Comment

Comment by Bob Hartunian on April 20, 2021 at 4:20pm

Geoff; The reduction in performance for normally aspirated engines is well known and we fly from Big Bear with all that in mind. My complaint is that the Zenith factory must have known that the 701 with the 912 engine does not perform safely at these higher DA airports but said nothing. It is a lower altitude plane with that engine and should not be flown high. I wrote a note to Sebastian Heinz but no response yet. I'm going to sell it to someone who flies at low altitudes.

Comment by Geoff Klestadt on April 20, 2021 at 3:12pm

Bob, a quick look at an old C172N handbook performance section indicates that, compared to sea level performance, at 7000ft takeoff distance is doubled and climb performance is halved. Unless you have a turbo charged or turbo normalised engine, I would expect that the 701 performance would behave pretty much the same because it’s a carbureted engine with a fixed pitch propeller.

The charts only go to 12000ft even though the ceiling is advertised as 14000. The Cessa power loading is about 14.3 pounds per horsepower. while your 701 is about 12.2, so maybe it is drag that is doing it. The 701 isn’t a streamlined design and I guess the margin between cruise and stall at 7000 ft isn’t that large for a 701.

Comment by Bob Hartunian on April 19, 2021 at 10:32am

With a higher hp engine, the plane would fly much safer with a better rate of climb. The 912 UL is just too underpowered for performance at higher altitudes. It would operate fine if you stay at lower altitudes but it is insufficient for takeoff at DA above 5000' at anywhere near gross. Zenith should make that clear to future builders flying out of high airports.

Comment by Keith Gempler on April 18, 2021 at 10:44pm

Well that is a huge bummer. Zenith advertises a “service ceiling” for the 701 of 12000’. Pretty sure that definition is 100 fpm at designed gross weight at that altitude on a standard day. 100fpm ain’t much. I need to look into this a bit as well, as I’m planning on being able to take a 750 STOL to my folks’ house in Colorado. Nearest airport ANK is at about 7500’. Thinking a 150hp Yamaha will be the combo for that. 

Comment by Bob Hartunian on April 18, 2021 at 10:30am

The Pulsar XP has a wing loading of 11.6 psf and the 701 has wing loading of 7.3 psf, so on paper, the Zenith should climb much better. But it doesn't which I attribute to drag of the design. Zenith requires much higher power to even modestly climb with some degree of margin. Had I known that, would have either opted for a more powerful engine or not chosen the 701 project with the Zenith recommended 912 engine.

Comment by Geoff Klestadt on April 18, 2021 at 5:13am

Bob, with respect, you might like to look at the wing loading of the 701 compared to the pulsar as well as the power loading.

Comment by Geoff Klestadt on April 17, 2021 at 7:05pm

Sadly Keith is right. The POH from Rotax says you are down around 58hp at 7000ft DA, So you don’t have much excess power to take off. A loooooong runway may help.

Otherwise you need much more power. Say at least 80 hp at 7000 ft DA. Even a 912ULS won’t give you that much at 7000’. You need a turbocharger if you sea level performance, plus you will still need more runway.

‘’It ain’t Zenith’s fault. It’s the laws of physics.

Comment by Bob Hartunian on April 13, 2021 at 10:26am

I also fly a Pulsar XP out of Big Bear with essentially same engine and the performance is completely different. Pulsar climbs at 700 fpm with passenger on 8000'DA day with no effort. Have been flying it for 17 yrs from this airport. The issue with the Zenith is the parasitic drag that requires more engine power than the 912 produces.

Had I known that, would not have built it with the 912. My reason for discussing this problem is so other high altitude builders won't discover the poor performance after years of construction. Zenith needs to inform potential builders about the limitations of the 701 with a 912 at high DA airports. If you fly from flatland airports below 4000', the plane will perform acceptably. But don't bring it up high.

Comment by Bob McDonald on April 12, 2021 at 4:05pm

You would be better served with a Rotax 914 engine. You need a turbo to deal with your density altitude. Talk with Rotax.  Your existing engine is and 80 hp unit which has had high compression pistons installed. Installing a Rotax 914 turbo & pistons will solve the issue and produce 115 hp at altitude. Its not the airframe its the density altitude.

Comment by Keith Gempler on April 12, 2021 at 3:32pm

Not 701 or Rotax specific, but at a field elevation of close to 7k’, on a 75 degree day, density altitude is around 9700’. Your 90hp engine is only producing 75% of that, about 67 hp. The prop is also less efficient at that altitude. You’re just not going to have much margin at that altitude. Imagine taking off at half throttle at a lower elevation, and that’s what you’re experiencing. Best to takeoff with much less fuel, and as early in the morning as possible before it gets hot. Get somewhere lower and fill up there.  I’d personally be very hesitant to change the design wing profile, hoping for an improvement there. In fact, your stall margin without the slats would probably be much smaller, and lead to a more abrupt and potentially less coordinated stall. That could be a recipe for disaster. 
Reduce weight. Fly when it’s cold. Keep speed up. Don’t make unnecessary turns.