# Improving Take Off Performance?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Armilite, Dec 10, 2019.

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1. Dec 10, 2019

### Armilite

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Ok, I have a friend who has a Kolb Firestar 2 which is tandem configuration, with a 503UL (50hp) with C Gear Box with a 3.0 to 1 ratio and a 71/72" Warp Drive Prop. 6500rpm / 3.0 = 2166.6 rpm for Prop. It's a (3) Blade.

A 72" x 22 at 6500/3.0 = 2166.6rpm Pitched for 2066.6rpm (3) Blade = 430.83 lbs needs 49.632 hp!

He wants to put a 670 (93hp) on it to increase his takeoff Performance for better Stol in the OutBack using the same 72" Prop & Grear Drive.

A 670 (More Weight & Cooling) with same 72" x 41 at 6500/3.0 = 2166.6rpm Pitched for 2066.6rpm (3) Blade = 430.83 lbs needs 92.496 hp!

A 72" x 33 at 6500/3.0 = 2166.6rpm Pitched for 2066.6rpm (4) Blade = 523.15 lbs needs 90.401 hp!

With a Take-off distance with a 503UL: 250 ft. Will you really Gain that much Shorter Takeoff Distance with the Higher 93hp Engine using the same 72" Length (3) Blade Prop or does he need a better prop?

Plane Specifications: https://www.pilotmix.com/firestar-ii

MTOW: 725 lbs (329 kg) 725 lbs = 328.9 kg / 10kg = 32.8 kw is needed = 43.98552 hp (44hp) needed.

Empty weight: 325 lbs (147 kg) <---- A little Less since he took out the 2nd Seat.

Useful Load: 400 lbs.

Stall speed: 30 kts (35 mph) (56 kph) <--- Can that be Improved! How much for a better Bush Plane?
Cruise speed: 61 kts (70 mph) (113 kph)
VNE: 74 kts (85 mph) (137 kph) <--- 503 50hp is probably pushing it close to the VNE.
Climb: 800 ft/min (4.1 m/s)
Take-off distance: 250 ft (76 m)
Landing distance: 300 ft (91 m)
Engine: Rotax 503 (50hp)
Fuel consumption: 4 GPH (15.1 LPH)

Using Static Thrust Calc at 77F:
http://godolloairpor...c_eng/index.htm

Last edited: Dec 10, 2019
2. Dec 10, 2019

### Vigilant1

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Do your friend a very big service and let him know that 670cc engine will not reliably produce 93hp in aviation service. I hope you haven't encouraged him to believe it will. If he chooses to use it anyway to try to make anything close to "93 HP", advise him to stay within gliding distance of a landable surface all times.
If you put a prop on it that requires 93hp to be efficient, it may make very little thrust at power ratings that are reasonable for that engine in aviation service (maybe 30 HP, for short periods).

Last edited: Dec 10, 2019
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3. Dec 10, 2019

### stanislavz

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Plus - rotax912 80hp version, is not that expensive as it was. More weight could be compensated by less fuel. It is still one third more economical compared to 503.

4. Dec 10, 2019

### Armilite

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Rotax Rick's 670 with his Tuned Pipe Dynoed 93hp@6350rpm and has a 450hr TBO if you run his 3-4 different Oil Choices at 40:1 Oil Ratio as he advises. My Stock 670 in a Sled Dynoed 115.7hp@7750rpm. So I don't know where you're getting your Info from, but it's not right.

But NO, I didn't Advise him to use a 670, which I think is Over Kill for his Airframe account I think it's a Waste of HP and the Extra Weight using the same (3) blade 72" Prop. He wants a Stol Type Aircraft. His MTOW requires 44hp and he has a 503UL 50hp now. Which he could make more hp with the 503. With just R&D's Mild Tuned Pipe it made 62.3hp@6500rpm.
http://www.rotaxservices.com/dyno.html#7

5. Dec 10, 2019

### Vigilant1

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Okay, it's a 2-stroke 670. It can make more than 30 HP. Still, stay over landable terrain.

And this thread is definitely in the wrong sub-forum.

don january likes this.
6. Dec 10, 2019

### Aerowerx

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No more cheeseburgers and fries???

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7. Dec 10, 2019

### Armilite

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=================================================
The 912 80hp is $15,813.00 from CPS and is even Heavier than the 670 which makes 93hp. I don't know Rotax Ricks current Price but I'm sure it's at least 1/3 the Cost of a 912's 80hp. Since he wants to go in the Outback Fuel will be needed. https://www.cps-parts.com/categories/rotax_parts/rtx/menus/rtx/engines.html 8. Dec 10, 2019 ### stanislavz ### stanislavz #### Well-Known MemberHBA Supporter Joined: Aug 22, 2016 Messages: 373 Likes Received: 96 Location: Lt My sorry. Used rotax of course. 9. Dec 10, 2019 ### Dana ### Dana #### Super ModeratorStaff Member Joined: Apr 4, 2007 Messages: 8,767 Likes Received: 3,124 Location: CT, USA 93hp is way too much for a Firestar. If your friend isn't on the Matronics Kolb list, he should be, that's where all the Kolb experts hang out. But I'm thinking what he really needs is a smaller redrive ratio, like 2.58, the 503 should be adequate. Didn't Victor Bravo just go through this (trying to force the wrong ratio to work)? Topaz likes this. 10. Dec 10, 2019 ### Armilite ### Armilite #### Well-Known Member Joined: Sep 6, 2011 Messages: 3,111 Likes Received: 251 Location: AMES, IA USA =============================== The C Drive he has only come's in 2.68, 3.00, 3.47, and 4.00 to 1 ratio. But, Yes, I think you Nailed it, that might give him some more thrust with the same 72" Prop. New gears aren't cheap but neither is a New Prop. A 72" x 22 at 6500/3.0 = 2166.6rpm Pitched for 2066.6rpm (3) Blade = 430.83 lbs needs 49.632 hp! A 72" x 15.5 at 6500/2.68 = 2,425.3rpm Pitched for 2,325.3rpm (3) Blade = 545.44 lbs needs 49.812 hp! Is there a Clac for figuring Takeoff Distance based on these thrust Numbers? 11. Dec 10, 2019 ### Armilite ### Armilite #### Well-Known Member Joined: Sep 6, 2011 Messages: 3,111 Likes Received: 251 Location: AMES, IA USA =================================== Yes, built from a good used Skidoo/Rotax 670 it's all Rebuilt with all new Parts, Rods & Bearings, Pistons & Rings, Gaskets & Seals. He installs New Bing 54's and New Ducati Dual Ignition and gives a 450hr TBO. Rotax never made it into a UL Motor account People wouldn't have Paid$20,000 for that first 80hp 912UL in 1993, and $5000 more than the 618UL(73.4hp) for +6.6hp which is Why the 618UL was Discontiuned. IF, you turned a 670 the same 6750rpm as the 618UL was turned it makes 102hp with the Proper Tuned Pipe, that 912UL (100hp) was$25,000. 914UL 115hp was \$35,000. A 670 with a Turbo 7 lbs of Boost, 140.5hp@6750rpm! For Airplane use about 4 lbs of Boost would be plenty.

A 670 with R&D's Tuned Pipe made 103.8hp@6800rpm!
http://www.rotaxservices.com/dyno.html#89

Last edited: Dec 10, 2019
12. Dec 10, 2019

### Victor Bravo

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I would be happy to tell Armie how to improve takeoff distance on a Kolb, but I'm afraid my answer would steer the discussion away from giving him an opportunity to post detailed engine specifications. And who am I to interfere with his life's work?

The short version is that engine thrust is one of the things that determines takeoff distance. But there are other things that make more of a difference. Otherwise, a 250HP Piper Comanche would take off in less distance than a 65HP Piper J-3.

The long version is that the Kolb's takeoff distance suffers from a very useful design compromise that Homer Kolb made way back when. Kolb uses very a short height landing gear and a very long tailboom tube. This makes the Kolb much more docile on the ground, for low-time and/or zero-training ultralight pilots. But the wing angle of attack in the three-point attitude is far below it's maximum lift angle. So the Kolb has to accelerate to a faster speed to create enough lift to take off.

The other problem is that the Kolb has a high thrust line, which means the engine thrust is trying to force the nose down instead of up. The more power you have, the more nose-down force is created. Also, the more power you have, the larger propeller you need to make thrust, and on a tailboom design like this it means you have to raise the engine higher above the airplane... worsening the problem. This requires enough wind (speed) over the tail and elevator to overcome this.

So you go roaring down the runway at full throttle, with the stick full back. The airplane has to accelerate to a speed that allows the tail to push down and raise the nose, against all the thrust-related forces. But the nose can only comeeup so far before the tailwheel hits the ground. The wing was capable of making enough lift if only you could have gotten the nose up to X angle of attack. But you have to keep accelerating until airspeed over the wing makes that lift at .8X angle of attack.

What I did was to put a much taller landing gear on it than the Firestar has. I tookthe gear off of a Kolb Slingshot. Then I put giant 21 inch Bubba tires on it. This allowed the wing to be at about an 18-19 degree angle (using the flat bottom of the airfoil as a reference, not the true chord line).

Then I put the stick-on vortex generators on it, which allows the wing to fly at a higher angle of attack. (Note, the VG's also add a significant amount of safety, because the Kolb wing has no washout or twist, and has a very abrupt sharp stall for this type of aircraft. Putting the VG's on, and leaving the inboard 18 inches of each wing "clean" without the VG's, made the wing stall from the root outward instead of all at once. I highly recommend this for safety, regardless of the reduction in stall speed).

So after all this, the simple answer is that to shorten the takeoff roll on a Kolb, you want to accomplish as many of the following things you can:

Highest angle of attack on the ground, by using taller gear legs, larger tires.
Smallest diameter propeller that will make the full thrust from your engine.
Shortest distance possible from center of propeller (thrust line) to CG.
Lightest takeoff weight
Greatest amount of thrust (HP, torque, etc.)

By the way, my efforts were only partially successful. As Dana mentioned, my engine gearbox would not allow the small propeller to spin up and make enough thrust. Increasing pitch didn't compensate enough. I would have loved to have been able to spin a smaller propeller much faster, and also move the enginee downward to get rid of some of that nose-down force on takeoff.

With the amount of horsepower I had available, with the landing gear modifications and wing modifications I did... if the engine would have been on the front of the fuselage instead of on top of the wing... all other things being equal I believe I would have been able to take off in 1/3 to 1/2 the distance that I did.

13. Dec 10, 2019

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