912is performance?

[FastEddieB]

WAY out of my ken, but...

 

...rather than mass, per se, are we not dealing with drag instead?

 

Leaving mass out of it, isn't it drag that increases as the square of velocity, needing an equal increase in thrust?

[chanik]

Math is awesome. BTW, you were right the first time. Drag force is proportional roughly to V^2 but the power needed is F*v so v^3

[Ed Cesnalis]

Math is awesome. BTW, you were right the first time. Drag force is proportional roughly to V^2 but the power needed is F*v so v^3

 

Ahhh, I had a moment of clarity but was unable to defend it :huh:

[FastEddieB]

Math is awesome. BTW, you were right the first time. Drag force is proportional roughly to V^2 but the power needed is F*v so v^3

 

Are you saying that to double the speed you need eight times the power?

 

That does not sound right. Let me check out my trust Aerodynamics For Naval Aviators and get back.

[FastEddieB]

From the horse's mouth:

 

"For example, if an airplane in steady flight is operated at twice as great a speed, the parasite drag is four times as great but the parasite power required is eight times the original value."

 

I stand corrected.

[Ed Cesnalis]

From the horse's mouth:

 

"For example, if an airplane in steady flight is operated at twice as great a speed, the parasite drag is four times as great but the parasite power required is eight times the original value."

 

I stand corrected.

 

 

I get it, you need 4 times the power to equal 4 times the drag as well you need twice the power to achieve twice the speed. To do both at the same time you need 8 x power.

 

So back to bigs, he needs first 44% more power to equal additional drag plus he needs 20% more to account for the additional speed. So now my guess is he needs 100 x 120 x .2^3 = 172.8hp engine at 75% to go 144kts.

 

looking at my chart, weight would become a huge factor over 150kts which would require 1 more factor increasing required power to achieve targeted speed. Also over 150kts I'm sure you would introduce drag reducing speed mods that would offset the need for additional power.

 

100hp 120kts

173hp 144kts

237hp 160kt

400hp 190kt

616hp 220kt

800hp 240kt

[S3flyer]

So now my guess is he needs 100 x 120 x .2^3 = 172.8hp engine at 75% to go 144kts.

 

I love it when math meets the real world. The Diamond DA40 is a dynamically slick aircraft with a 180hp engine. Max speed is 157kts and 75% cruise is 150kts. Compares favorable with the 180hp Tiger with LoPresti speed mods -- cruises at 143kts (if I remember correctly).

[Roger Lee]

Ed's,

 

Keep this complicated math and engineering solutions up and you'll have to go out and buy more brain cells and I don't think they make them for your ages any more. They were discontinued a long time ago.

:)

[mocfly]

I heard the following statement which I feel fits the bill perfectly.

"It is better to be silent and be thought a fool rathern than speak(post) and confirm it.

 

Nough said!

 

[FlyingMonkey]

 

 

 

I get it, you need 4 times the power to equal 4 times the drag as well you need twice the power to achieve twice the speed. To do both at the same time you need 8 x power.

 

So back to bigs, he needs first 44% more power to equal additional drag plus he needs 20% more to account for the additional speed. So now my guess is he needs 100 x 120 x .2^3 = 172.8hp engine at 75% to go 144kts.

 

looking at my chart, weight would become a huge factor over 150kts which would require 1 more factor increasing required power to achieve targeted speed. Also over 150kts I'm sure you would introduce drag reducing speed mods that would offset the need for additional power.

 

100hp 120kts

173hp 144kts

237hp 160kt

400hp 190kt

616hp 220kt

800hp 240kt

 

Sounds very correct. A Grumman Tiger will do about 140-145 knots on 180hp, so it sounds like the right ballpark.

[Ed Cesnalis]

Dan,

 

If the cooling systems have thermostats than the cooling from the extra fuel can be negated. An extra gallon or more per hour will provide cooling otherwise.

 

Bigs, everything is a compromise and your injected motor will lean at the expense of cooling, not a bad thing overall.

 

Saying it doesn't save fuel by leaning is oxymoronic. If you are using 20% less fuel its because you are using less fuel. Thats a lot of fuel savings, if you filled a 5 gallon bucket with it you couldn't even pick it up!

[Jim]

Kinetic energy is the energy of motion. The formula for Kinetic energy is 1/2 M x V squared. That is one half of the mass of an object times the square of the velocity.

 

For non-relativistic speeds only. Just saying...

[FastEddieB]

As a data point, my Tiger did about 132k.

 

Loved that little plane.

[GlennM]

fuel efficiency is not due to running lean, its due to running at a computer-real-time corrected ratio

 

S3Flyer posted the video from the spokesman at Rotax that mentioned the 912is runs lean (1.05 lambda) less than 96% or so of full throttle. That means the computer is correcting the fuel flow real-time to a ratio lean of peak and that is the reason for the fuel economy differences.

 

I am happy you put these little zingers in your posts. It has done wonders for my interest in this forum.

[Ian]

.......I am happy you put these little zingers in your posts. It has done wonders for my interest in this forum.

 

and there was me thinking you Americans didn't do irony

[Ed Cesnalis]

The 912i does not LEAN at the expense of cooling. carbs are inefficient and make fuel/air based on crude vacuum and bernoilli effect. the air/fuel roughness is the result of both the pilot , air density and the lack of optimal carb settings as time goes by.

 

all of that goes away with computer controlled fuel injection with active air and fuel pumps delivering metered mix to each cylinder in real time. the computer knows what the air density is, and what the cylinder temp is when it mixes up the recipe.

 

THERE IS NOT WAY A HUMAN can duplicate this process. nor is there a way to get the optimal fuel/air mixture delivered to EACH cylinder using a manifold.

 

it is PURE BALONEY to suggest that the fuel injected engine runs any hotter or colder based on fuel injection. the fuel economy and added power to weight in the 912i are achieved by the computer and the design of the fuel delivery via the rails and injectors

.

 

There you go again.

 

Its well known in the world of aviation that fuel is cooling, even more so in the world of Rotax. We both live in the lee of the same mountain range so lets compare. My 912ULS flies WOT utilizing the main fuel circuit and employs no leaning by design. I plan on 6 Gal / hour, your expected plane will burn far less flying the same route. even at WOT because your system will employ leaning and mine does not, unless the needle circuit has partial control at altitude as some contend.

 

You are again alleging a metered mix to each cylinder? Didn't Roger already explain that your mixture is delivered to a common rail?

 

Manually adjusting mixture is not a super human task as you would suggest. That is why planes are equipped with a EGT gauge that permits you to fine tune your mixture.

 

No baloney here, an extra gallon or two / hour has a cooling effect that would take thermostats to negate, just a fact.

 

And again you close by claiming extra power to weight when the reverse is true. You will have no more power but you will have additional weight which amounts to subtracted power to weight.

[markmn]

I started this thread and it seems I have more flying experience comparing the 912i and 912 carb than anyone here so will give some real world experience.

 

Rotax shows the 912i has lower horsepower and torque than the 912 ULS except at full RPM. My real world experience mimics this. Takeoff and climb performance are slightly reduced but full cruise speeds are very similar. The rotax performance charts I looked at showed approximately 10% lower numbers at takeoff RPM and thats about what it feels like.

I really could not tell any difference in smoothness between a well balanced carb and the fuel injected and you wont be able to either. Obviously we wont have to balance carbs on the FI.

As far as fuel economy at full power settings the FI is about 10% better. At lower throttle settings (like half) is where you will see the 30% savings. Not going to help me alot because only a small percentage of my time is half throttle. I am a 120Kt get there sort of guy and the 10% savings at that setting is only half a gallon per hour.

 

I have not noticed any differences in operating temperatures.

 

As many of you know I am a sales rep for Tecnam and i appreciate you all letting me post here. I have learned alot here over the years and I hope I can occasionally give back.

I dont really see a big advantage to one engine over the other at this point. I believe in selling customers what they want. That said we have ordered most of our new aircraft with the FI engine but I wouldnt fault anyone who decided to choose the ULS. Obviously rotax offers both for a reason.

 

Now when rotax comes up with a 120hp fuel injection it will be a different story!

 

Mark

[Ed Cesnalis]

Mark,

 

I suspect WOT at altitude will show good fuel savings because the bings don't go there leaning wise.

[markmn]

Will not know until someone takes one up to altitude and reports back. I have not done that.

Next time i go up I will climb up there and see what the fuel flow is at WOT.

 

The bings will partially lean at altitude but not as much as they should for optimum efficiency.

 

I have been told the 912i runs a richer mixture at the WOT setting but I have been told alot of things about the 912i that have turned out to be untrue!

Its called marketing!

 

Mark

[WmInce]

"I started this thread . . . . . . different story!"

 

Mark

 

Thank you Mark.

[FastEddieB]

Thanks, Mark.

 

The most objective analysis I've seen.

 

Fuel injection has the potential to provide some benefits - it has become the standard on the vast majority of cars and mootcycles for that reason.

 

But carbs have been around for a long time and have over a century of development and refinement. IOW, they're not exactly dog meat either.

 

BTW, my general experience is that modern fuel injection systems are, for the most part, stone reliable. Carbs are simpler, but a little more finicky. But both remain clever and elegant ways to get gas and air together in the right proportions. But we better get used to fuel injection - it is the future.

[Doug G.]

Shouldn't that be, "It's the way of the future...the way of the future...the way of the future..."

[FastEddieB]

Grin.

 

To paraphrase the old joke...

 

As you climb out, your fuel injection computer talks to you through your headsets.

 

"Good Morning. My name is Fiona, and I'm your engine's fuel management computer. I will take care of every aspect of fuel ratio, making sure that ratio is perfect - not only for the flight conditions and power setting, but I will even fine tune it for every cylinder. Now, just sit back and relax. You have nothing to worry about...worry about...worry about...".

 

Thanks for reminding me of that hoary old joke. My mom told it to me nearly 50 years ago!

 

 

[Ian]

....... its because they squeeze more horsepower and more fuel efficiency out of an engine. ..........

 

Rotax says the engine gets better gas mileage and it seems those who have tested the engine both on a dynamometer and in the air report that it's all true.

 

You're like a dog with a bone !

 

Despite all the evidence you are still saying "more horsepower" - cobblers!

 

The one thing you probably have right is better fuel economy

 

You know the nice thing about banging your head on a btick wall? It's when you stop

[FlyingMonkey]

As a data point, my Tiger did about 132k.

 

Loved that little plane.

 

That's probably a more realistic real world number, though I have heard several Tiger owners claim 140kt+ flat out. Seem everybody that has flown one loves it. If I was not a Sport Pilot the Tiger and Cheetah would be on my short list.