Jab twin

[Peter008]

http://www.jabiru.net.au/images/Jabiru_TwinWhy_South_Africa.pdf

 

I found this quite interesting.

 

 

[facthunter]

True. Minimum change to achieve twin configuration Props close together. Two six cylinder engines would sound nice.Nev

 

 

[Patrick Normoyle]

Which engine will fail first ! Typical jab hater comment !

 

 

[Peter008]

I was waiting for the first negative comment even if it was in jest!! It is starting to be similar to the Holden - Ford war of words amongst the fans....... I shake my head at the passion put into some of the posts about Jab engines, both for and against.

 

I do like the twin however. Wonder how it would handle.

 

 

[Patrick Normoyle]

I would love to give it a go, and yes Peter, it was in jest and thanks for recognising that. P.S. please send us some warm weather.

 

 

[Peter008]

Was below 30 deg here again today. Still a bit cold for the locals to endure. Should warm up soon but we may lose the great flying weather that winter brings.

 

On the bright side, the warmer weather will send all the Grey Nomads home and the roads will be less congested.

 

 

[Deskpilot]

Nice BUT........I struggle to get out of an LSA55 and I doubt I could bend my legs sufficient to avoid those throttle 'levers(?)'. I bet a few of them will get bent.

 

 

[facthunter]

I agree. Having two push-pull stalks like that looks ridiculous and your knees are gong to connect with them. Single quadrant in the middle should have been better, then something needs to be done about the centre control. A point of contention with Jabs anyhow( as I see it) Having a twin engine plane with no feathering capability is a bit performance limiting as far as engine out performance is concerned. IF it won't fly on the remaining engine It wpn't be certified at that weight, and the weight that it would fly on one may be very low. It still might be OK for training as the engine is not feathered in a training situation. It is set at a RPM that approximates the engine being feathered performance. It would need rudder trim and a bigger fin/rudder area. Nev

 

 

[PKnight]

It's some years since I flew a twin but I recall that all the skill is in flying the aircraft with one engine out of operation. As the props will both rotate clockwise (from the pilot's view) the left engine will be the critical engine in all phases of flight. So the questions that will have to be explored will include issues such as whether or not the aircraft can maintain a positive rate of climb at MTOW with a left engine failure and will there always be sufficient rudder authority to overcome the tendency to turn to the left if the left engine has failed, particularly at low speed with a non feathering prop while climbing. Some serious twins cannot always achieve this in the gap between take-off speed and best climb speed.

 

The feature that really helps the proposed design is that the two engines are comparatively close together and near the aircraft centre line. This means that the amount of asymmetric thrust from a failed engine, even the critical engine, may be modest and easy to overcome with Jabiru's reasonably powerful rudder. In level flight I would have thought that the amount of asymmetry was sufficiently small that it did not require a rudder trim. If your leg starts to get stiff land the thing and work out why the engine stopped!

 

Best of luck with this innovation. Nothing ventured; nothing gained.

 

Peter

 

 

[rocket1172]

"......then something needs to be done about the centre control. A point of contention with Jabs anyhow( as I see it)"

 

Please explain?

 

 

[winsor68]

Video says "First flight". What is with the pax?

 

 

[facthunter]

Rocket. It's a pretty questionable dual control. Nev

 

 

[Deskpilot]

Rocket. It's a pretty questionable dual control. Nev

Please explain. Lots of small planes have a central stick, don't they?

 

 

[facthunter]

The key word is "dual" control. There's ONE stick and your arm has to be right behind it. Gives the instructor a bit of difficulty. If we place the dual throttles in the centre on the dash, both pilots have easy access to them. This is the "normal" way, rather than having two sets of "organ stops " near each door. Perhaps have a Y shaped control stick? You need to place these things carefully. I'm highlighting the problem. Not giving all the answers..

 

Twins can be more to manage than a single. Nev

 

 

[winsor68]

IMO it has real potential to fulfil the stated design brief perfectly.

 

 

[bexrbetter]

Ahh they are Australian trying to move forward, good on them.

 

 

[rocket1172]

I love my central stick and have had plenty of people comment on it....in the Sav that is

 

 

[facthunter]

I love the central stick (In a TANDEM) or single seat.Nev

 

 

[winsor68]

I too reckon the central stick is generally the superior cockpit configuration.

 

 

[damkia]

I would wonder about the prop tip vortices interfering with each other and giving some strange aerodynamics over the fuselage and wing root.... Not an expert at all but just curious as to others views.

 

 

[facthunter]

Some twins have an option of having opposite direction motors. This means a lot of different spares have to be carried Contra rotating is best and that way there is no critical engine. Having the engines forward and closer together ( as has been pointed out) is better because there is less assymetric effect. Mosquito and Beaufighter aircraft had that feature. All multi engine prop aircraft would have some interference effect but any out of sync is worse than any thing else. The L-188 Electra had "PHASE" synchronisation where not only did they run at a synchronised speed but the prop blades passed in a selected relationship with. each other. Clever Eh!. Nev

 

 

[damkia]

I was thinking also about possible "pulse" damage to the prop tips from from two prop tips passing each other in close proximity at greater than Mach 0.5 giving a supersonic "boom" to the tips, affecting both the structural integrity of the tips, and also interfering with the thrust generation. Approx prop tip velocity 815km/h at 60" dia x 2850 rpm if my calcs are correct, equaling tips passing at 1600+km/h

 

 

[facthunter]

Compressibility is only related to airflow. Sound is a different matter. I have never heard of any problems but there could easily be vibrations in thin sheet causing cracks etc Fabric wing skins vibrate lots.. nev

 

 

[Keenaviator]

I would wonder about the prop tip vortices interfering with each other and giving some strange aerodynamics over the fuselage and wing root.... Not an expert at all but just curious as to others views.

With 2000' per minute and an easy cruise of 110 at 2200 rpm indicates to me that there is no problem with weird aerodynamics :) PS, I too like the centre control stick and throttle between the legs. Looks strange but my arms are on the side of my body and I find these controls ergonomics to be very comfortable. It can be exciting when the passenger kicks the throttle though!

Laurie

 

 

[alf jessup]

With 2000' per minute and an easy cruise of 110 at 2200 rpm indicates to me that there is no problem with weird aerodynamics :) PS, I too like the centre control stick and throttle between the legs. Looks strange but my arms are on the side of my body and I find these controls ergonomics to be very comfortable. It can be exciting when the passenger kicks the throttle though!Laurie

Laurie,

 

Just a question on the 2200rpm at cruise, I thought Jab like their engines to be run hard like 2900-3100 so how would the 22oo rpm go you think in the twin?

 

Alf