E-Props ...who has first hand experience with these

[skippydiesel]

MY apologies Kyle - I reviewed the video and see that I was mistaken - it is the same aircraft.

 

Rest of my observations stand.

[spenaroo]

can we all just agree that they win in the all important wank factor.
lets face it, at the end of the day being able to look back at the aircraft and have bragging rights important.
we are all a little vein. and there is the old adage, if it looks good - it'll perform good

 

[skippydiesel]

1 hour ago, spenaroo said:

can we all just agree that they win in the all important wank factor.
lets face it, at the end of the day being able to look back at the aircraft and have bragging rights important.
we are all a little vein. and there is the old adage, if it looks good - it'll perform good

 

Weeellll sort of. We now have plenty of aircraft in the light sport/RAA class, that are so sexy you might just want to date them but do they deliver the expected performance?? Some but most don't.

 

If you prioritise ramp appeal over performance (form over function), you will be super happy with the ultra sexy ones. They still fly of course,  but your expectations in the performance area -  stall speed, empty weight, fuel consumption & capacity, range, climb, & cruise speed may not quite come up to expectations.

[Kyle Communications]

Well I thought the usual one was "You can sell anyone a sh#t sandwich so long as it looks good"  "😇

[spenaroo]

yeah, I learnt it from a chat with my brother.
He wanted to buy a motorcycle, I had a couple spare at the time that I offered him full use of (just pay rego)
his response is that while he knew it was reliable and higher performance... it just didn't have the look he wanted when he saw his reflection in the window.

And he is completely right, at the end of the day we do this stuff because of how it makes us feel. if we were chasing numbers we'd jump on a commercial flight instead.

Edited August 25, 2021 by spenaroo

[skippydiesel]

28 minutes ago, spenaroo said:

yeah, I learnt it from a chat with my brother.
He wanted to buy a motorcycle, I had a couple spare at the time that I offered him full use of (just pay rego)
his response is that while he knew it was reliable and higher performance... it just didn't have the look he wanted when he saw his reflection in the window.

And he is completely right, at the end of the day we do this stuff because of how it makes us feel. if we were chasing numbers we'd jump on a commercial flight instead.

Really!!!!!?????

 

I guess it takes all sorts - give me function over form any time.

[spenaroo]

would you rather fly a spitfire or a cirrus jet?
would you rather have an Ferrari 308, or a modern hot hatch?

the newer stuff is far more functional and performs better....
but it doesn't have the same emotional appeal

Edited August 25, 2021 by spenaroo

[skippydiesel]

3 hours ago, spenaroo said:

would you rather fly a spitfire or a cirrus jet?
would you rather have an Ferrari 308, or a modern hot hatch?

the newer stuff is far more functional and performs better....
but it doesn't have the same emotional appeal

Fair comment - hard decision.

 

I would love to have a go in a Spitfire (even better would be a DH Hornet) but I suspect the reality would soon catch up (noise , vibration, fumes and uncomfortable seat) and the Cirrus Jet would be my preferred mount to actually  go places.

 

Cant say I give a toss about the car selection. One of the most fun cars I ever drove, was probably had the lowest HP - a "Bug Eyed Fright"  (Austin Healy Sprit) that I briefly owned back in the 1970's. The illusion of speed (your arse was only inched off the ground), the almost ridged rear suspension, making every ripple, pebble, pot hole a distinct memory, the wonderful precise (non power) steering, slick little 4 speed box, the mighty roar of the 1099 cc, 4 cylinder, twine carb, fanging through the Cotswold lanes at some horrific speed - peeeerfect!

[RossK]

13 hours ago, skippydiesel said:

Cant say I give a toss about the car selection. One of the most fun cars I ever drove, was probably had the lowest HP - a "Bug Eyed Fright"  (Austin Healy Sprit) that I briefly owned back in the 1970's. The illusion of speed (your arse was only inched off the ground), the almost ridged rear suspension, making every ripple, pebble, pot hole a distinct memory, the wonderful precise (non power) steering, slick little 4 speed box, the mighty roar of the 1099 cc, 4 cylinder, twine carb, fanging through the Cotswold lanes at some horrific speed - peeeerfect!

get yourself an early MX5, old british sports car dynamics, with japanese reliability, and one of the sweetest manual gearboxes you'll ever shift.

[RFguy]

I had an NA and NB MX5... 1992 and a 1998. they are sweet vehicles. the 1998 (NB) was much stiffer around corners. 

[Rastus]

Thread drift much?

[RFguy]

Thread is 10 pages long now. Drift is expected after 3 pages.

[RFguy]

The online comparisons on fixed e-props with other fixed brands yielding little 75% cruise difference, but many anecdotal reports of better performance over whole range  lead me to think the E props guys are doing some very clever controlled deformation over the airspeed/power range,

 

IE effectively finer pitch at lower speeds, coarse at higher speeds. It would not  be pitch that is changing precisely ,but  something very special and clever being done with the blade shape deformation, which might have a bit to do with the thin blade.

 

I say this because these techniques are being used in other industries (that have props) .

 

Maybe.

 

Edited August 27, 2021 by RFguy

[Bruce Tuncks]

Their advertising sure is good. But I suspect that many of the planes listed are Rotax things, and of course the geared down business is easy on the prop. What does the Jabiru factory say?

I like the idea of a flexing prop providing a bit of pitch change. Has this ever been done before?

[Kyle Communications]

Bruce they are developing and testing now a prop for the Jab engines. It has been a long time in development so I think they are doing something similar. I know with the Eprop if you push the throttle too fast on takeoff the prop can almost cavitate which means it does have some flex. This is why it seems to work more like a constant speed type. We noticed it with the very first prop I got in. After a couple of flights that was the comment "this is more like a CS prop the way it works"

 

[RFguy]

Bruce, I am not sure at all if that is what is happening - a complete guess on my part, nothing more, but its possible that's what they're doing.. 

 

With a sophisticated carbon 3D layup- they would be able to control the flex in 3D !

 
composite props compared wooden props as you know on the jab are much stiffer- (composite prop has ~ 5% better overall performance)  the wood jab props tend to deflect and lose efficiency, so flex is usuallly unwanted in terms of efficiency.

Edited August 28, 2021 by RFguy

[JG3]

JG3 you have a few errors ! 

TECHNICAL DATA FROM THE ROTAX BOOK

 

max continuous performance : 69kW at 5500 RPM.

fuel consumption at max continuous performance : 25 litres per hour.

**** specific consumption at max continuous performance is 286g/kW/hour ****

premium ULP caloric value ~ 46 MJ/kg, so translates to 13.15MJ/kW/hour, efficiency about 3600/13150 = ~ 27%

probably +/- 7 %

depending on how you value the weight of the fuel, the calorific value of that specific fuel etc

 

Yes, I realize that Rotax publishes 25 litres/hr, but I do not get anywhere near that consumption in real life. I get 20 litres/hr at max continuous of 5500rpm when pitched at 5800rpm WOT. Tested and retested many times and calibrated and rechecked my fuel flow gauge many times.

 

Long ago I did fuel flow calibration using a calibrated container and stopwatch at various power settings and airspeeds and t/o weights, several times over. These results were very consistent and matched the gauge readings. Those results were very interesting and I'll post those them on another thread presently. Then several times on Xcountry trips I checked that calibration on three hours legs and refilling from a metered bowser. A couple of months ago I did a 30.5hr trip and carefully logged all fuel used. Flying all the time at my usual leisure cruise at 5100rpm with prop pitched at 5800rpm WOT, which gives a speed of 80-83kts at 2500 QNH carrying my usual load, and fuel flow indicating 17 litres/hr. Total fuel used was 520 litres so that gives an average for the whole trip of 17 litres/hr. Can't get much better check than that.....

 

It's the real life results that I seek in all my testing. I take testing and analysis very seriously. I've studied the factors involved and tried to make all comparisons as valid as possible.

 

As for the thermodynamic efficiency, I concede that my estimate may have been overly optimistic.... That efficiency for car use on the road with varying speeds is usually estimated at 25% so I upped it for continuous operation and good Rotax design.... Maybe too optimistic.... So that means that even more waste heat has to be dissipated...... 

 

[RFguy]

Hi JG3
I agree with your analysis. It is thorough.

 

I guess if you only consume 20lph at 5500 in the slow cruise, you are pitched fine, not really making the prop work , and hence it doesnt take too much work by the engine to spin it at 5500.

 

That's an interesting point I guess. 5800 WOT (static?) you've lightly loaded the engine. sort of the opposite to say, 5000 WOT static

do you concur ? 

What's the fuel consumption at 5800 WOT static ? (or is that 5800 WOT cruise) ?

 

 

 

[Blueadventures]

9 hours ago, RFguy said:

The online comparisons on fixed e-props with other fixed brands yielding little 75% cruise difference, but many anecdotal reports of better performance over whole range  lead me to think the E props guys are doing some very clever controlled deformation over the airspeed/power range,

 

IE effectively finer pitch at lower speeds, coarse at higher speeds. It would not  be pitch that is changing precisely ,but  something very special and clever being done with the blade shape deformation, which might have a bit to do with the thin blade.

 

I say this because these techniques are being used in other industries (that have props) .

 

Maybe.

 

I recon it’s the resin type they are using that allows the blade shape to work.

Edited August 28, 2021 by Blueadventures

[JG3]

That's not static; it's 5800rpm wide open throttle straight and level, just as Rotax recommended and their charts are based on.

That puts me at 5200rpm at 60kts best climb, and 5100rpm for best cruise right at the top of the Rotax torque curve between 5000 and 5200rpm. Not pitched fine at all, just correct by Rotax recommendation.

I found static to be not a consistent base line for props comparison due to cavitation and whatever effects at no airspeed, straight and level far more consistent.

Fuel consumption at WOT straight and level is 26 litres/hr.

[facthunter]

On a  fixed pitch what difference you get static and cruise RPM with WOT depends on.the cruise speed..

  Good rule of thumb unless you are going above 130 knots approx  cruise, don't bother with constant speed. Weight, cost and complexity.. C/S prop blade shape is a compromise, (twist) in any case.

  3 blade is used where ground clearance and limiting tip speed are a problem.   In multi engines the feather feature is almost mandatory to achieve the engine out performance.. Nev

[RFguy]

Hi JG.

ahhh,  OK. not 'WOT static'

 

well if your cruise is 17 lph AND 5100 rpm,  (close to the 18 lph "75%" ) that's pretty close to where you want it I guess.

 

slap bang in the middle. Rotax book as max performance @ max RPM  =  consumption at 27 lph.

 

so its all pretty close I reckon !

 

 

 

 

[Bruce Tuncks]

I like nev's point about how tip speed and ground clearance considerations are the only reasons to consider a 3 blade prop.

If it was wings, the 3 blade is a triplane and the 2 blade a biplane. If the balance forces were not so big, we would see more single-blade props I'm sure.

[skippydiesel]

3 hours ago, Bruce Tuncks said:

I like nev's point about how tip speed and ground clearance considerations are the only reasons to consider a 3 blade prop.

If it was wings, the 3 blade is a triplane and the 2 blade a biplane. If the balance forces were not so big, we would see more single-blade props I'm sure.

 

It is always the ability of the prop to efficiently deliver engine power within the limitations of clearance & tip speed;

 

Should tip clearance (ground or fuselage) be a limiting factor, additional shorter blades, describing a smaller swept circle (increased clearance) may be required.

High engine powers, require larger props  that then may result in excessive tip speeds (less efficient)  and the above problem, so multi blades may be the solution.

High RPM direct drive engines (eg Jab) may require shorter blades to stay within effective tip speeds - the shorter blades may need additional blades to make best use of available hp.

 

 

[RFguy]

Skippy, assuming your are not considering a STOL plane, and not one that is factory rated for cruise > 130 kts, then it is highly likely the airplane will already be pitched for best efficiency at the specified 75% cruise speed, in which there will be nothing to gain from a CS or IFA prop as far as cruise performance is concerned.

 

The only gain would likely be, in my estimation would be on TO roll and climb.

I estimate, loosely based on prop curves and engine curves :

In TO roll and climb :

1) overall 5-7% higher engine power by being able to operate at peak engine output (dont do that on a Jabiru) .
 

2) 15% improvement in propeller efficiency on TO roll.

 

3) 10% improvement in propeller efficiency on Climb.

 

So, overall you might expect 20% on TO roll and 10-15 % on initial climb.

 

That might well be worthwhile if you have a marginal TO strip which as I understand it is your critical issue. 

 

As for higher cruise speed? just get out of bed 7 minutes earlier for each hour of flying....

 

Those numbers are for soemthign like a J230, rotate speed ~60 kts, cruise 120 kts. A 2:1 envelope. Something like a Faeta (33 kts rotate??, 125 cruise ????) 4:1 envelope the TO/climb improvements would be be greater

 

Edited August 28, 2021 by RFguy