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NEW PROJECT TIME Q1 with a Hirth engine

5 years 7 months ago #1594 by Backcountrydrifter
Hello fellow quickheads,

Overview- I have the opportunity to buy a previously flying Q1 (750 hrs TT) that has been relieved of its original engine.
I happen to have a 650cc Hirth engine of the same vintage complete with an oddball 3 blade cast aluminium prop. (No gear reduction). Electric start. It’s single plugs now but I would modify it for dual plugs /dual CDI ignition.

I have researched a ton about what doesn’t work. But in doing so I keep seeing 28-40hp as the ideal range for a Q1. But I would love for someone to educate we as to why. If I can build an engine that puts out 50-60hp and keep my prop top speeds sub sonic and the engine weighs the same, what is the downside to having more horsepower?

I’m not against changing any components in the equation. I just don’t want to spend a lot of time doing something that someone has already cut the learning curve on. I haven’t run the math but my gut tells me that I will need to change props, possibly add a reduction drive and do a fair bit or tuning.

Has anyone here had any experience with HIrth engines as this is my first experience with them?

Thanks in advance,

2200 hours total time
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1 year 9 months ago #1702 by Armilite
First, you should always Power your Plane for its MTOW. The Original Q1 used an 18hp Onan, and the Onan was soon upgraded to make more 22hp. Still, people weren't satisfied, so they used bigger hp, mainly Rotax 377UL (35hp), 447UL (40hp), 503UL (50hp), Kawasaki 340/440, etc!

What is the Max Lenght of Prop that can be used on a Q1?

I don't know the Original Onan 4 Stroke Engine Weight, but it was Heavy?

I think there was a Carbon Fiber Spar upgrade for the Q1 & Q2 that raised the MTOW, but don't know what that Weight was?

With an MTOW of 485 lbs = 219.9923 kg / 10 kg = 21.99923​​​​​​​ Kw needed to Fly Well = 29.50145 hp!

The Rotax 277UL (72mm x 66mm) 268.8cc using a Bing 54 36mm Carb should have been the most logical Engine since it was much Lighter than the Onan and made 28hp@6400rpm. If turned 6500rpm (Industry Standard) for 2 Strokes used on Planes) it would have made the required 29.5-30hp! The 277UL was also rated 26hp@6250rpm and with an R&D Aero Mild Tuned Pipe 268.8cc/30.5hp = 8.813cc used to make 1hp made in the early 90's it made 30.5hp@6250rpm. Tuned Pipes are Specific for the Bore & Stroke = CC and Max rpm Designed. Tuned Pipes used on Sleds first showed up around 1970. Both 277ULs were the same Core Engine it was the Exhaust used. The 277UL was made in (3) different Configurations, 277UL Fan Cooled, with a Gear Drive, Exhaust, and Electric Start at 65 lbs. Then the 277UL Free Air with a Gear Drive, Exhaust, and Electric Start around 60 lbs. Then the 277UL Free Air with a Belt Drive, Exhaust, and Electric Start around 55 lbs.

Any 2 Stroke using 11.5cr, the proper Size Carb for Max rpm used, turned 6500rpm with a Good Tuned Pipe it takes on Avg 7cc to make 1hp. That little 185/185UL is 184.2cc/7cc = 26.3hp@6500rpm is possible and came out in 1945. A +/- 1.0cr = 1hp. You have had 52 Years of Tuned Pipe Technology now, you have had 20+ years of Engine Ceramic/Moly/Heat Dispercent Coatings, you have had Hybrid Crank Bearings with Steel Races with Ceramic Balls for 10 years, Cageless Bearings since the 90's to Build these Engines better, make more hp. You have much better 2 Stroke & 4 Stroke Oils today, some can be used at 100:1 Rato, which means fewer Decarbons needed. That 277F/277UL is 268.8cc/7cc = 38.4hp@6500rpm is possible.

Skidoo/Rotax had many other Singles out in the '60s, '70s, the 143, 163, 185, 250(1963), 292, 299/300, 318/320, 335/340 before the 277F/277UL came out I think in 1978. 1971 340 Blizzard Single was 36hp. 292 Blizzard was 31hp. In 1969 Skidoo/Rotax had a Proto Type (78mm x 70mm) 334.6cc Rotary Valve that made 45hp! On Top of the 33+ other Brand Name Snowmobile Engines that were being made. Each of them Brands made many Singles, JLO alone made (14) different Singles 100cc to 395cc, so did Sachs.

Today, you also have these 22+ different Honda/Clone Singles based on the Honda GX390 that go up to (100mm x 86.5mm) 679.6​​​​​​​cc! These Engines are very popular in the Racing Go Kart World so lots of HD Racing parts made for them. Common found Single Cylinder Clones in Stores are the 420, 440, 460, and a 500 has just come out on Duramax Generators.

Honda/Clone Singles & V Twins used for Aviation use!
1. Disable Governor.
2. Disable Low Oil Sensor.
3. Install Billet Aluminum Rod.
4. Install Billet Aluminum Flywheel to Save on Weight.
5. Install a 34mm Carb on Singles.
6. Install K&N Type Airfilter.
7. Install a better CAM .307 Lift+.
8. Install better HD Valve Springs & HD Pushrods, Locks, Keepers.
9. Install Head Milled for a higher 9.0-11.5 CR.
10. Install Bigger Valves for more hp.
11. Install a Tuned Header Exhaust
12. I would install the HD Billet Needle Bearing Rocker Arms.
13. USE Mobil 1 Synthetic Oil
14. I would use all the different Ceramic/Moly/Heat Dispercent Engine Coatings!

All the Upgrades Avg $570 to $600.

ACE Aviation makes the best Belt Drive for them, was $569 shipped to USA.
Specifications (Onan engine)General characteristics
  • Crew: one pilot
  • Capacity: 20 lb (9.1 kg) luggage and 240 lb (108 kg) useful load. <-- I question this!
  • Length: 17 ft 4 in (5.28 m)
  • Wingspan: 16 ft 8 in (5.08 m)
  • Height: 4 ft 5 in (1.35 m)
  • Wing area: 53.8 sq ft (5.00 m2)
  • Empty weight: 245 lb (111 kg)
  • Max takeoff weight: 485 lb (220 kg) - 245 lbs = 240 lbs!
  • Powerplant: 1 × Onan opposed four-stroke piston engine, 18 hp (13 kW) at 3,600 rpm
  • Maximum speed: 126 mph (203 km/h, 109 kn)
  • Cruise speed: 115 mph (185 km/h, 100 kn)
  • Stall speed: 47 mph (76 km/h, 41 kn)
  • Range: 570 mi (920 km, 500 nmi)
  • Rate of climb: 420 ft/min (2.1 m/s)

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