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Doggzilla

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I know I’ve heard that many times, but I suspect it’s a math error.

For instance, comparing speed of aircraft with different engine variants appears to be somewhere between the square and twice the square, but not anywhere near the cube.

For instance, compare the variants of the Gloster Meteor. The speed to thrust ratio is almost exactly squared.

And if it were cubed, swapping a 200hp to 300hp would only give a 15% speed increase, but it’s actually far more in real life.

Comparing a 200hp Mooney M20 to a 300hp shows a 25% increase in speed for that 50% increase in power. Much closer to square than cube.

Only if the aircraft is far larger and heavier does a 50% increase in power only produce 15% extra speed. For instance. The Cessna 206 has 50% more power and is 15% faster than a 172, but it is 1000lbs heavier and also much larger.

I can’t find any real world examples where power requirements cube. I am almost certain that it’s an error or misunderstanding.
 

wsimpso1

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More power is nice - it really raises climb rate and ceiling. If you can maintain power as you go higher (turbo-normalize the engine) you get lower and lower air density which reduces form drag at any given speed, which then allows substantial increase in true airspeed. But compare more and less power down low, and you do not get the huge differences in speed. You get the big speed improvement by going higher. Look at the 75% power speeds of fixed gear Cherokees from 140 to 235 hp - the PA28-235 gets off quick and climbs faster, but it does not cruise much faster down low. It can go higher and then cruise quite a bit faster. I am not making up the physics here, you can find it in the books under predicted performance...

With engines spinning props (or tires for that matter) at constant power, the thrust (or tractive effort) available goes down with speed. You only have so many ft-pounds/sec of energy available, and you divide by your speed in ft/sec to get the pounds of thrust available at that speed. This is all assuming your prop area and pitch are set to make that power available at those speeds. There are so many ways to have the wrong prop. So, available thrust drops off with the inverse of airspeed... When the thrust available equals the drag - you stop accelerating. Add a lot more power, go a little faster. Reduce drag with same power, go faster. Take the same draggy ship to higher altitude and you do have quite a bit less drag at any true airspeed so your true airspeed where drag and thrust become equal goes up.

Go to jets (or rockets) and the picture changes a bunch. Jets have about the same thrust at all airspeeds (assuming the engine can get a full dose of air). Nice feature. Some airplanes with fixed inlets sized for supersonic flight and diverging ducts (F-100, T-28/F5, and others) are air starved at the start of the takeoff run and have kind of anemic acceleration until they approach flight speeds. When they can begin to get enough air then acceleration picks up. With constant thrust, the drag going up approaches thrust at higher speeds than with props. That is why even the rather wimpy engines on the Me262 resulted so much higher flight speeds than the prop planes they were flying against. That is also why we do not typically talk power in jets, we talk thrust in jets. Take the same jet engine and put a power turbine on it to turn props or rotors, and we talk horsepower.

Want your RV4 to go faster? Swap the IO360 for an IO540. It will get off the runway faster and climb like crazy, but level flight down low will only be about 15% faster. Want it to go still faster? Reduce the wing area and tune the cooling air inlet, clean up other drag sources, which drops the total drag curve, and now it is significantly faster down low. Take that IO540 to 16000 feet go still faster ... RV4 to Harmon Rocket.

Billski
 

Vigilant1

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To have a useful discussion on this topic, it is important to first understand that thrust (a force) is not the same as power.

(Edited to add:. This post is overcome by billski's more thorough reply)
 
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davidb

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Empty weight of the final product will be the defining metric. It looks like they are well on their way to achieving 340kg. Anyone care to speculate about stall speed?
They claim 70 mph stall speed. So that’s about 90 mph/75 knots approach speed?

I spent a few minutes at their booth at OSH this year. I’d say they have a very good chance of meeting their performance goals. As pointed out, keeping the weight and drag down is key and they are certainly working smart to achieve that.

It should be flying early next year. The big questions for me are flight characteristics and comfort.
 

DaveK

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Anybody else notice the resemblance to Mike Arnold’s AR-6, but with a wider fuselage? Even has the minimum Formula I wing area of 66sq.
 
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Rik-

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I’ve been fascinated by this plane for a while. I don’t know how they are doing it but that’s not my problem.

There are a lot of variables in success. Two LSA’s, for example, with the same engine can have vastly different performance envelopes. So just looking at L/D/HP isn’t reflective of what’s possible nor reality.

So, when someone is designing a plane (yes there is weight, yes there is HP, but these are not totally reflective of reality) does everyone just open a NASA wing book like the yellow pages and choose a wing design? Or does someone say this looks good so it should work?

The saint Mike Arnold, proved that it’s more about design than it is HP. It’s more about everything being perfect down to the smallest detail.

So, from someone who doesn’t know jack **** about what’s a good design, vs a bad airplane design, what does everyone think of the overall design itself?
 

wsimpso1

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By their specs, It is a tight cockpit, modest baggage, high speed travel airplane that lands hot on retractable gear.

If history teaches us anything, after development, it will still be a tight cockpit, modest baggage, high speed travel airplane that weighs more, lands hotter, and is not quite so pretty.

I remain skeptical that they will actually be as light and strong in finished design, because weight always seems to drift up and CG drift aft, which then causes the engine mount to get longer.

Sounds like you better like speed more than you like comfort and bags on your trip. We have had Glasairs and Lancairs before. This is a little better on room and bags than them, but not by much. I wish them well, but I am not putting money down for a production slot.

Billski
 

Rik-

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I noticed that they haven’t designed the landing gear yet and thus is strange as with the cad software one would have to think they would had to design this into the fuselage already as they said they will be using a Cessna style landing gear.
 

BJC

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I’m pulling for them, but they have not been realistic in their assessment of what is involved in designing, building and debugging a totally new design.


BJC
 

BBerson

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Well, just like pants, airplanes should be sold in small, medium and large.
A small person can roll up the cuffs and wear big pants. Big guys with small pants not so much.
For some reason big guys want small airplanes.
 
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mcrae0104

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So just looking at L/D/HP isn’t reflective of what’s possible nor reality.
Sure it is.

Better airfoil? That's covered by L/D.
Less weight due to carbon or innovative structure? That's covered by L.
Less frontal area or cleaner aerodynamics? That's covered by D.
More power? Well, you get the idea.

Throw any design improvement you like at an airplane, L/D and power comparisons are useful for a performance claim credibility check. It's simply saying, if you claim X performance out of Y hp based on Z lb gross weight, you need to achieve such-and-such L/D. The reality check is then to see how favorably this compares to the best known performers. This plane could beat them all--but we need to wait and see.
 

Andy_RR

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I applaud the guys for giving it a go but it seems way too conventional in its design to be, as a package, anything spectacular in the end. I'm sorry that they are only beginning with a wind tunnel test program when they've already fully tooled the airframe. Like Peter's Raptor, I wonder whatever happened to engineering...?
 

BoKu

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Where will this plane fall short?

Speed
Stability
Strength
Control
Cost per unit all those things. They have adopted a needlessly complicated structural system that offers few discrete benefits. If they get lots of paid orders and buy lots of robots to build them, it might pan out. Otherwise they would have been much better served with a sandwich rather than cellular approach.
 

aeromomentum

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There are actually very good reasons for "conventional". The main reason is it works best. Especially for fast GA aircraft that have low induced drag and high parasitic drag fraction at cruise. The claimed cruise speed may be a little high but keep in mind it is a very small aircraft with a tiny wing. This helps reduce parasitic drag.

Running the numbers that they have on their website I get about 241mph at 8Kmsl and 75% power assuming they do just about everything right. Not bad at all.

But that small wing will have a high stall speed of about 77+mph clean and about 70+mph with good flaps. Approach speeds of about 91+mph. So it will be hot but not as hot as some others.
 
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