Topics for Thinking Big Column

[Gerryy]

Hey, here's a chance to add your interest to my Thinking Big column in Model Airplane News. Please take a few moments to give me your comments about past Thinking Big columns and what you might like me to cover in future ones. Past popular topics have been CAD drawing, basic aerodynamics for designing giant planes, Flight maneuvers, Getting Started, gas engine tips, Smoke systems, Walbro carbs, basic construction techniques, fabric covering.

I'd love to hear from Thinking Big readers, if I get some feedback, I will be happy to tailor future columns to address your questions.

I'd also love to see some of your projects so feel free to post some photos here.
thanks for reading!
GY

[GHL]

Hey Gerry, Just a thought, but is there a way to relate the gas engine sizes to prop size for particular weight of planes. Would a G-62 engine be a good one for a 100" Ziroli Skyraider, if the Skyraider weighed in at 27 lbs? Would the G-62 be a good choice for a Yellow Aircraft P-47 with a 82" wingspan weighing in at 24 lbs? Is there a better formula for configuring engine size, wingspan, weight of plane, etc? Gas engines are still somewhat new to me. Regards, George

[Gerryy]

Hi George, thanks for joining the forum discussion. Yes, there is a quick way to zero in on what engine would be preferable for a given airplane weight and size. Sorry in advance for this reply being a little on the long side!

Before you can ask the question, "How much power do I need?", you first have to determine how big your model is and what wing loading it is likely to have. This is important because it directly affects the model's stall speed. The heavier the model, the higher the wing loading and stall speed for a given wing size.

My rule of thumb for a sport-model wing loading is to stay between 20 and 30 ounces per square foot. For a light model, such as a Piper Cub, Taylorcraft, Aeronca Champ, etc., the loading is at the lighter side of the range. WW I models also fall into this group. Higher-performance models, such as CAP 232s, Extra 300s, etc., will be at the higher side. Very large, high-performance aircraft and fighters like Mustangs and Thunderbolts, will perform very well with wing loadings of 40 ounces per square foot (or more). A convenient formula that will keep your model in a safe wing-loading range is to maintain one pound of model weight for every 100 square inches of wing area. This works out to 23 ounces per square foot. Once we have the general weight and wing-area figures, we need to apply them to a given horsepower to figure out the model's power-loading

I first learned about power loading from the book, "Giant Steps" (1980). In it, Don Godfrey offered a good rule for engine/aircraft matchmaking: never exceed 12 pounds of model weight for every cubic inch (ci) of engine displacement. This is very general and does not account for the model's aerodynamic sleekness, wing aspect ration or the efficiency of the engine and prop combination, but it does give us a model-weight-to-engine-displacement (ci) ratio of 12:1.

For a conservative equation, I use one horse power for every cubic inch of engine displacement (for 2-stroke gasoline engines). Some 2-stroke and 4-stroke glow engines can produce more than this, but to keep it simple, I always use the 1:1 ratio.

This gives values of 1.3hp for a Zenoah G-23, 2.5 hp for a G-38 and 3.7hp for a G-62.

For an example, lets take a warbird with a wingspan of 85 inches, roughly 1,275 square inches of wing area and weighing about 15 pounds. This equals 27.10oz./sq. ft. wing loading. Using a power-loading value of 10:1, I can assume that I will need roughly 1.5hp to fly the model in a scale manner (on the wing). Either my Saito 1.50 4-stroke or my RC Showcase RCS 1.4ci gas engine should work out nicely. I'm not suggesting that other engines won't work as well or better, but I do know that my engine choices will work.

Using some basic power-to-weight ratios, you can be confident that the engine you choose will get your model safely off the ground and fly it properly. By also having your model's wing loading at an acceptable value, you know that your model will be able to land safely at a reasonably low airspeed.

Hope this helps!
GY

[tailskid]

Gerry, how about a paragraph or two on what size bolts you should use to bolt the engine to the plane? Now that we have small gas burning two cycles (Evolution) all the way up to 3w's 150's.....lot of newbies out there....just a thought.

[Gerryy]

sounds like a good topic for discussion. I just finished talking to good friend and MAN contributor, Jerry Smith and he is going to write up an article about engine mounts and ways and techniques to install engines. Hard and soft mounted and what other things to consider when hanging glow and gas engines!
GY

[xtraflyr]

Hey Gerry,
How about proper setup of battery reduntant systems and running 2 RX on the same planes,power boxes ect. Using batteries that require the use of Voltage regulators.Do this for planes from 50cc size all the way up to the 150cc size giants..
Cya Frank :wave:

My latest project. BME Yak54,DA50,KS Header and Canister muffler!!

[Gerryy]

Hey Frank, nice photo. You are in luck! We just finished up an article called "Bigger is Better" Setup Secrets for Giant Aerobats.... (working title!) It is a getting started article written by John Reid our west coast Sr. Editor and it includes many of the items you are speaking about. It will be in the March 2007 issue.

GY

[Kmot]

Please take a few moments to give me your comments about <snip> what you might like me to cover in future ones. GY
Okay Gerry, here is a really good subject. :D

What giant scale ARF's are available that will support the large multi-cylinder radial model engines? In particular, the ASP/Magnum 400 five cylinder radial. And also, the OS 5-cylinder radial. The Saito 450 three cylinder radial and the 325 five cylinder radial.

Thanks!

PS: Feel free to include the OS Pegasus flat four and Saito 200 inline twin. ;)

[Gerryy]

Hmm! that sounds interesting! I did do an article on radial engines a while back, I included all the specs for the popular radial engines so it should be easy to find which engines would be available for certain ARF giants. thanks for the suggestion.
GY

PS: I will look up the Radial Engine article and let you know which issue it was if you are interested

[Kmot]

I did a search on man.com and found it: July 2003

Is there a .pdf online anywhere?

[Gerryy]

no unfortunately not. But we will be having a new website redesign coming up and we will be doing an article archieve so could be done in the future!
GY

[vfr pilot]

Hi Gerry,

Your "How much power" post was timely and helpful. I've got a Zenoah G26 on order for my Top-Flite Stinson Reliant...on the low end of the range, but I'm used to building light with electric parkflyers.

I'd like to see a future article on the best ways to reduce electrical noise with the gasoline 2-strokes, and a comparison of how magneto and electronic-ignition engines compare in this respect.

--Doug

[Gerryy]

thanks for writing Doug, this sounds like a good subject. I will consider it for 2007. Several builders I know have different ideas for the need of noise supression and elimination. So I will talk it up with them at the next opportunity!
thanks
GY