How to calculate glow engine size for a model?

[Airwolf911]

Hi, I know that there are some charts that have engine sizes for a given wing area. But what I want to know is that there must a way to calculate the aproximate glow engine size for a given model weight or something or I am wrong? If there`s a way to do it please let me know. I`m trying to build a program to compute rc models with a given set of parameters. Thanks for any help.

[BMatthews]

The range of model sizes is huge. A lot depends on the way you expect the model to fly.

Wing loading also enters into it.

For example a basic .40 can be used on a racer with 400 sq inches (or the Quickie class with a mandated 500) or it can be used to fly a Senior Telemaster at over 1000 sq inches. I'm using a .35 to fly an RC old timer that has around 1300 sq inches and an 84 inch span and it isn't underpowered by any means. I also flew a Quickie ARF with a .25 and it was a delightful sport model.

.049's are in a similar boat. They have been used on racers with under 100 sq inches and 2 meter gliders with 650 sq inches.

The point here is that what you will get if you go for a narrower focus will be a narrower focus. I don't know of any such chart but you can generate your own by doing a sort of market survey of the arfs and kits available pretty easily but then you're parameters will be limited to that range of models. There will obviously be a lot of overlap as well. Models intended for .15 sized engines can often fly quite nicely with a .074 to a .30 depending on the performance wanted.

[Tom Harper]

Airwolf,

A program will compute the solution to an engineering problem. What are your inputs. What do you expect the output to look like?

As Bruce points out, there is an array of answers to the "give me a model for a .40 engine" question. But, the program can be a good interactive tool. Give it a 'what if' description and let it tell you what the model would look like. I find that watts at the prop is an easier number to handle than displacement. There are those blue .40s and there are Norvel .40s. There is a big difference.

My first 'software engineered' model did exactly what the numbers indicated. I had calculated the various areas and loadings. The model flew exactly as predicted. It would not always take off and it could not return to the field if there was the slightest breeze, but it flew at the speed predicted with the engine specified.

[nmasters]

It would be nice if glow manufacturers would publish horsepower ratings.

[Wayne V]

they do, but most inflate the HP numbers or are rated at an RPM that most people will never see on a plane. My TT PRO .46 is close, it says 1.43hp and based on my thrustHP calculator i'm getting 1.325HP (12.25x3.75@13000), but my OS LA.40 says 1hp but i only got .605HP (11x5@10500).

As for most of my planes i bench test the engine with a given prop, put the numbers thru the ThrustHP calculator and go from there, i shoot for 40-60 MPH pitch speed and i like at least a 1:1 thrust ratio. This method has worked great for me, my 80" B2 bomber, weighed 70oz no fuel, my .40la said 49.72MPH and 77oz thrust and with a good hand launch it flew no problem, this was 1158 sq/in of wing area. Now my 5lb twist with the TT PRO .46 said 46.17MPH and 11.42lbs of thrust so no wonder this plane would fly straight up right out of my hand until i could barely see it.

So i would suggest downloading the ThrustHP calculator at the bottom of this link under the software section, get your self a tach and get some numbers and then go from there.

http://www.geistware.com/rcmodeling/link_club.htm

Also if you looking for rpm numbers for a given engine check here, that's what i usually do if i don't have it on hand

http://www.rcuniverse.com/forum/forumid_173/tt.htm

Good luck and happy flying

[Tom Harper]

You can build a simple torque reaction dyno. It does not have to be very accurate to yield engineering numbers. There was one on the market a while back. It does a reasonable job.

My Norvel .40 approaches the manufacturers claim of 1.6 HP on 15% nitro and a 10x4 prop. That gives me a measured 48 oz of thrust. I use 1000 watts for design purposes.

[vintage1]

Use the watts per pound. You can just about get 100bhp per liter. Or one bhp per 10cc, or .61 cu in.

A bhp is aroiund 750 watts,so it equates to around 100watts for every .1 cu in.

So take the weight in lbs and divide by 10 for the displacement in cu in for '100W/lb'

Or about 6 for a 4 cycle.

That gives you a 4lb model=0.40 cu inches or 0.667 for a 4-stroke. That's a shade overpowered, but in the ballpark.

[macboffin]

AP plane, 68 ounces with 4 ounce fuel, goes straight up from the hand, "rapidly" to say the least, O.S.32 with 10" x 6" APC prop, 10% nitro.

[gregs78cam]

Vintage, you know that is the exact same thing I have been seeing as I look at different ARFs, kits, and some PNPs. It makes a real simple tool for thinking about what outrunners to use for Glow->E conversions.