    How much is a new OS91 Surpass these days?.
    
    Regards,
    
    Eric.
    

Unless Dave Walter goes with you 8^)

    Arrrrggh!!! Why did you have to remind me that I could have had a
    .91 for the price of a .70? *Grumble, grumble*
    
    

    Well, the plot thickens.
    
    A member of my club has offered me his NIB Saito FA80 for $200.  Now
    I'm going crazy trying to decide which way to go.  Here's what's on the
    table right now.
    
    FS-70 Surpass	NIB	1.1 Hp		$190
    
    FS-91 Surpass	Used	1.6 Hp		$220
    
    FA80 Saito 		NIB	1.3 Hp		$200
    
    I am not familiar with the Saito line since nobody at my field flys one
    of these engines.  WHat is the scoop on Saito??  Anyone have any
    experience with them?  Has anyone seen an engine review on the Saito
    FA80?
    
    As usual, any help will be appreciated.
    
    -Joe

    The 70 surpass is right in the middle of nothing plane wise. I'm not
    even shure why OS made it. I wouldn't use it for more than replacing
    a 45/46 2 stroke.
    
    Saito is supposed to be good, but as you noted, you'll get no local
    help. I don't even know anyone in the CMRCM club that flies anything
    with a Saito.
    
    I'd go with the 91 surpass in a heartbeat.

    I've owned Saito and O.S. (the .91-4 in particular).
    
    As the the previous reply said about O.S. .91's - "In a heartbeat."
    
    Saito is O.K., but I feel you get better performance, reliability, and
    all around less fuss with the O.S. Surpass series engines.
    
    Another 2 cents...
    

    
      The O.S. 70 fits a very specific nitch. It works out great for planes
    like the Goldberg Cub, the Great Planes Decathalon, the SIG
    Citabria, etc where the 4 stroke type of power better matches the
    planes characteristics than a .46 2 stroke. It also works well in some 
    Telemasters, Kadets, etc where a .91 would really be a bit too much 
    (Did I say that? :).
      I have had a .70, and it runs as well as the .91 Surpass, but for
    overall versatility and power, the .91 is the way to go. As Joe has
    said though, a .70 might be a bit taxed in a 10.5 pound Taube. 
      
      I have no experience with Saito, but the previous comments regarding
    the Saito's are consistent with what I have heard about them.
    
      Joe is also fighting another variable here. The specific Saito he can
    purchase is the "Gold" edition. The engine is black with "gold"
    rocker covers, etc, and would look GREAT in the Taube.
    

    Joe,
    
    	I think your choice is really very simple. If you want to go for
    the LOOKS and take a chance on performance, then go with the Saito.
    Like I said earlier, I havn't really heard anything bad about Saito
    (except) for the twin. So you might be ok and get good looks AND good
    performance.
    
    If, however, you want a good reliable engine that isn't going to let
    you down and possibly destroy your new scale project, then go with the
    91. I've been through enough engine headaches that I'll go with
    performance and reliability anytime.

John (wrksys::) Carl has a Saito .50 that he likes. MAN this month (August) has
a review of one of their twins

Jim

    I'm another for the 91, having two of them for some years. As has been
    said before they are reliable power houses.
    
    Eric.