Uses for Common Engines

Sometimes people ask me which plane to build for a specific engine that they’ve had sitting unused.  This is a good question, so I’ll try to come up with several recommendations for some of the more common engines that the average modeler may have lying around.

OS 40 FP


For decades OS was the premiere model engine manufacturer.  Almost everybody had several OS engines, and OS made an engine for every application.  In fact, they made many engines for every application.  In the 1980s they created the FP series for ease of handling, fuel economy, purchase economy, easy maintenance, and of course dependability.  The FP series was aimed at beginners.  The 40 FP soon became the number one beginner’s engine, and just about everybody had one.  But OS had a trick up their sleeve.  The 40 FP was de-tuned a bit to make it easier to start as well as a bit less powerful.  The official position of OS (and the conventional wisdom among the RC public) was that when you put your trainer away and graduate to a real man’s toy airplane you should buy an OS 40 SF, which is the sport/pattern engine that OS manufactured at the time.  It had higher power, greater weight, and faster fuel burn.  But the 40 FP is nevertheless quite a desirable choice for powering a sport model, because with a much lighter weight than the 40 SF it has a very respectable power to weight ratio.  So look for something in the lighter end of the 40 range.

Old timers:  A 40 FP is the perfect engine for a Buzzard Bombshell.  It will idle all day, it’s very lightweight, and the plane will climb like crazy when you want it to.   I can get 20 minute flights on mine with a 3 ounce fuel tank.  The Buzzard Bombshell has around 850 square inches of wing area and a target weight around 64 to 75 ounces.  Any old timer with specs in this range would be a fine choice for the 40 FP.

Trainers:  Any plane marketed as a “40 size trainer” is perfect for the 40 FP.  My son was having a ton of fun flying his PT 40 with an old 40 FP, and it was able to do outside loops and sustained inverted flight.  You can put a hot-rod 46 or more on a 40 size trainer, but the magic of the 40 FP is its light weight.  When you add engine weight you increase wing loading, which increases turn radius, sink rate, etc.  Try it light, and you’ll like it.  And if you’re an experienced pilot trying to figure out what to do with an old 40 FP, don’t be afraid to try flying a trainer again.  You’ll be amazed at how many bad habits you have.  Teach yourself to land correctly, do coordinated turns, or try those weird maneuvers you always wanted to learn.

Sport planes:  If you’re a fan of old magazine plans, try a plane originally designed for a 30 size engine, such as the RCM Basic Bipe.  Any plane that seems too big for a 25 and too small for a 40 will be about right for a 40 FP.  Also, any plane that’s specifically designed to be very light, such as the Terrier or the Cloud Dancer, both of which I’ve flown very successfully with a 40 FP, will be suitable.  If you go to the trouble of building a lightweight Cloud Dancer, don’t weigh it down with a boat anchor engine.  Also, keep your eye open for sport planes specifically designed for the 40 FP, because others have had the same thought in the past.  Light and Up is a great example.


OS 40 Surpass


The 40 Surpass is an enigmatic engine that’s essentially just like a 40 FP.  It has the same weight, power output and fuel efficiency, and it’s easy and satisfying to run, just like the 40 FP.  All of the above recommendations for the 40 FP can be applied to the 40 Surpass.  Just make sure to select the correct propeller, because both of these engines may have trouble overcoming handicaps imposed by improper propeller selection.


OS 70 Surpass


The OS 70 Surpass was a popular choice for 40 size war birds in the 90s and 00s.  It is a similar size and weight to an average 60 2 stroke with muffler, and it has similar power output although it comes in a different RPM range.  Because this engine was so popular 20 years ago, they come up for sale secondhand frequently.   My top choices for the 70 Surpass are the Senior Telemaster, Cloud Dancer, Astro Hog, and any 60 size trainer.


15 Size engines in general

Some engine sizes have had many planes designed for them, especially 40 and 049.  But after looking around for good 15 size designs I noticed that you have to go back to 1960 to find a variety of interesting sport models for your 15 engine.  In the 1970s 15 size pylon racing was popular, so there are lots of pylon racers in the online plans archives.  The Underdawg, Top Dawg, etc. come to mind.  Other than that, it’s pretty slim pickings.

One of my favorite engines is the OS 15 FP, and I’ve found a few really outstanding combinations.  One is the Wicked Wanda.  This is my top choice for an easy to fly, aerobatic, small model with a good glide.  A 15 is also good on a Baby Buzzard, Classmate Biplane, or Esquire.  I even used a 15 FP on a Drake II seaplane.  Just watch out for the Enya SS15 because the crank case is wider than average, and it’s somewhat heavy.  I had to do a lot of digging to come up with these matches, so any of you guys out there who can suggest a good plane for a 15, send me a note.  I’d like to hear more ideas, and I’d like to build some myself.

I received an email from David Metz, who suggested the RCM Honker Bipe for a 15 engine.  That’s a great idea.  It’s easy to build, durable, and is reputed to fly very well.  Dave also suggested the Number 7 and the Baby Bipe by Harry Stewart.  I haven’t built either of these, but if you want a 15 size plane designed for a proportional radio they seem to be just what you’re looking for.


10 Size engines in general

Like the 15 size class, 10 size can be somewhat hard to find a plane for in modern times.  09 and 10 engines were very common in the 50s and 60s, when the magazines were trying to attract large numbers of new guys to the hobby on an affordable budget.  In those days radios tended to be one channel and unreliable, planes were slow and wallowing, and engines were frequently chosen at the lower end of the scale.  When digital proportional radios became available, suddenly everybody started building bigger, and 40 size became “normal”, leaving the 10 in limbo.

In spite of the generally low popularity of planes in 10 size, the engine companies continued to sell 10 size engines into the 2000s, probably because they’re just so cute.  Today there are tons of them sitting around with nothing to do.

If you build one of the old 10 size plans from the 50s and 60s for your OS 10, you may find that they don’t always fly right with a proportional radio and 3 or 4 channels.  A 10 size plane in 1965 was designed for a pound of radio gear and rudder-only control.  You would hold the rudder to do a spin, and when you let go the plane would straighten out and do a loop.  If it wasn’t trimmed for an outrageous climb, it wouldn’t be any fun to fly on one channel.  With a super light modern radio, if you build the plane according to plan it will shoot to the moon, and you’ll be holding the elevator down to keep it from climbing like mad all the time.  If you’re good at engineering you’re welcome to reconfigure the wing and tail incidence from any of these old designs to make them fly better today.

The number one concern I have with 10 size airplanes is the drag of the landing gear.  Flying a 10 from grass is a bit of a challenge.  Even if your plane flies well, the landing gear may cause enough drag to prevent taking off.  And due to the scaling effect and Reynolds numbers, a 10 size plane in the air has very little airspeed difference between top speed and stalling.  So it’s wise to build light, make sure you have enough wing area to keep the plane in the air but not enough to cause a ton of drag, and keep the landing gear light and low profile.  One really good way to overcome the challenges of landing gear load on a 10 size plane is to eliminate the landing gear entirely and build a sea plane instead.

If you’re itching to get your 09 or 10 in the air, you’re in for a treat.  These engines are lots of fun because they’re none too loud, they idle all day, they don’t use much fuel, and they’re remarkably easy to start and run.  And I just happen to have quite a few 10 size planes on my list of short kits.  Look for the QB10H, Excalibur, Baby Buzzard, Q-Tee 10, Cloud Dancer 10, Daydreamer, TNT, Das Mini Stik, Real Thing, Super Whiz Kid, Ace Alpha, and Seafoam.  These are all on my kit list, along with a few others.  As you can see, I’ve been trying to produce kits to help you get these engines flying.

UPDATE:  The thing I said about the seaplanes was theoretical because I had never actually flown a 10 size seaplane, but today that changed.  I flew the Seafoam with an OS 10 FP, and it turned out that a seaplane and a 10 are the perfect match.  The plane made a beautiful takeoff run and rose easily into the air, and I didn’t have to worry about pesky landing gear dragging the plane down on a grass runway.


60 pattern engines


Everybody knows that the hot rod 60 engines were the standard for pattern competition for many years, and there are plenty of smooth flying aerobatic designs for this class of engine.  For 20 years the RC magazines regularly featured construction articles and plans for 60 size pattern planes.  I almost don’t want to name any because there are so many, but the most famous and lasting designs are the Super Kaos and the Kwik Fli.  If you want to have a good time with a strong 60 engine just browse the old magazine plans for pattern designs.  These planes are a pleasure to fly.

There were also several scale models of championship aerobatic planes made in 60 size, such as the Extra, Lazer, etc. These planes will do all the maneuvers, but they generally require your full attention at all times.

Somewhere in the middle is the Super Sportster 60, which looks more sporty than a pattern plane, is capable of all the aerobatic maneuvers, is easy to land, and can slow down without tip stalling because of  the thick constant chord wing.  It’s a classic design that everybody should build sooner or later.

There are other great 60 size designs such as the Das Ugly Stik, which would be a lot of fun with a hot rod 60.  But there are others that don’t seem quite as appropriate, such as the Cloud Dancer, Astro Hog, Telemaster, etc.  Excessive speed can lead to control flutter and eventual destruction, so be careful to avoid overpowering the planes at the slower end of this size range unless you are a very good builder and know what you’re doing.

If you’re not ready for the challenge of a hard core aerobatics plane, or if building a model with a canopy is above your pay grade, check out the Basic 60 from RCM Plans.

If you have an engine lying around doing nothing and you can’t figure out what to put it on, send me an email.  If I get enough questions about a specific engine I’ll see if I can come up with some good recommendations for this page.  If you have a good plane suggestion, you can send it along, too.