Hirth F23 plans: Let's go racing

[mrpilotron]

I've seen the Hirth F23 mentioned in a few other threads and it seems to enjoy some popularity with other small aircraft, but has anyone actually finished a flying Badland with one of these engines? My F3 is fully expected to not make Part 103 weight and that's okay. I'm building it to be a purpose-built STOL competition toy and it will get an N-number. I watched Jonas Marcinco's winning run in the Sport Class last month at Pinedale WY in a very lightweight Kitfox4 running a simple 100hp Rotax. Density altitude that day was over 10k feet so he was only making maybe 65hp. He beat out some airplanes with 2x the horsepower and 5x the money with just a simple, no-frills, lightweight plane. I'm planning to go even farther into that realm and run a half-naked F3 with a slightly modified wing and the absolute maximum horsepower I can safely stuff into it. This is what Steve Henry does in the Unlimited Class and it works really well, but I don't have his income stream to support a machine of that caliber.

[LA F2 Flyer]

I'll have to look for that Jonas video...it's been a minute since I followed his adventures on YouTube. It would have been fun to see him try the competition in his Airbike, assuming he still has it. haha

[Flex]

Not aware of anyone actually mounting an F23 to their Badland, yet. Closest you'll get to a review of that combo would be the previous Belite. Good write-up in Kitplanes April 2011.

FYI, I got a reply yesterday from Hirth supplier NW UAV. For the F23, current cost is approx $13.5K fuel injected, $11.7K carbureted.

[mrpilotron]

Those are the numbers I got 10 months ago too. The carbureted version was 6,200 5 years ago. I was really hoping the prices would drop a bit now that supply chain chaos is settled down.

[Bruce_L]

I will be installing a new F23 engine in my F2. Had a few major setbacks as life can throw you a curve just when you don't really need it. Close to getting back on the project.

A few things about the F23 install. There is not a ready made engine mount out there for the twin. My biggest issue is trying to estimate
the distance I need to place the engine from the firewall to maintain proper CG balance (the F23 is significantly heavier than the Polini 303) Luckily, Hirth has threaded holes (both vertical and horizontal) all over the engine case to allow plenty of possible mounting solutions. The other is selecting the proper Hirth mufflers when you order your engine as there are a lot of choices.

The engine mount will be 1/2 inch 4130 tubing connected to solid rectangular aluminum bar stock via rubber isolators. The solid stock will be bolted directly onto the bottom of the crankcase with 4 metric bolts. I have already sourced these fasteners and had to determine what thread pitch Hirth used for the holes.

I will take pictures as I make progress and post them here. Right now, I am focusing on assembly of the wings and testing the folding mechanism.

The F23 is not a cheap engine but with a 1000hr TBO and 50Hp on tap, I made it my 1st choice. I like the reliability of air cooling and dual ignitions.

Will also have to determine prop size as well.

[mrpilotron]

I just did some rudimentary math for mass, acceleration, time, and distance. At a realistic operating weight of 495 lbs and running the 38hp Polini engine, the acceleration should be roughly 5.229 m/s^2, take 2.56 seconds to accelerate to 30 MPH, and cover 56 feet. When I cross reference these numbers to some YouTube videos, it seems my numbers are quite optimistic. I'm seeing closer to 6-8 seconds and covering well over 100 feet.

I'm not sure where I goofed on my acceleration estimate, but I reverse engineered the video results and get closer to 1.9 m/s^2. Feeding that back into some ratios, it suggests the extra ~26 lbs for a Hirth F23 (521 lbs operating weight) should give acceleration of 2.38 m/s^2 and get up to speed in 5.6 seconds while covering 125 feet. For some reason, this actually sounds less impressive than I expected.

[LA F2 Flyer]

Thanks for making me feel a little better about choosing the Polini...for now.

[mrpilotron]

I think we need a few more built so we can get better real numbers.

[Badland-F5 Pilot]

I'm not sure where I goofed on my acceleration estimate,

You forgot to subtract the two JATO rockets.

Now THAT, would be a heck of a ride!

Really though, don't other things come into play - prop size/pitch and such? I still think your corrected calculation is very reasonable. There may be other things that can help....VG's maybe?

I'm not even close to being able to work the math you're doing, so please forgive my ignorance.

Todd

[Badland-F5 Pilot]

I think we need a few more built so we can get better real numbers.

Doesn't torque also factor in? I was under the impression that a high HP engine with lower torque can be outperformed by an engine with lower HP and higher torque for take off. Yes, No?

Todd

[mrpilotron]

Horsepower is a magical thing derived by math and based on torque. At the end of the formulas, it's the ability to do work over time. Lifting a 1000 lb block of aluminum 1000 feet can be done in half the time with twice the horsepower. This hold true regardless of the torque. If that (X) horsepower is delivered by an engine spinning 12,000 RPM, it will produce very little torque but go through a massive gear reduction unit (aka: torque multiplier) to get the prop speed down to ~2700 RPM. A direct drive Lycoming engine can deliver the exact same (X) horsepower without the gearbox by spinning the prop 2700 RPM. In both cases, the torque delivered to the prop at 2700 RPM is exactly the same. Where the perceived differences happen is different prop efficiency and perhaps some "It feels faster" factor that has no basis in science.

As a wildly variable estimate, most props can deliver around 4-4.5 lbs of thrust per horsepower. If we're doing STOL competitions, the prop diameter should be as big as practical and spin as slow as practical. This will result in a climb prop that is nearly useless for X/C travel, but will likely exceed that 4.5 lbs estimate. For the sake of my math exercises, I'll just stick with the upper bound of 4.5.

Polini 303: 38 hp * 4.5 = 171 lbs thrust.
Hirth F23: 50 hp * 4.5 = 225 lbs thrust.
Rotax 912: (come on, you don't really thing we'd hang one of those on there do you? )

BTW: I found my math mistake. Gravitational acceleration (1G) is 9.8 m/s^2. 171 lbs of thrust / 495 lbs T.O weight = 0.345. 9.8 * .345 = 3.38 m/s^2 of forward acceleration, not counting for friction.