Looking for suggestions!

[jmiedwig]

Hello everyone.

I have a Sport Hornet that was equipped with a 100HP Hirth 3703 2 stroke and Warp Drive 3 blade by the gentleman who originally started assembling the kit. The airplane was designed for the 100HP Rotax 912 with IVO so we had to reverse the offsets but everything else matches the factory prints. Currently flying off the first 40 hours and tweaking the prop pitch trying to find the sweet spot and struggling to even get close to the factory numbers. We tried pitches from 12* to 17* in half degree increments looking to get the 110-115mph cruise (smooth air) speeds some of the guys with the 912s were reporting but best we could get was 100mph at 16.5* of pitch. Additional pitch didn't yield better speeds so we worked our way back down to 14* which provided a 90mph cruise with good climb performance (so we thought).

Initial testing was all done with just the pilots weight and full fuel so this weekend we added a few hundred pounds to check the numbers at gross with the CG near the forward limit. Long story short I'm glad we had 4,000' with open field at the end because she didn't want to climb. Taking the pitch out and bringing up the RPM should help with the climb issue but anything less than 14* drops the cruise speeds quickly.

I'm not sure if the Warp is the best match for the plane/engine and am looking for suggestions/feedback from anyone running the Hirth 3703.

Thanks in advance,

John

[Bob H]

You provided no info about max static rpm. I'd set pitch as a function of static rpm and the power band of that engine. You won't get any performance if takekoff rpm is out of power band.
On my 912 with Warp Drive, static rpm needs to be above 5000-5200. Don't have data on Hitrh so determine powerband from hp vs rpm curve and set prop pitch to get rpm in that band.

[Infidel]

You provided no info about max static rpm. I'd set pitch as a function of static rpm and the power band of that engine. You won't get any performance if takekoff rpm is out of power band.
On my 912 with Warp Drive, static rpm needs to be above 5000-5200 (tel:5000-5200). Don't have data on Hitrh so determine powerband from hp vs rpm curve and set prop pitch to get rpm in that band.

Agreed. Tie that baby down and run her up to see what your rpm's are. That'll give you an idea which way to go with your prop. Then it's a matter of deciding if you want the performance for climb or cruise. Sorry I can't help with any suggestions on the Hirth engine as I've never operated one.

[martymayes]

What other said, need more info. What kind of PSRU numbers? The Hirth 3703 has a relatively flat torque curve. For max takeoff performance, should be ~5500 static engine RPM. The prop speed should be ~2/3rds of that. Can tweak from there to get the cruise in the range you want but it's always a compromise. I would not expect to be able to match the performance of a Rotax 912 powered ship.

[planecrazzzy]

OK... yeah Tie it down ... check Static RPM's

Top end.... My Kolb Firestar II has a FASTER cruise with two blades ( IVO )

But it climbs better with three Blade... ( Same IVO )

But... I set the STATIC RPM on the ground...

Then Fly... land.... and Tweak it.... Fly ... land... tweak it.... Ect...
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Gotta Fly...
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PS So you need to ask those guys with faster cruise... WTF...
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[1600vw]

I believe he is seeing under 6000 tied down. I told him to bring it up, myself I would turn it at around 6300 static and look for a cruise of around 6500. I think he told me he was seeing 5400 static or 5600, I just do not remember.

Tony3859

[martymayes]

According to published Hirth 3703 specs, max HP is at 6000 RPM. Beyond that, both HP & Torque decrease while the fuel burn increases

[1600vw]

Do NOT set this up as one would a rotax looking for 6500 in cruise flight? Interesting.

Here is a very small video of John running his engine up. I believe he re-pitched the prop after this video and added some bite to it and reduced the rpm's some but he will have to chime in on that.


https://www.youtube.com/watch?v=0tE8_0ACK0Y

Tony

[jmiedwig]

Thanks everyone for the feedback. Here is some of the data that was collected while trying the different pitches.

3703 Engine
100HP @ 6,000RPM
88 ft.lbs @ 5,500RPM
Redline 6,500RPM

Gearbox
G50 2.59:1 Ratio

Prop
66" Warp Drive 3 blade straight tips

Recorded Performance (600' field elevation/ 80* day / cruise at 3,000')

Prop at 12* pitch
Static RPM 5,800RPM
Climb RPM 6,400RPM (full throttle)
Take off weight- 1,015lbs
Climb- Estimated 1,500 ft/min
Cruise @5,500RPM 75-78MPH

Prop at 14* pitch
Static RPM 5,400RPM
Climb RPM 5,900RPM(full throttle)
Take off weight- 1,015lbs
Climb- Estimated 750-1000 ft/min
Cruise 5,500RPM 88-92MPH

Same pitch as above
Take off weight- 1,295lbs
Climb-Look out for the trees! Estimated 250-300ft/min
Cruise at 5,500RPM 80MPH


Prop at 16.5* pitch
Static RPM 5,100RPM
Climb RPM 5,700RPM (full throttle)
Take off weight- 1,015lbs
Climb- Estimated 500ft/min
Cruise 5,500RPM 100mph

I spoke with the folks at IVO and it sounds like the Hornets were delivered with IVO three blade mediums that ranged from 60-66" in diameter.

[martymayes]

So I'd go for 13 - 13.5 and let 'er rip.

[planecrazzzy]

Sounds like your on the right track...

WOW, That's a lot of weight...
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Sounds like take-off weight for my Buttercup...and it's got a 0-235
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My two seat Kolb Firestar is about 625 lb with me on take-off...( 50 hp )

Does your Hornet compare with Challengers in the weight area ?
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Gotta Fly...
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[jmiedwig]

I think the early Hornets were very similar to the Challengers as far as gross weight goes. At some point they redesigned the wing, added a second wing strut, stretched the fuselage 5-6", bolted on the Rotax 912s and increased the gross weight to 1320lbs. Mine is a bit of a mutt because I have the modified wing, second strut, short fuselage and a Hirth 3703! W/B is below...I'm using the old short fuselage CG range and the increased gross weight.



Sport Hornet


N428LG














Empty Weight
677 lbs




Gross Weight
1,320 lbs




Useful Load
643 lbs




* Fuel loading in gal (US)

(for calculation into lbs: x 6)















Weight
Arm
Moment


(Empty Weight) Airplane
677 lbs
99 ''
67,067


Pilot
lbs
52 ''



Passenger
lbs
80 ''



Fuel (max 18 gal) *
lbs
88 ''









Total
lbs










Center of Gravity Forward Limit:

84.0 ''



Center of Gravity Rear Limit:

90.5 ''

[planecrazzzy]

That's a good useful load...
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Do you know what your fuel burn is now that you have your prop pitch adjusted ?

Actually... you might pitch it for climb around the patch...

and bump it for cruise if yer going somewhere.......?
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Two seater are great for packing up the Camping gear and Heading out...

Don't forget the Dog...
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Gotta Fly...
Mike & "Jaz" the Flying Dogz
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3861

[jmiedwig]

Prior to the repitch we were burning about 4.5gph at 3,000' at 5,000RPM. I'm running a custom triple exhaust in lieu of the factory single and they were able to lean it out a bit when they tweaked the engine program. Looks like you two were packed and ready to roll in the pictures. Where are you based at? I'm in Springfield IL at 3IS5 (Holmes Airport).

[planecrazzzy]

Yeah, That was before I started building my Wittman Buttercup...
The Guys would gather at Baraboo WI before O$HCA$H ...
Mostly Wittman W-10 Tailwinds...
I was Flying from the Twin Cities... it was a couple hundred miles away...
We'd camp over night and fly back the next day...
As I'd wander around... Jaz would go back to guard the plane...
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Here she is getting tucked in... Sweater on for 3,000 ft and Mutt Muffs
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Gotta Fly...

[1600vw]

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