ROTAX 912 - VARIABLE PITCH

[johnm]

This is the thread I want

 

Some say a vaiable pitch on a Rotax 912 is ineffective (take off or top speed)

 

If you go fo a long fly - you just can't help thinking that if the blades were tweaked - something good might come of it ?

 

Anyone out there that wants to share their experiences

 

What efficencies can be gained - if any ??

 

 

[Guest High Plains Drifter]

johnm - if you mean an in flight adjustable prop, they are more suited to the faster airframes and wasted on the slower airframes. What aircraft are you thinking of putting it on.

 

HPD

 

 

[johnm]

hey HPD

 

I'm thinking high wing Tecnam Bravo - say 100 to 115 knots

 

This thread could even apply to a Jabiru - about the same cruise and power

 

I'd like to hear from someone who's got one of those variable 'wood comp' (or similar) props - what does it do for climb or cruise ??

 

Are they much chop (no pun intended)

 

JM

 

 

[Matt]

Hi John,

 

We had an in-flight variable pitch (manual operation, not constant speed) Kaspar prop on the Sportstar. I found it to be good for being able to operate the engine in it's 'sweet spot' throughout the various stages of flight i.e. take off, climb / descent & cruise.

 

It's operated much like a manual gearbox in a car and as a result adds workload to the pilot over a fixed or constant speed solution, particularly during turbulent conditions as the pitch can need to be varied regularly to maintain performance.

 

I'm not sure the variable pitch made much difference on peak cruise speed compared to a fixed pitch but it did offer more options to suit varying conditions and performance requirements i.e. runway length, airfield elevation, density altitude etc.

 

Having flown fixed pitch, manual variable and constant speed - my preference would be for constant speed (any day!), followed by fixed pitch and then manual variable (mainly due to workload).

 

Ultimately it comes down to what type of flying you'll be doing, what conditions (i.e. short field, "hot & high" etc.) and choosing a solution that meets your requirements.

 

Cheers,

 

Matt.

 

 

[Guest pelorus32]

Hi John,

 

Couple of comments: The Rotax guff for the 912 ULS shows it producing 100hp at 5800rpm. It produces corresponding less hp at lower revs.

 

On the Tecnams with the GT props it's hard to get more than 5000-5100rpm wide open. That says to me that the props are pitched fairly aggressively - in other words towards a cruise setting rather than a fine take off pitch.

 

However it is reasonably common to climb at 1000 fpm initial climb so they must be fine enough to maintain reasonable climb performance. Compared to the Pioneer 200 which climbs at a much flatter angle with the same engine.

 

What I'm getting around to is that I think that Tecnam have a pretty good compromise going on. Anecdotally the word is that you might get 3-5 knots in the cruise with an in-flight adjustable prop. That's not much for $3,000+ cost and the additional workload Matt talks about.

 

Regards

 

Mike

 

 

[Guest High Plains Drifter]

John, of the aircraft you mention, I've only flown the Jabiru and that was a fixed pitch wood prop.

 

One thing I'd keep in mind is that the variable pitch props are not as robust as the fixed pitch props. If you intend to go to airfields that may have long grass or woody weeds I would recomend a fixed pitch prop.

 

 

[Yenn]

I havn't flown with a variable pitched prop on a Rotax, but there is moree to it than speed. If you can coarsen your prop at cruise you should be able reduce fuel consumption, which means less refuelling stops on a long trip, less climb and descents also.

 

 

[PaulN]

My CT (fitted with Rotax 912ULS) was del'd with a 3 blade Airmaster AP332 constant speed prop with Warp Drive composite blades and elect controller, retrofitted by its previous owner. Top bit of gear ;). Ex factory it had the a 2 blade var pitch Neuform prop as seen on Ian's just departed CTsw. The only downside of the Airmaster in our case is its extra 12kg of weight on the nose needing ballast at the tail to achieve safe CofG.

 

Re operation, we have set the constant speed parameters as T/off pitch = 5500rpm; Climb = 5200; Cruise = 4800.

 

Now with T/off selected and with AUW of 435kg I barely finish applying full throttle and she's off the ground and climbing at around 1400fpm (+ or -), 300ft up I switch to Climb and by now the momentum is there and she continues the rapid climb. At 1000 agl I drop the pitch a little, set to Cruise and the climb continues comfortably at 800-900fpm and gaining speed. I don't really wish to try this, but the Airmaster has the advantage of feathering in case of engine failure thus enhancing the glide ratio. Also, I can select manual control allowing me to vary the pitch at will at any given rpm, but honestly, I haven't found this to be of any advantage with either speed or economy. That's with a careful watch of the ASI and FS450 fuel flow instrument. Maybe it's just me.

 

By way of comparison I reinstalled the Neuform for a period (107 hrs) and have to say that the Airmaster is way smoother and affords better flight control. I can't say that I have noticed any significant variation in fuel burn between the two. The Rotax is happier running at higher revs so in my view cutting the throttle (and manifold pressure) back to cruise at lower speeds (say around 90kt) the rpm holds at 4800 while the prop pitch fines up. Using the Neuform to cruise at 90kt the engine was back around 4000rpm from memory, a bit quieter though.

 

Another advantage of the Airmaster is on approach to land. With the pitch set fine it works a bit like a brake making for short steep finals and easy short landings ... and reacts really smartly when the throttle is pushed on again.

 

In short, I like the Airmaster's performance :) ... just wish it wasn't so heavy .

 

Hope this helps.

 

Paul

 

 

[johnm]

Thanks aviators

 

I think the early concensus is - costs some money - not much extra top end speed (if any ?) - better take off and climb - smoother ride - more drag when you want it - lower cruise revs so less gas

 

I do 2 x 300 nm trips in a Bravo (Roatx 912) every month - for the past 20 months - and have found that 5200 rpm seems the best setting. The 300 mile trip from go to wo is usually 3 hours - so I'm happy about how I get there and always enjoy whats happeing outside the window

 

I'll sit back and watch the subject 'props' develop - and maybe make a change when the ball stops rolling - if it ever does

 

The best thing about these forums is that the 'user' can comment - and can be heard by lot's of others (users and potential users)

 

JM

 

 

[Guest pelorus32]

G'day John,

 

I'd be interested to hear what you true out at with those settings and what the fuel burn is.

 

Regards

 

Mike

 

 

[facthunter]

In-flight variable pitch.

 

The point Mike makes is very valid , ie If you don't get the revs on take-off , you don't get the rated horsepower. All the performance (if the claims are anywhere near true) figures are based on the full horsepower figures.

 

You are compromised, if you fit a "climb" prop (Fine pitch), tends to over-rev at the fast end of cruise, or, alternatively fit a "cruise" prop (coarse pitch) where you will not achieve rated horsepower on take-off. ( at sea level etc.etc )

 

The efficiency (air nautical miles/ litre ) or some such measure has a lot to do with the lift/drag ratio of the total aeroplane, at certain speeds, and the best speeds (for economy) are a lot slower in some aircraft, particularly the overpowered ones, than we tend to fly them at. After all one of the reasons why we are flying is the reduced time of travel. It is also pointless applying a lot of power to an essentially draggy aeroplane. You run into a brick wall,( metaphorically),and just waste fuel.

 

If you require maximum take off performance, and cruise above say 120 knots you could need a variable pitch prop, otherwise just get a well suited fixed pitch unit. N...

 

 

[Bruce Robbins]

John,

 

Drop a line to flywanaka.co.nz.

 

I did a few hours af mountain flying with Wayne Allanson, their CFI last year in a Tecnam Bravo fitted with a variable pitch prop.

 

Wayne claimed that he could get a reliable 120 knots out of it on a cross country by getting "up on the step" and increasing the propellor pitch.

 

Cheers,

 

Bruce

 

 

[Guest pelorus32]

John,Drop a line to flywanaka.co.nz.

I did a few hours af mountain flying with Wayne Allanson, their CFI last year in a Tecnam Bravo fitted with a variable pitch prop.

 

Wayne claimed that he could get a reliable 120 knots out of it on a cross country by getting "up on the step" and increasing the propellor pitch.

 

Cheers,

 

Bruce

Right Bruce,

 

now that you've let that bit of info slip we want a FULL report on the mountain flying you did with these guys

 

Lots of interest around here in this course.

 

If you could just organise a full, detailed trip report to be on the forum within 24 hours that would be satisfactory :big_grin:;):big_grin:

 

Kind regards

 

Mike

 

 

[vk3auu]

I have an 80 HP 912 and it will rev to 5800 when straight and level at mid 80 knots, but on climbout just gets over 5000 but the rate of climb one up is around 1600 fpm and two up still over 1000. That is with a 3 blade GSC ground adjustable prop. I have also flown with a 2 blade prop with about 12 laminations from Richard Sweetapple and that prop was absolutely magic. It revd. to 5500 on climb, but still didn't over-rev in cruise. I can't remember what the rate of climb was, but it was better. The blade seemed to be very flexible compared to the GSC blades. I had to give it back though.

 

I suspect that a 912S pitched for cruise would still have plenty of grunt on climb in any of the aircraft thay we currently fly, so a variable pitch is probably a bit of overkill.

 

David

 

 

[Bruce Robbins]

Right Bruce,now that you've let that bit of info slip we want a FULL report on the mountain flying you did with these guys

 

Lots of interest around here in this course.

 

If you could just organise a full, detailed trip report to be on the forum within 24 hours that would be satisfactory :big_grin:;):big_grin:

 

Kind regards

 

Mike

Sorry Mike,

 

Guess you'll have to grade me "Unsatisfactory"

 

I didn't do the full Mountain Flying course that Fly Wanaka offer, but based on the abridged "taster" I did do, it would be money very well spent. Just the couple of hours I did were enough to pick up quite a few valuable tips, and instill a healthy respect for the conditions you can encounter flying around in and around mountains and valleys.

 

Even when sitting there looking sunny, clear and peaceful, the mountains can still bite!

 

I think I have some shots tucked away somewhere, will see if I can dig them out.

 

Bruce.

 

 

[Guest ROM]

The discussion above has all been on fitting adjustable and inflight variable pitch props to Rotax engines.

 

Has anybody fitted the Sensenich 2 blade ground adjustable prop to a Jab 3300 engine or an inflight variable or constant speed prop to the same engine?

 

If so what were the results and your thoughts on doing this?