Raptor Composite Aircraft

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Voidhawk9

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I'd like to know what a typical Velocity XL sees for wheel weights. With the engine being 150-200 pounds heavier than a 540, it seems Raptor may need more than 100 pounds of ballast up front unless something in the layout is a lot different from an XL.
Those were empty weights, right? Remember the pilot sits well forward of the CG, so it will move forward in a flight configuration.
 

pictsidhe

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These were generated using BW's own tool with air density correction. I will concede they are close to the edge of their capability from a mass flow perspective.
Aha, a magical online tool, it seems about right. I can only assume that along with Peter, you used it incorrectly. Garbage in: garbage out.

It needs the numbers fudged to use it for staged turbos. You have to calculate the engine size that it thinks it sees before the HP compressor. If you aftercool down to the LP intercooler output temp, that is the HP stage PR. Using the turbine tool also needs fudging.

I had a quick play with some rough fudge factors. I failed to download maps with points on, so here are the URLS:

LP
HP

Points 1,2, and 3 are 3800rpm 250h, 300hp and 350hp. The HP turbo needs the altitude fudged, so I couldn't put those on the same map.

Notice that these are not small turbos. Look at the turbine sizes the tool has spat out, bigger than the compressors. This is normal in big genset diesels, where they don't get loaded until they hit operating RPM. probably the most similar ground application to what an aero engine wants. These are first approximation results, I'd need to run some proper numbers to proclaim actual sizes. But they are in the ballpark.

If you don't understand the theory behind some calculator tool and be able to do some rough sanity checks, you are extremely likely to end up with a Raptor sized engineering error... Part of my scientific and numerical aptitude is that I sanity check my own numbers, using an alternative method. I catch a lot of mistakes doing that. Mistakes happen. It doesn't need to be exact, it just needs to be in the same ballpark. When you size turbos over a factor of two too small, you have dropped a pretty big clanger that will strangle your engine. If someone who has done similar engineering that you are attempting the first time says check your numbers, then check your dam numbers until you find out why!
 

FarmBoy

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Aha, a magical online tool, it seems about right. I can only assume that along with Peter, you used it incorrectly. Garbage in: garbage out.

It needs the numbers fudged to use it for staged turbos. You have to calculate the engine size that it thinks it sees before the HP compressor. If you aftercool down to the LP intercooler output temp, that is the HP stage PR. Using the turbine tool also needs fudging.

I had a quick play with some rough fudge factors. I failed to download maps with points on, so here are the URLS:

LP
HP

Points 1,2, and 3 are 3800rpm 250h, 300hp and 350hp. The HP turbo needs the altitude fudged, so I couldn't put those on the same map.

Notice that these are not small turbos. Look at the turbine sizes the tool has spat out, bigger than the compressors. This is normal in big genset diesels, where they don't get loaded until they hit operating RPM. probably the most similar ground application to what an aero engine wants. These are first approximation results, I'd need to run some proper numbers to proclaim actual sizes. But they are in the ballpark.

If you don't understand the theory behind some calculator tool and be able to do some rough sanity checks, you are extremely likely to end up with a Raptor sized engineering error... Part of my scientific and numerical aptitude is that I sanity check my own numbers, using an alternative method. I catch a lot of mistakes doing that. Mistakes happen. It doesn't need to be exact, it just needs to be in the same ballpark. When you size turbos over a factor of two too small, you have dropped a pretty big clanger that will strangle your engine. If someone who has done similar engineering that you are attempting the first time says check your numbers, then check your dam numbers until you find out why!
Ahhhh - I was wondering if there was a trick to the tool regarding staging. I did manage to fudge some things that weren't obvious but I didn't completely understand the issue with the LP turbo - makes sense now. It looks like the turbo I picked could still work ok for HP. It may be best to optimize for the 3000-3500 RPM range though - and 3700 is probably the highest I would bother with unless I was modifying other components in the 3.0TDI.

Yes, the LP is NOT small! Given the even smaller turbos that Raptor has and its poor IC setup, I'm surprised it is running as well as it is. I warned Peter to at least program an EGT limit - I hope he at least heeds that one small piece of advice as it could prevent a really bad consequence in the air (not that blowing the engine on the ground is great but at least it would be on the ground).
 
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pictsidhe

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That's an S500 sized turbine. S500s are not small, or light. No surprises there. It is probably going to be easier to root in a catalogue for a suitable genset turbo or parts than try and kludge aftermarket parts together. The HP side is not as unbalanced as I first thought. The low TIT (which was a guesstimate) to the LP turbo means that the pressure ratio across the compressor and turbine are very similar. The HP is still going to need a much larger than normal turbine than it's required compressor would usually use.
 

flyboy2160

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... Part of my scientific and numerical aptitude is that I sanity check my own numbers, using an alternative method. I catch a lot of mistakes doing that. Mistakes happen. It doesn't need to be exact, it just needs to be in the same ballpark. .....
Yes, sir! One of my pet peeves nowadays is Googlehead computer jockeys who just push the buttons and believe whatever the computer spews out. You need some alternative hand calc to get a ballpark number. An engineering professor I had said "The first rough calculations you do are the most important. The answer only has to be the correct order of magnitude and one significant digit. You have to exercise 'engineering judgement.' What are the primary principles and equations? Which are secondary or lower order concerns? I want to know how much a rocket to Mars will weigh to the right order of magnitude and one digit. 60 tons or 600 tons or 6000 tons or ?
 

pictsidhe

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Somebody posted some Raptor test data on youtube. I attacked it with a calculator. LP turbo is 50% efficient, thought the mass flow I calculated points to the 60% choke line. The HP compressor is in the efficient zone. I calculated a 538K (510F) HP outlet temp and 50% effective intercooler.

I also, out of interest, had a look at what would happen with one turbo removed:
EIT down 75F at the same mass flow but boost dropped from 3.15 PR to 2.8. The EGT should drop a healthy amount too with the backpressure of a single turbine running more efficiently. As an added bonus, that's a good few lb out of it's fat butt. It's still going to suck at altitude, but not as badly. 25k is not happening without different turbos.
 

Marc Zeitlin

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I'd like to know what a typical Velocity XL sees for wheel weights. With the engine being 150-200 pounds heavier than a 540, it seems Raptor may need more than 100 pounds of ballast up front unless something in the layout is a lot different from an XL.
I can't say what a "typical" Velocity XL sees, but I can tell you what the XL sample POH shows, and what the XL that I've done a W&B on shows. The POH indicates:

Nosewheel Arm: 31"
Nosewheel Weight: 40 lb.
Main Gear Arm: 144.6"
Main Gear Weight: 1600 lb.

Total Empty Weight: 1640 lb.
Empty CG: 141.6"​

So the CG is about 3" forward of the axle, hence the weight on the nose and no need to kneel like EZ's.

The XL CG box is 127" to 134".

On the plane I weighed:

Nosewheel Arm: 32"
Weight on Nose: 113 lb.
Main Gear Arm: 143.1"
Weight on Mains:1758 lb.

Empty Weight: 1871 lb.
Empty CG: 136.4"​

Now, the issue here is that other than some VERY rough comparison, this doesn't tell you much because we have no clue where the Neutral Point of the Raptor is, how the main gear/nose gear arms relate to the Neutral Point, nor where the CG range of the Raptor SHOULD be (or are projected to be). But with a empty weight 71% more than this Velocity XLRG (and 95% more than the factory XL's supposed weight as shown in the POH), having 300 lb. on the nose gear isn't crazy out of range. Move the main axles a few inches aft, while keeping the NP of the airplane in the same place and there will be way more weight on the nose gear while not changing the aerodynamics one bit from whatever it already is.

What may be problematic (other than the zillion other things we've beat to death) is takeoff rotation - Velocities generally need a bit more speed than run of the mill EZ's to rotate, because the main gear was moved aft a bit to allow them to sit on all three and not need to kneel when empty, but that affects rotation speeds. Without knowing a lot more about where the supposed NP of Raptor is, where the main axle FS is, and what the CG range is supposed to be, we're just waving our hands around here. Could be that the calcs are fine - could be that they're complete nonsense, like they were on the Cobalt Valkyrie.

As an aside, Mike Melvill moved the main axles on his Long-EZ aft 1" to allow him to let the plane sit on the nose when empty without retraction. Although he's got over 4700 hours on the plane (I've got about 5 hours in it), he kind of regrets moving the axles, as he says it definitely increased his takeoff rolls from the other Long-EZ's he's flown with equivalent power.
 

drstress

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Yes, sir! One of my pet peeves nowadays is Googlehead computer jockeys who just push the buttons and believe whatever the computer spews out. You need some alternative hand calc to get a ballpark number. An engineering professor I had said "The first rough calculations you do are the most important. The answer only has to be the correct order of magnitude and one significant digit. You have to exercise 'engineering judgement.' What are the primary principles and equations? Which are secondary or lower order concerns? I want to know how much a rocket to Mars will weigh to the right order of magnitude and one digit. 60 tons or 600 tons or 6000 tons or ?
Still waiting for that hand calculation for the aircraft neutral point.
 

BJC

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There is an important omission in the calculations for the "Constraint Analysis". Put there on purpose to see what would happen. All of the calculations are simple "hand, closed-form" calculations any high school algebra student should easily follow. Since no one commented on any of them, I think either no one bothered to look through them or no one has the expertise to understand them.

Either way this is my last post. I'm sure the "good ole' boy" club will pointlessly blabber on and on about any and all of my attempts to bring a small amount of order to what has to be the most random assortment of opinions I have ever experienced.

Since I don't fit in to this culture, I will welcome it as a badge of honor to be exorcised from HBA.
Still waiting for that hand calculation for the aircraft neutral point.
Still waiting for your last post here.


BJC
 

Richard Schubert

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As far as the neutral point goes, the issue is the huge amount of area ahead of the cg due to the 61" wide fuselage. The cozy MKIV had the recommended canard length reduced due to stability concerns from the wider fuselage that it had compared to the long-eze.
 

rv6ejguy

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In looking at the fin area of an XL vs Raptor, it looks quite a bit less on Raptor which also has a lot more area forward of the C of G. That won't aid directional stability either. I guess we'll see soon if it flies.
 

Marc Zeitlin

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As far as the neutral point goes, the issue is the huge amount of area ahead of the cg due to the 61" wide fuselage. The cozy MKIV had the recommended canard length reduced due to stability concerns from the wider fuselage that it had compared to the long-eze.
Minor nit - it wasn't stability concerns that led to the reduction in canard span of 3" per side - it was deep stall susceptibility. With the longer canard span, the aircraft could deep stall fwd of the rearmost CG limit. Rather than move the CG range forward and slightly compromise takeoff distance, Nat chose to shorten the span of the canard and move the deep stall susceptibility out of the approved CG range. The fuselage area fwd of the CG is not only destabilizing, but it's also non-linearly so, so it's hard to predict what's going to happen.

Some folks have severely extended the noses of Long-EZ's and Variezes - they like the looks, apparently - and having flown a couple of these, it tends to take a plane that used to be a 1-2 on the Cooper-Harper scale to a 4. Not unsafe, by any means, but less fun to fly :).
 

Marc Zeitlin

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In looking at the fin area of an XL vs Raptor, it looks quite a bit less on Raptor which also has a lot more area forward of the C of G. That won't aid directional stability either. I guess we'll see soon if it flies.
According to folks intimately involved in the early Long-EZ testing, Burt used the Varieze winglets on the first Long-EZ. It was perfectly happy to fly sideways, until they increased the winglet area in (apparently 5 - 6 steps) to what it is now, where it's very happy and directionally stable. Even geniuses don't necessarily get it right on everything the first time :).
 

pictsidhe

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According to folks intimately involved in the early Long-EZ testing, Burt used the Varieze winglets on the first Long-EZ. It was perfectly happy to fly sideways, until they increased the winglet area in (apparently 5 - 6 steps) to what it is now, where it's very happy and directionally stable. Even geniuses don't necessarily get it right on everything the first time :).
Geniuses know that they may not get it right first time!
Well, that idea makes me feel a whole lot better about the size of my 'round file' ;)
 

rv6ejguy

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On the Raptor YT channel today: A fellow posted rotation speeds for the Velocity which were quite a bit higher than what Peter was projecting for Raptor (70 knots). Peter commented that Raptor has more wing area (167 vs. 145 for Velocity). Peter seems to have forgotten that his design weighs over 1300 pounds more than a Velocity. Wing loading calcs are the simplest math you ever do in aircraft design, yet he fails here... Scary.

The latest setback was severe nosewheel shimmy at about 49mph. He confidently feels a damper will fix this and has a design drawn up. Another 2 week delay. I'm doubtful this will fix the problem looking at the layout he has. He went on to say at 6:09 this should be the last mechanical problem to fix after declaring the redrive issues solved. I don't think so. There hasn't even been a single 5 minute full power run done yet to have any confidence in the propulsion system.

Several people commented on the ailerons moving up and down when going over bumps in the runway. Wonder what will happen there in flight with the wings flexing?
 
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BBerson

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So, what would happen if a brief liftoff with far aft CG was attempted? Would the main wheels hold up the aft end until liftoff and then suddenly pitch up when the wheels are free of the ground?
 

Kyle Boatright

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So, what would happen if a brief liftoff with far aft CG was attempted? Would the main wheels hold up the aft end until liftoff and then suddenly pitch up when the wheels are free of the ground?
Presumably, the canard would stall at some point and you'd get the inherent canard nose bob cycle.
 
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