monocoupe

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Asking as an experienced "single engine" modeler, I'm curious to know if there are any rules of thumb, or formulas to determine the correct engine sizes for a multi engine model. For example:

For a single engine model of a known type, overall size, weight, and wing loading etc, a modeler can determine from experience what size engine will be required to achieve the desired performance. Let's say that in this case, our subject model is powered by a 60cc engine, and performance is satisfactory.

If we were to build another model of the same general specifications as the first, only this time the model is a tri-motor, would it simply be a matter of using the same amount of total combined displacement, of say three 20cc engines? Or even one 30cc and two 15cc?

I guess the question is, Are three 20cc engines going to put out more or less thrust than a single 60cc engine?


Cheers, Nigel

JohnBuckner

My Feedback: (1)

When converting a single engine to a multi and using similar total displacement as a multi to the single engine version there will be a performance gain realized. This is because when divideing the same relative power into, in your case three propellers has the effect of reducing the propeller disc loading which is similar to the effect of using a larger diameter lower pitch prop on a single. This results in better low speed acceleration (take off, climb) but not speed.

In my opinion it is not neccessary or possibly not even a good idea (for other reasons) to match the total displacement to the comparable single engine aircraft.


John

twinman

My Feedback: (2)

I have bashed several planes into twins.
Here are my two cents.
Example one. The Ultra Sport 1000. It had a St 3000 rated at approx 3.5 HP and an 18" prop. Did not have unlimited vertical but flew well.
It now has two ST 90's with 14x6 props......however....each engine is rated at 2.5 HP and as stated above, much larger disc area. The plane will fly straight up and much faster. Yes, I did reinforce the wings and control surfaces.
The wing loading will almost always go up on multi engine planes due to the additional controls, nacelles, tanks, ect.
Initially the plane flew "Heavy", but with so much power, was only a problem at low speeds.
I added a total of 6" to the wing span and wing plates to increase lift.
Example 2. I also bashed an ugly stick 60 from one ST 90 to two ST two forties. This did not fly well as the wing loading sky rocketed. Good power, but engine out meant it was coming down hard.
Changed to two ST 60's, added 8" of wing span, and wing plates. It then became a real blast to fly.

From these experiences maybe we can get a few ideas.
1. A single to twin conversion will increase the wing loading and that must be considered.
2. Don't underestimate the need to address the increased structural and control surface stress.
3. As a general rule, using two engines, one half (Or in your case one third) the original, will also increase the available horsepower and disc area. All of this will increase the performance of the plane far beyond the original. This is particularly true if the new plane's wing loading is similar.
4. Don't forget that longer wings also add drag that must be over come, to at least some degree, with the new design.
I will now get off my squeaky soap box.
Good Luck

JohnBuckner

My Feedback: (1)

Sounds like a well educated soap box to me. Nice examples indeed twinman.

John

Vince

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The man knows his stuff.

Vince

twinman

My Feedback: (2)

If you go to page three of this forum and the post marked "Looking for my first twin " I kind of detailed bashing twins a little more.
I just want to keep others from making the mistakes I made and enjoy multi-engine planes.
Good Luck,
Twinman

monocoupe

My Feedback: (40)

Thank you guys!
This is exactly the sort of input I was looking for. I wasn't actually planning on bashing a single into a tri-motor, but for the sake of the discussion, it's an ideal illustration. Besides, come to think of it, bashing a sport type single into a simple non scale tri-motor is probably a good idea. I can use it to familiarize myself with multi-engine operations prior to flying a "bigger investment" scale model.
I'm a fan of 'Golden Age" aircraft, so any scale multi that I build will be from that era. Fortunately, those aircraft usually had generous amounts of wing area and indeed some are even biplanes. Their fabric covered structures and fixed gear also allow for some weight savings as compared to a warbird for example.

I can't help but think that a tri-motor would have an advantage over a twin in an engine out situation. It seems to me that with the loss of an outboard engine, the tri-motor would have less assymetric thrust for the given remaining available power than a twin.
My only experience with multi's so far was rather comical. I was standing on the flight line supervising a student one day, and the fellow next to us who at that time was still a relative newbie himself, was flying a twin for the first time. He had an engine out and the plane went out of control. He yelled that he needed help and jammed the transmitter into my hands! I instinctively closed the throttle and managed to get it straightened out and back to the runway. If it wasn't for the altitude he had at the time, things might not have turned out so well!

Cheers, Nigel

twinman

My Feedback: (2)

monocoupe

Anyone who can react like that on an unfamiliar twin is a natural in my opinion....Welcome to the world of multi's

monocoupe

My Feedback: (40)

twinman,

Thanks for the kind words! I'll consider myself really a part of the "multi world" when I've successfully built and flown my own!
I likely owe my quick reaction to the fact that it was training night at the field, and I had been recently and repeatedly reminding students to pull the power off if they were out of control and headed for the ground!

Cheers, Nigel

Uncle Heinkel

I'm with you, Nigel. I like multi's myself but they have to have 3 or more engines for me.

I would dearly love to build a twin and am probably missing out by not building them, but I've seen the situation you described quite a few times. As far as I'm concerned about engines, the more you have, the safer it is!

I've had a bit of experience with 3 engined aircraft so I thought this info might help.........

I built and flew a Junkers Ju-52 about 6 years ago, built from enlarged Graupner plans. It was 1/14th scale, 90 in wingspan, weighed about 12 lbs and was well-powered by 3 O.S. 26 four strokes.

I'm now almost done with a bigger one, also from the same plans, about 1/9th scale, 120" span, on track for 30 lbs. There is a Ju-52 kit on the market that is about the same size and weighs 35 lb that uses 3 .70 four strokes.

Since 90's cost just a little more than 70's, and are the same size(Saito), I'm going to go with the 90's and have a bit of reserve power. I have been told that reserve power will help if you do have an engine out.

Right now I'm flying a Stafford B-24 with four OS 26's...weighs 14 lbs. and it flies great. I lost an inboard (#2) engine and didn't even know it until I did a low pass and saw it! Didn't even affect left turns...felt normal.

Good luck with your project! Ford Tri Motor? Ju-52? Savoia SM 82?

H.

monocoupe

My Feedback: (40)

Uncle,
Thanks for sharing your experiences, I'm finding this to be very interesting. The example models you have described really do illustrate well what John B. and Twinman have both said earlier.
Your 120" JU-52 at 30lbs has a total engine displacement of just 34cc. I wouldn't expect a single 34cc engine in the nose of your JU-52 would get the job done! Of course we're talking in general terms and not addressing wing areas and power loading etc.
Speaking of which, can you tell me what the wing area is on the JU-52?
To answer your question, the model is a Stinson Model U at 130" span.
Just for fun, here are a few more golden age multi's I like (they're not all beauty queens but still appeal to me for various reasons):

DeHavilland Dragon Rapide, Dragonfly, Comet
Kreutzer Air Coach
Stinson Model A
Miles Peregrine
Airspeed Ferry
Handley-Page HP-42
Boeing Model 80

Cheers, Nigel

JohnBuckner

My Feedback: (1)

Monocoupe you are a man after my own heart. The HP-42 would make a truly magnificient project as well as the Kreutzer would be so unique. The 'Hannibal' class HP's has always been a bit of a dream project for me. I also have a soft spot for the one and only Kreutzer after having looked it over closely at Oxnard CA.
during the period it was stored there after its recovery from its long sleep buried in a mine somewhere in south america. That was maybe ten years ago.

Two other dream projects I would like to add to the list are the six engine Tarrant Tabor (lost on its first flight) and the four engined Barling bomber. Both triplanes and both designed by the same man in two differant countrys.

I also have a Stinson Tri motor, mine is eighty inches that I scratched out of my own plans. It is the SM6000B Highwing version. Its finished up through cover now. Just last weekend I finished some strong pylon commitments so today just started back to work on the multi's. first up will be completeing and flying the four engine bash of a Kaydet. First flight should be in a few weeks then back to the Stinson.

John

monocoupe

My Feedback: (40)

John,
I'm certain we could come up with one huge list of airplanes worth modeling without trying too hard!
I see that I'm not alone in being fond of aircraft that well...wouldn't win any beauty contests! I am familiar with both the Tarrant Tabor and the Barling Bomber, which I think makes us the exception rather than the rule. Old Walter liked to think big!

Greg Herrick has since done a fantastic job restoring the Kreutzer didn't he?

I actually have plans for the "Hannibal". There are many sheets of drawings, and as I recall it's around 80" span, but I haven't looked at them for quite some time. It's on the "list"...you know the "list"...you have one too.

How far along are you with the Stinson, and what are you using for power?

There's a Danish twin that I'm quite fond of, but the name escapes me for the moment. I'm going to look for a picture of it, and post it here when I find it.

O.K. John, here's a rare British twin...I'm curious to see if you know this one.....a Reid and Sigrist "Snargasher" If you don't know it, I'll post a picture.

Cheers, Nigel

monocoupe

My Feedback: (40)

Here's a picture of that Danish Twin. It's a S.A.I. KZ IV. It first flew in 1944, and has recently been restored. I think it's real pretty.
Sure like to locate some 3-views of this one!

Cheers,Nigel

JohnBuckner

My Feedback: (1)

Don,t know much about the Snargasher other than the fact it was the model RS-1 Hehe!

Wow that Danish airplane is beautiful and a new one on me. The fuse looks a lot like some DH products. Yes indeed there goes that legendary list agine.

My Stinson is setup for .25's but have not decided which yet. The airplane is complete through cover waiting for finish, all the servo installation and linkages are complete. In one of my weaker moments I used a single servo and bellcranks for all throttles instead of a three way 'Y'.

I must confess to not following my own power advise with the Kaydet and using four Magnum .25's. I really wanted .18's but I trust the reliability of the Magnums and the K&B or MDS .18's are an unknown. The ship is 11.5 lbs. and with 1300 square inch (span was increased) has a load of around 19 oz per square foot so its overpowered floater time. But to help with the situation I installed midspan spoilers at the spars.

John

Pelle Gris

monocoupe... I have a good 3-view of the KZ IV. It is on A3 format so I have to find somewhere to get it scanned. If I can find someone to do it I could e-mail it to you if you want it.

Jimmy Bananas

My Feedback: (13)

I was told that rule of thumb on multi engines... ( example)if your plane weights 20 Ibs,you need 10 Ibs of static thrust...This was what this person told me..he's built 3-B-17's,so I figure he knows what he's talking about...good rule of thumb

monocoupe

My Feedback: (40)

Thanks for the input Jimmy.

By the way, I never did post a picture of the Reid and Sigrist "Snargasher" since I told John B about it. Well, just in case anybody thinks I made it up, here it is:

Cheers, Nigel

scalebldr

My Feedback: (7)

30 years ago when i did my first twin r/c design after numerous twin controline models in the late sixties was a twin had 3 times the power of a single engine not double because of the propeller area of the 2 engines so back then if you had a plane that used a 40 size engine instead of 2 .20's you used 2 .15's .example would be the C.G skylark of the period for both single engine and twin engine specs.

JohnBuckner

My Feedback: (1)

Scalebldr:

Do love those Ukie multi's. In the sixties being heavy into combat and we were a dedicated group of kids flying combat every weekend at the school and needless to say they were not exactly rulebook events. I built a Combat Cat with two Cameron .15's and you should have heard the guys grumble. Performance was no better but the thinking was to get a lot more prop disc area and increase the chance of cuts. Still got the petrified remains and engines of that old airframe.

Another one that I loved was a Douglas A-26, can,t remember whos kit it was but may have been Berkeley. Had two Fugi .29's without throttles (could not afford a three line system). A great airplane but never could get the outboard to quit first even going to the extreme of swapping sides with the engines or using a slightly smaller tank on the outboard. Of course the resultant single engine 'Backward Boogie' shuffle was fun for every body to watch each time.

John

scalebldr

My Feedback: (7)

I had/have a cleveland models p-61 two line who could afford r/c type engines and the 3 wire system when you only earned 2.00 for cutting a neigbors yard.had k&b.23 inboard and k&b .19 ,both tprpedoe's ,outboard and did use like a 1 oz larger tank inboard.i guess i was lucky,never had the outboard quit first.wish i had a cammera back then and took pictures.oh well.would fly a few minutes on the 23 with reduced line pull ,after about 5 minutes with both running i remember arm getting tired from the pull seemed to pull more than my winder ,flite streak and ringmaster.