# Best Bang for the Buck STOL

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#### cluttonfred

##### Well-Known Member
HBA Supporter
Mike W, since two-stroke engines have largely fallen out of favor except for true Part 103 ultralights in the USA, would you recommend any of your designs for conversion to four-stroke power? If so, any thoughts on which model and what engine might be suitable?

#### Mike W

##### Well-Known Member
Mike W, since two-stroke engines have largely fallen out of favor except for true Part 103 ultralights in the USA, would you recommend any of your designs for conversion to four-stroke power? If so, any thoughts on which model and what engine might be suitable?
I am mainly an airframe man. The engine is just a weight and thrust to me. Eddie Clapham an ex Rolls Royce engineer is the engine expert. Eddie has been test pilot on most of my designs including the present Plank aircraft. The power to weight ratio of four stroke engines is usually worst than a two stroke. If fitted to an MW the weight would need to be similar to the approved two stroke normally fitted.

However saying that, Eddie once fitted a Honda V twin motorcycle engine to the 6 and flew it for quite a few hours. He had a theory that changing gear would work like a variable pitch propeller. The engine and gearbox was so heavy, that the poor MW6 could be only flown one up from the rear seat.

#### Mike W

##### Well-Known Member
My thoughts are to get an abandoned MW5 single seat Microlight and modify the wing with a full span leading edge slot and a large TE flap and with a few mods to the fuselage and undercarriage it could make a good STOL aircraft.
Hi Mike W ,Thats what I plan for my LMA build, here in the UK.
I look forward to seeing it.

#### flywheel1935

##### Well-Known Member
I look forward to seeing it.
Hi Mike, ( Mr Whittaker) its the SSDR LMA build in Tube and Fabric,

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#### flywheel1935

##### Well-Known Member
Hi Mike, I'm at that stage where 50% is done, only another 99% left to do. We spoke about 18 months ago, was looking for tubing, now resolved.
the Planks looking good BTW. Get to the Hangar most days for a couple of hours, so progress is slow but steady, hope to fly in late 2021.

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
View attachment 100462

Most of us are familiar with the well established engineering term "TLAR".

...Looking at the cantelever loads on that prop shaft, I cant get there.

To the OP-

I'd only reiterate that you:

Fly before you commit;
Expect to change your mission once flying;
Don't expect to keep that airplane forever - there will be others.

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#### Iwerk

##### Active Member
I was in the market for a recreational STOL aircraft.
I bought the factory plans for the CH701 and CH750.

The following youtube videos are what inspired me in my choice.

I wanted to scratch build both the engine and airframe. A tall order.
I researched a lot of engines and thought a turbine would be fun.

I bought some model turbine plans from WREN and wanted to scale them up for a full sized aircraft.
This turned out to be bad idea, mostly because of the fuel burn and excessive noise levels.

I bought a C-85 engine and matching McCauley prop after watching David Orr put one in his CH701.
This seems like the most reasonable solution to an inexpensive STOL aircraft project.

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#### Victor Bravo

##### Well-Known Member
HBA Supporter
I believe you will have to take several steps to reduce the weight of the C-85. The stock C-85 is too heavy for the 701 by 25 or 30 pounds. You will wind up with a nose-heavy airplane, and the firewall / nose gear structure is a little light for that much weight.

This is the reason that Chris Heintz (701 designer) built the CH-750, which uses the heavier firewall from the CH-601/640... to accept the O-200 and similar engines.

To use the C-85, you might consider using electronic ignition instead of 12 pounds' worth of magnetos, and the light weight B&C starter and small alternator, instead of that enormous Delco automotive starter and generator that Continental used.

#### rv7charlie

##### Well-Known Member
I've got the hots for one of the Yamaha 4cyl sled engines. They seem to be coming in at around 150 lbs FWF (everything), and 140-160 HP depending on which model you choose for a core. Used engines are around $2K & the drives seem to be$2k-\$3k.

If I needed an affordable Rotax 912-915 substitute, it would be a Yamaha. I'm sorely tempted to use the 2 cyl version (~70 HP in a plane) to replace the R503 on the Kolb Twinstar I restored, but I'd have twice as much in the motor as I have invested in the plane now.

Charlie

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
I believe you will have to take several steps to reduce the weight of the C-85. The stock C-85 is too heavy for the 701 by 25 or 30 pounds. You will wind up with a nose-heavy airplane, and the firewall / nose gear structure is a little light for that much weight.

This is the reason that Chris Heintz (701 designer) built the CH-750, which uses the heavier firewall from the CH-601/640... to accept the O-200 and similar engines.

To use the C-85, you might consider using electronic ignition instead of 12 pounds' worth of magnetos, and the light weight B&C starter and small alternator, instead of that enormous Delco automotive starter and generator that Continental used.

I have a C-85-12F in the JMR. Slick mags, light weight block-off plates for the starter and generator pad. Replaced the heavy brass breather elbow with one made from aluminum, Stromberg carb, for non electric. Weigh is 188 lbs.

#### ToddK

##### Well-Known Member
HBA Supporter
I have a C-85-12F in the JMR. Slick mags, light weight block-off plates for the starter and generator pad. Replaced the heavy brass breather elbow with one made from aluminum, Stromberg carb, for non electric. Weigh is 188 lbs.
That’s pretty dang good.

#### PagoBay

##### Well-Known Member
I believe you will have to take several steps to reduce the weight of the C-85. The stock C-85 is too heavy for the 701 by 25 or 30 pounds. You will wind up with a nose-heavy airplane, and the firewall / nose gear structure is a little light for that much weight.

This is the reason that Chris Heintz (701 designer) built the CH-750, which uses the heavier firewall from the CH-601/640... to accept the O-200 and similar engines.

To use the C-85, you might consider using electronic ignition instead of 12 pounds' worth of magnetos, and the light weight B&C starter and small alternator, instead of that enormous Delco automotive starter and generator that Continental used.
There are two well known CH701's flying with 130HP engines. "Super 701" owned by Jon Humberd and "JUG" owned by Deane Philip, NZ backcountry, sandbar flyer and winner of STOL competitions in New Zealand. Deane swapped out a Rotax 912 for the larger engine. Spoke to Deane some months back about details. He put 20 lbs in the tail. Deane said he was totally happy with the decision to go with the bigger engine.

Both aircraft are easy to find on YouTube. It is obvious both gentlemen are super delighted with performance.

Don't know if either have other than the stock firewall, etc. Worth confirming.

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#### BJC

##### Well-Known Member
HBA Supporter
There is a well known aerobatic pilot (gold medal winner in WAC) who put a 300HP Lycoming in a highly modified Pitts S-1S. Up lines were impressive. The standard S-1S airframe is a 9+ g airframe. The modified airframe was a 5 g airplane, with the same Vne.

Be careful adding weight and available thrust. Even when balanced by adding weight in the tail, it can affect other parameters.

BJC

#### TFF

##### Well-Known Member
I wouldn’t put 20 lbs in the tail of a 701 stock. It already pushes engineering correction coefficient as small as it can. I would take the overweight Pitts over it anytime for strength. Not saying the 701 is bad, but don’t guess on mods, as not much fudge factor is left, to make it do what it does well. There is not much more of a minimalist plane than a701.

#### PagoBay

##### Well-Known Member
I wouldn’t put 20 lbs in the tail of a 701 stock. It already pushes engineering correction coefficient as small as it can.
Point being that the airframe will be at risk to warp under load on the longitudinal and/or lateral axis? Stress slowly leading then to possible failure in time and worse while in flight? That is serious. Particularly so when considering the added forces on hard landings in back country operations.

If that summary is correct, how to determine the acceptable load limits with confidence? Usually I only think about Max Gross Weight. But that is due to lack of engineering knowledge. I would have thought the Zenith factory would have been consulted first.

Thanks.

#### TFF

##### Well-Known Member
The one I have been around I would consider fragile if banged around. I consider the Aircam the same. The fuselage and wings are as light as possible. I can’t see a fuselage with any dent or ding handling 20 lbs in the tail. It would fold up. It’s performance advantage is lightness; you should not need to bang it around to get short field performance. I wouldn’t treat it like a Highlander. You tip toe with a 701. To me it’s a high performance airplane. I would not consider it a robust airplane.

#### rv7charlie

##### Well-Known Member
Lots of info here about 701 mods, engine choices, etc.

#### PagoBay

##### Well-Known Member
The one I have been around I would consider fragile if banged around. I consider the Aircam the same. The fuselage and wings are as light as possible. I can’t see a fuselage with any dent or ding handling 20 lbs in the tail. It would fold up. It’s performance advantage is lightness; you should not need to bang it around to get short field performance. I wouldn’t treat it like a Highlander. You tip toe with a 701. To me it’s a high performance airplane. I would not consider it a robust airplane.
Lots of info here about 701 mods, engine choices, etc.
Oil canning mentioned lots in this thread. I am up to page 4 now. Thanks for the tips and guidance.

#### PagoBay

##### Well-Known Member
I wouldn’t put 20 lbs in the tail of a 701 stock. It already pushes engineering correction coefficient as small as it can. I would take the overweight Pitts over it anytime for strength. Not saying the 701 is bad, but don’t guess on mods, as not much fudge factor is left, to make it do what it does well. There is not much more of a minimalist plane than a701.
An example of UL Power 350iS 130HP in a Zenith 701. Upsized the two top bolts for the engine mount and mentions here some "extra bracing".