Matching an engine to an airframe.

[David Hall 9]

I have recently tried my first IC powered plane for more than 20yrs.

My Ballerina flies well on the OS FS40S, I am enjoying the change.

I am drawn to the retro style of the Ballerina and am now thinking of building an Astro Hog.

What four stroke motor would suit the 72" Astro Hog and, as I have an SC52FS engine, what span might that suit?

[Jon H]

Astro Hog's will fly on anything from a 50-90 4 stroke. Depends how wild you want it to be! Generally a 70 or 80 fs would be my choice for a Hog

Its the same story with a 52fs. My 72'' Piper Tomahawk is lovely on an Enya 53 4 stroke, equally my old wayfarer biplane of 52 inch was lovely but it was more aerobatic. a friends 55 inch Se5a is also very well powered by a 52 4 stroke. Acrowot's have flown well on 52's, but also go well on a 70.

A great deal depends on how you want to fly. Do you want to fly with the propeller, or the wings? If you are happy to use the wings you can use a smaller engine and do large sweeping manoeuvres that preserve energy and minimise drag. If you want to smash the sticks around and use the prop to get you out of it, you need more power.

I have an 85'' model of around 20lbs that i use as a flying test bench. Its a big ugly old brick and its known as the flying park bench due to its rather beefy construction. I have flown this model on everything from a 120 4 stroke to a 360 4 stroke. Currently its got a 64cc radial engine on it, a few weeks ago an ASP 160 flat twin.

Even on the smallest of the engines my 20lb barge is still aerobatic. No its not as fast as it is with a bigger engine, but it is still able to loop and roll without any trouble.

Power isnt everything.

[john stones 1 - Moderator]

70 FS would be my choice, plenty Spacewalkers about or similar that would suit the SC 52.

[Robin Colbourne]

David, The Outerzone plan shows a K&B Torpedo .35 two stroke engine. That is from the days before Schneurle porting which greatly increased two stroke performance. Your SC52FS should be more than match for it. Having said that, I would expect most fliers these days to put a .70 four stroke in a model that size (72" span, although it would not represent the original in flight, except on low throttle settings.

If you make the engine bay area large enough to accommodate a .70FS, and start with your .52, you can always upgrade if you think you need it.

If you are scaling to suit your engine and thinking in current day performance, around 63" would be about right.

Personally I would just go with 72" span and keep the airframe as light as possible. Do put a servo on each aileron though.

Edit:   By the time I hit reply, Jon and John had already replied.  There is clearly a bit of a concensus going!

Edited By Robin Colbourne on 25/06/2020 11:50:18

[Martin Harris - Moderator]

My 8 1/2 lb 69" Chipmunk flies very nicely on an OS 52FS - as does another one from the same Airsail kit that I've flown on the SC equivalent.

Performance is much better than scale, allowing all scale aerobatics and a bit more - but of course 3D stuff is out of the question even if I wanted to do them with a scale model. It's a great example of a model that flies "on its wings" - I have to use half throttle to take off in order to extend the ground run to more than a couple of model lengths!

[Geoff S]

That's good to know, Martin. I've been trying to convince myself that my latest project,a 60" ws Sopwith Pup which is turning out to be rather porkier than I'd hoped (as always!) and looks like being around 9lbs, will fly OK with 65/70 watts/lb of electrical urge. With 10sq ft of wings the wing loading remains low. I know it's not glow or petrol but spinning a sufficiently sized prop is what counts. The OS52fs in my Flair SE5a spins a 12x6 and is more than adequate from what I remember (it's some years since it flew).

Perhaps the old saying "there's no replacement for displacement" isn't always true - in our case, wing loading is also a factor.

Geoff

[Richard Clark 2]

Posted by David Hall 9 on 25/06/2020 11:22:26:

I have recently tried my first IC powered plane for more than 20yrs.

My Ballerina flies well on the OS FS40S, I am enjoying the change.

I am drawn to the retro style of the Ballerina and am now thinking of building an Astro Hog.

What four stroke motor would suit the 72" Astro Hog and, as I have an SC52FS engine, what span might that suit?

The designer's original Astro Hog (1958) used a K&B 35 two-stroke and he won lots of US aerobatic competitions with that engine.

Even an SC52FS will be more powerful than the original K&B though performance will be very 'mild' by modern standards.

[Graham Davies 3]

I personally have ruined nice flying aircraft by overpowering them. My Wot's Wot flew well on a 46 back in the day, OK on a 61 but was horrible when I put a pipe on it. The Torque reaction was so fierce and it very quickly got very small if you weren't extremely careful. I know, I was young and rash...

I currently enjoy a 40" span plan built Wight Wizard with 390W for 2lbs1Oz. It is extremely fast, and whilst good fun, is not relaxing. I have a 36" span aerobipe with just 130W for 1lb 10Oz, and it is a totally different experience. Horses for courses, but there is something extremely satisfying with letting the wings do the work.

[Frank Skilbeck]

My Astro Hog has flown on an Irvine 53 and now has an OS55AX, both suit the model very nicely, plenty of power for big loops but not overpowered. I would think a 70-80 four stroke would be a good match.

Was quite interesting looking at the video recently posted on facebook from the nationals in the 1960s, all the aerobatic planes needed a long takeoff run on tarmac, probably a reflection of the smaller sized engines used.

BTW the Astro Hog is a delightful flier, build one, you won't regret it.