Marty d's CH-701 build log

[bexrbetter]

3 hours ago, Moneybox said:

That's the sort of accuracy I expect with Autocad but I have trouble with Sketchup.

 

 

Maybe it's just me but If I try to select a point of couple of intersecting circles, I cannot accurately select the intersection.

 

 

I understand your issue: While it is true SU doesn't have 'true' circles, when you click on 'circle' icon, the next thing you do is type the number of sides you want and press enter. Default is 24 sides, but I usually use 96, staying with divisions of 12, but just tested 960 and it was no problem. The more sides the larger the file of course. 

 

Thanks Marty, surprised to see you're still at it, many give up in shorter time, such tenacity, well done!

[skippydiesel]

Marty-D,

 

I am concerned about the management of your Rotax. 

 

It seems, from all the questions, that you are not referring to the Rotax manuals, rather advice from well meaning persons on this Forum.

 

Rotax have detailed instructions on priming the oil & coolant system (including specification for coolant type) fuel /oil pressure, etc etc.- stick with this if you don't want to risk expensive engine damage.

 

Further there is the Rotax Owners Forum https://www.rotax-owner.com/en/rotax-forum where the latest manuals are available,  Rotax specific discussion and  Rotax  mechanics.😈

[Marty_d]

Pulled the plane out again this morning and did another start - thanks to @nomadpete the oil pressure gauge is now working.

I bought a small fuel tank (generally used for working on motorbikes when the tank is off) and suspended it on one side of the fuselage going into the wing tank inlet to the header, put a couple of litres in then switched the line to the return barb for running.  (Forgot to take a photo sorry).

 

Ran it for about 10 minutes.  Look like the oil TEMP gauge is not working now, so will have to try to figure that out.  (or ask Peter to help which is more likely to result in success!)

 

Kept it at 2000rpm for most of it, after running around 5 minutes at 2000 and with the oil pressure stable on around 3.5 bars I increased throttle to 4000rpm for around 30 seconds, then tested magneto drop individually.  Then back to 2000 for another few minutes.  Idle with throttle pulled back as far as it'll go is around 1800rpm, at that point oil pressure is reading just under 2 bars. 

At the end of the run the CHT's are showing around 75 degrees on one gauge and just on 50 degrees on the other.  I'm assuming the lower temp is on the front cylinder, will have to trace the wiring on that (and then label the gauge!)

I need to tighten up a few nuts in the oil system, there's a slow drip from the bottom nut and I think also the oil cooler connections.

 

The other problem is one of the main gear tyres went flat - it always had a very slow leak but now you can hear the air whooshing out of it when you take the pump off.  I reckon the valve stem has a leak where it joins the tube - will have to undertake the painful process of replacing that (it's a Savannah wheel which splits, I've heard they're painful!)

[Moneybox]

8 minutes ago, Marty_d said:

Pulled the plane out again this morning and did another start - thanks to @nomadpete the oil pressure gauge is now working.

I bought a small fuel tank (generally used for working on motorbikes when the tank is off) and suspended it on one side of the fuselage going into the wing tank inlet to the header, put a couple of litres in then switched the line to the return barb for running.  (Forgot to take a photo sorry).

 

Ran it for about 10 minutes.  Look like the oil TEMP gauge is not working now, so will have to try to figure that out.  (or ask Peter to help which is more likely to result in success!)

 

Kept it at 2000rpm for most of it, after running around 5 minutes at 2000 and with the oil pressure stable on around 3.5 bars I increased throttle to 4000rpm for around 30 seconds, then tested magneto drop individually.  Then back to 2000 for another few minutes.  Idle with throttle pulled back as far as it'll go is around 1800rpm, at that point oil pressure is reading just under 2 bars. 

At the end of the run the CHT's are showing around 75 degrees on one gauge and just on 50 degrees on the other.  I'm assuming the lower temp is on the front cylinder, will have to trace the wiring on that (and then label the gauge!)

I need to tighten up a few nuts in the oil system, there's a slow drip from the bottom nut and I think also the oil cooler connections.

 

The other problem is one of the main gear tyres went flat - it always had a very slow leak but now you can hear the air whooshing out of it when you take the pump off.  I reckon the valve stem has a leak where it joins the tube - will have to undertake the painful process of replacing that (it's a Savannah wheel which splits, I've heard they're painful!)

 

Split rims are usually easier, just make sure you release all the air, valve out, and break the bead before you split the rim. There will most likely be a rubber sleeve to prevent you pinching the tube on assembly however a tiny bit of air in the tube can help prevent that.

 

Ps: Inflate the tyre very carefully, they don't take much air.

Edited November 2, 2025 by Moneybox

[IBob]

Hi Marty......all sounds like progress!

FWIW:
My front CHT always read lower than the other one: probably inevitable given the position of the sensors.
I think Rotax now recommend initial warmup at 2200RPM, dropping back to 2000 after a few minutes. And my 912 always felt better at 2200 when cold.
And I'm not sure about 1800 as the min setting. Mine went lower than that, which was kinder to the engine/gearbox when finally stopping (no big clunk). I set my idle jet and throttle stop as per the Rotax manual, it's quick and easy and one of them was out on a new engine, so worth checking.
 

[facthunter]

The Motor has essentially NO Flywheel whatever and won't be Happy at Low revs particularly cold. The carbs are NOT Phased evenly.  Nev

[rgmwa]

Mine will idle down to about 1450, but I don't go below 1800 unless stopping the motor. It also helps to cut one magneto at about 1800 and wait two or three seconds as the motor slows before cutting the other one. It stops with less of a clunk. A 2000 rpm idle is a bit slow when the engine is cold. I usually run it a 2200-2300 initially. 

Edited November 2, 2025 by rgmwa

[facthunter]

Stop the Motor when running as slow as Possible. 1 Switch OFF wait, then Cut the second.  Nev

[skippydiesel]

47 minutes ago, IBob said:

Hi Marty......all sounds like progress!

I think Rotax now recommend initial warmup at 2200RPM, dropping back to 2000 after a few minutes. And my 912 always felt better at 2200 when cold.
And I'm not sure about 1800 as the min setting. Mine went lower than that, which was kinder to the engine/gearbox when finally stopping (no big clunk). I set my idle jet and throttle stop as per the Rotax manual, it's quick and easy and one of them was out on a new engine, so worth checking.

 

On cold start up, my 912ULS goes to 2500pm - 2700 rpm . I reduce the "choke" by 1/2 and the rpm comes back a little. After a few moments I remove all "choke" & rpm drops to about 1800 and I increasing, with throttle, to 2500 rpm.

 

As engine warms, rpm rises - I reduce throttle to maintain 2500 rpm, until oil temperature reaches 50C.😈
 

[Blueadventures]

5 hours ago, IBob said:

Hi Marty......all sounds like progress!

FWIW:
My front CHT always read lower than the other one: probably inevitable given the position of the sensors.
I think Rotax now recommend initial warmup at 2200RPM, dropping back to 2000 after a few minutes. And my 912 always felt better at 2200 when cold.
And I'm not sure about 1800 as the min setting. Mine went lower than that, which was kinder to the engine/gearbox when finally stopping (no big clunk). I set my idle jet and throttle stop as per the Rotax manual, it's quick and easy and one of them was out on a new engine, so worth checking.
 

Yep; they recommend 2200 or a little higher about 2300 to ensure smoothness.  Some engines can be a bit rough running at 2200.  This is mainly to reduce effect to gearbox plates.  (Also not ever mentioned is that rough running of engine results in the scuff marks on the carb piston slide sides, due to the carbs shaking.  Any marks are not detrimental to the carb performance.  So in short best after start to be at 2200 rpm minimum then after a few minutes increase to say 2500 to get to recommended lowest temp for take off.

[Marty_d]

After dragging the plane in and out of the shed a few times, I decided to construct a tow bar to make the process a bit easier.

 

Bent it up out of some 0.064" scrap, attached an old Bunnings table leg to it and put a cross piece on the end.  I thought it may have been too short, but just tested it and it works a treat.

Because it slides over the axle just inside the fork, it's easy to turn the plane too.

 

[planedriver]

Good job Marty

No leaning elbows on the table now, eh? 🤪

Gotta get our prorities right!

[Blueadventures]

3 hours ago, Marty_d said:

After dragging the plane in and out of the shed a few times, I decided to construct a tow bar to make the process a bit easier.

 

Bent it up out of some 0.064" scrap, attached an old Bunnings table leg to it and put a cross piece on the end.  I thought it may have been too short, but just tested it and it works a treat.

Because it slides over the axle just inside the fork, it's easy to turn the plane too.

 

I don't understand why builders and owners don't make or invest in a tow bar before first flight as it makes moving aircraft out and into hangar so much easier and accurate in the movement.  Well done.  Cheers.

[Marty_d]

Big thanks to @nomadpetefor spending the morning helping with engine runs.

On the first longish run the cylinder head temps did slowly come up but the oil temp stayed low.

We blanked off half the oil cooler and some of the water radiator with duct tape, on that run the oil temp gauge showed movement (at least getting to the 50 degree mark).

I have a list of things to correct, so will work on those before the next engine runs.

[skippydiesel]

Word of caution: Ground runs / engine temperatures are not necessarily indicative inflight performance. 😈

[nomadpete]

On 10/02/2026 at 11:08 AM, skippydiesel said:

Word of caution: Ground runs / engine temperatures are not necessarily indicative inflight performance. 😈

Point taken. Thanks. 

With outside ambient temp around 16 and engine only running at about 2500 rpm and cowlings off, we were happy to just get the head temperatures up off the stops. 

Accuracy of readings are yet to be confirmed. Realistic temps might be seen on full power ground runs.

[skippydiesel]

"Realistic temps might be seen on full power ground runs."

 

They may or not. 

I speak as someone with an experince, where satisfactory ground  temperatures, were not translated to flight. The first test flight was cut short due to excessive in flight temperatures.

We tried all sorts of cowling mods, ducting, etc with only minor success.

In the end, a complete cooling system rethink/design was sucessful.

In your favour, you are  building well know/tested aircraft. If you have not strayed to far from the plans, all should be well. If on the other hand you have allowed your inner experimenter to take hold, be prepared for the unexpected.😈

Edited February 11 by skippydiesel

[Marty_d]

Well the difficult thing there Skip is that the 701 plans only go to the firewall (plus the bed engine mount if you're going that way).

So engine setup, cowl and cooling is all experimental.

[facthunter]

For Most engines, running with the Cowls off unless Just very Briefly is a NO NO. A thorough warm up for Most engines is about 45 Mins in the AIR. An engine is best Left Inhibited till Put in normal  regular use. Nev

[nomadpete]

On 12/02/2026 at 8:09 AM, skippydiesel said:

They may or not. 

What part of "may" did you not understand?

[nomadpete]

On 12/02/2026 at 8:09 AM, skippydiesel said:

In your favour, you are  building well know/tested aircraft. If you have not strayed to far from the plans, all should be well.

Except, few builders will exactly copy an airflow tested cowling. This is no criticism of any builder. Least of all Marty.

Obviously, lacking a wind tunnel to assist airflow assessment, the best we can do is ground run, and reserve final judgement until actual real ground runs, and assuming promising results, actual takeoff. If we still harbour doubts, we can temporarily fit extra temp probes, and even airflow measurements. 

Affordable instruments are now at a level only dreamed of in the past.

[Marty_d]

My cowl is from a mould another 701 builder made. For me it was simply a means to get a semi streamlined shape from prop hub to firewall. There are quite large openings in the front, especially compared to many Rotax cowls, and an even larger outflow hole around the noseleg - so I'm hoping enough airflow circulates to work probably.

If I have difficulty managing temperatures I'll have to experiment with baffles and more targeted direction of airflow.

As soon as I work through the glitch list Peter and I noted, I'll start doing runs with the cowl on and see what difference that makes.

[BurnieM]

With airflow do not forget about the airflow coming out of the radiator and oil cooler.

The exiting air also needs to flow smoothly.

 

[skippydiesel]

Marty_d

"I'll have to experiment with baffles and more targeted direction of airflow."

 

Check out a few diffrent Rotax 9 instalatons. I think you will find that few, if any, have baffles (as in an air cooled engine).

 

Possibly oversimplifying:

Rotax oil/coolant heat exchangers facilitate locating these in a good air flow.

Inlet air ports need not be large however exit ait is all important. 

Your target undercowl temperature should be about  + 10C  above ambient or lower.

😈

[nomadpete]

Thanks for a number, Skippy.

As a starting point I plan to put a air temp probe under the cowl. And in the exit stream.

That will give an idea of what is happening. Any other ideas?