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Raptor Composite Aircraft

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Jay Kempf

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You say current wife like you are working your way through a series. Agreed on capitalizing Wife and clairvoyance. It's uncanny how she always knows exactly how wrong I always am.
 

cheapracer

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It's uncanny how she always knows exactly how wrong I always am.
Never absolutely sure, but I think I might have been right when I find it's seamlessly blended into a completely new argument ...


Why is wife capitalized?
She's not, she's Communist, only you Americans are Capitalized.
 

jet guy

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I agree 20 seconds is kind of long.

Here is a clip of me flying the old P210N in the Texas STOL comp.

I am taking off from grass and am in the air in about 10 seconds from a dead stop! The plane is light with 1POB and ~200# in fuel. I estimate around 3200# gross. I am 310HP takeoff power.

As its comp flying I am yanking the plane off the ground as soon as it can fly so Vr is probably only 55KIAS. Still 10 seconds is a long long way from 20 seconds and grass is harder than smooth asphalt.
Thanks for the video. That's a very good short takeoff.

If we crunch the numbers we get takeoff acceleration and net thrust force that is very much in line with the other airplanes already analyzed, namely Hammer's Saratoga and the SR22.

Looking at the video it may be more like 11 seconds from brake release, and perhaps your weight estimate is a little heavy, since the Centurion 2 only weighs 2,400 empty, and I assume you weigh somewhat less than 600 pounds!

So here is what results if we assume empty weight of 2,800 and an 11 second run to a liftoff speed of 60 knots, which is 101 ft/s.

The average acceleration will be 101 ft/s / 11 s = 9.2 ft/s^2

Average thrust will be 9.2 ft/s^2 * 2,800 lbm / 32.2 = 800 lbf

Takeoff distance will be 101 ft/s * 11 s / 2 = 557 ft

So that is excellent takeoff performance, especially from grass. We see that the average net thrust is a little higher than the Cirrus and a fair bit higher than the Saratoga.

If we now look at the CLmax achieved by your wing at rotation speed we find 1.9. Again that is somewhat better than Cirrus and a fair bit better than Saratoga.

CLmax at Vr = W / (Vr / 1.2)^2 / rho / S = 1.9, with wing area S of 175 ft, and ISA SL air density rho of 0.00237 slugs/ft^3.

The Cirrus gives a rotate CLmax of 1.88 with half flaps, and the Saraoga 1.67 but has a little bigger wing. The Cessna has a bigger wing too, plus very big Fowler flaps, so its takeoff performance is going to be the strongest out of these three airplanes, as we see in Lelievre's 500 ft grass takeoff.

Now we can take a good look at the Raptor takeoff performance. This airplane doesn't have flaps and has 167 ft^2 of wing area. From the Saratoga without flaps we get a rotate CLmax of 1.06. This is perfectly in line with typical design numbers for Vr without flaps. The flapless wing will only make a maximum CL of about 1.3, so obviously you don't want to rotate at the edge of stall [in normal circumstances]. Since Vr is typically 1.2 Vs [as we see in the above equation] the flapless Vr CL is going to hover typically around unity.

Peter's latest video gives some more data, which we didn't have before. At about the 4:20 mark, we see this airplane wants to rotate at about 92 knots.

We had worked out previously the Raptor's takeoff acceleration and net thrust, from the 20 seconds it took to go from 20 knots groundspeed to 79 kt...video here from 3:40 to 4:00.

Even from that rolling start at 20 knots, this airplane managed an acceleration of only 5 ft/s^2, about 50 percent lower than the SR22 and 30 percent lower than the Saratoga. The P210 has nearly twice the measured acceleration of Raptor, 9.2 ft/s^2.

With that acceleration number [which is actually giving Raptor a break, since he was already at 20 knots when we started measuring], and now knowing Raptor's rotate speed of 92 knots we find his takeoff roll is going to be a whopping 2,400 feet and over 30 seconds.

Takeoff distance Slo = 155 ft/s^2 * 3,400 lbm / 32.2 / 2 / 526 lbf = 2,417 ft

That 526 lbf being Raptor's measured net acceleration force [thrust] over that 20 second run from 20 to 79 knots. Again, this is cutting Raptor a break, since the net acceleration will decrease both as we lengthen the takeoff run, and when we start from brake release.

Now add a passenger or two plus a little more fuel and the Raptor is going to take 3,000 ft to get airborne. And what happens when the density altitude goes up?

This airplane is a complete farce. He needs an honest 350 hp at minimum, preferably 400 in order to come anywhere near what is acceptable and safe takeoff performance.

That kind of power is possible even from that Audi engine [Edit: if built properly], but how reliable that will be is another question entirely.
 
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jet guy

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A minor point, Marc Z. has let us know that the website is incorrect, and that the actual wing area is 190 ft^2.
Thanks. It's good that the wing is a little bigger, considering the very high empty weight and the very low power available.

The added wing area means that his 92 knot rotation speed is going to give a wing CL that is slightly lower, 0.93. Let's also bump the weight to 3,500 since that's what his last video seems to show.

These factors change the numbers slightly: his average thrust goes up by 16 pounds, to 542 lbf, but his takeoff run remains the same.

F avg = 3,500 lbm * 5 ft/s^2 / 32.2 = 542 lbf

CL @ Vr = 3,500 lbm * (155 ft/s / 1.2)^2 / 0.00237 slugs/ft^3 / 542 lbf = 0.93

Slo = 1.44 * 3,500^2 / 32.2 / 0.00237 / 190 / 0.93 / 542 lbf = 2,417 ft

If he didn't have prop strike concerns he could rotate more aggressively at perhaps a few knots lower speed. His V rotate CL could easily be in the 1.1 range as it is on other airplanes in no flaps takeoffs, like the Saratoga or the Cirrus. But we see from that video that he really does not have much room to rotate the required 12 to 15 degrees. Someone mentioned earlier that Peter claimed to have measured only 9 degrees of rotate prop clearance. That's why he's seen in that liftoff video going to 92 knots.

This airplane could be literally transformed if you simply bolted in a Continental IO550. There is a good chance this would shave maybe 300 or even 400 pounds of his firewall weight, which I believe is about 900 pounds, with all those heat exchangers, the iron block engine, twin turbos, heavy reduction drive etc.

That weight reduction, combined with the increase in power from his current 200 hp [I gave the Raptor a lot of breaks with the numbers; an honest accounting would show about 200 hp], would at least bring this airplane into line. Also the big sword hanging over the test program is when this engine is going to let go, or the PSRU.

And that's even before we get into the control and stability flight physics, which has me very worried, since every indication is that this was a slapdash effort [like just about everything else on this airplane].

So many issues, so little knowledge.
 

TurbAero

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With that acceleration number [which is actually giving Raptor a break, since he was already at 20 knots when we started measuring], and now knowing Raptor's rotate speed of 92 knots we find his takeoff roll is going to be a whopping 2,400 feet and over 30 seconds.
If he didn't have prop strike concerns he could rotate more aggressively at perhaps a few knots lower speed. His V rotate CL could easily be in the 1.1 range as it is on other airplanes in no flaps takeoffs, like the Saratoga or the Cirrus. But we see from that video that he really does not have much room to rotate the required 12 to 15 degrees. Someone mentioned earlier that Peter claimed to have measured only 9 degrees of rotate prop clearance. That's why he's seen in that liftoff video going to 92 knots.
From what I understand, that 92 knots "rotate speed" is where it was calculated that with no elevator input, the nose-wheel would fly off by itself and the aircraft would "self-rotate". It has been shown in his videos that this is not the case and it does not self-rotate at 92 knots with the current cg, but the nose-wheel does look close to lifting off at that speed. In his "flight" video, the aircraft is still flying at 82kias (with whatever errors he has in the pitot static system). If he was to use elevator input to rotate rather than using the self-rotate method, there could be a lower than 92 knots lift off speed with a commensurate reduced take-off roll (prop strike angle permitting).

I think that most of us agree with your suggestion that fitting an IO550 would really help this airframe.
 

BBerson

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Brake drag increases ground run and suppresses self-rotate. I don't think the Tri-Pacer I soloed in would ever self-rotate.
 

lelievre12

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So clearly Peter's idea is nonsensical. And what about the accuracy of his fuel flow numbers to begin with? Does he actually have proper fuel flow instrumentation installed? The only way to accurately measure fuel flow is by means of a small turbine wheel, like those used in the Shadin systems. Most light airplanes don't come with those; they simply work by measuring fuel pressure, which is not accurate at all.

Is this how Peter's aftermarket ECU comes up with those fuel flow numbers? Some kind of algorithm that reads fuel pressure and then spits out a flow number? Anyone familiar with elementary fluid mechanics knows that pressure actually has nothing to do with flow, which is a function of flow velocity, density and flow cross section area. Which is why everyone flying a Bonanza has at some point bought a Shadin with a proper little wheel to measure flow.
Its hard to argue with your acceleration logic when it takes so darn long for the Raptor to get to Vr.

However I dont agree that the Audi TDI BSFC is wildly out from the 0.38/lb/hr range, for reasons I laid out before.

What satisfies both issues is the point you raised about FF measurement. The OEM ECU method is rock solid, however the Motec system needs to be calibrated as the Bosch diesel injectors are unusual for this ECU. The calibration procedure is pretty complicated (including fuel temp/viscosity) and if PM made any errors then it's quite likely that indicated fuel flow is incorrect. This would explain the lack of power as PM is basing his estimates on FF alone.

As you say, the inclusion of a traditional flow meter would cross check the Motec calibration (injector time and injection pressure) however with a Diesel TDI, the supply line AND the injector return line need to be measured so that fuel consumption can be calculated. Not sure if Garmin can handle those calcs or not.

By the acceleration calcs, a rough number of 220HP at the prop seems likely. If we add a (generous) 20% losses for ancillaries and PSRU would see around 275HP at the flywheel (which BTW is very close to the max Audi rating for this engine). If we plug in the 0.38lb/hp/hr BSFC we get an actual FF of 15GPH not the 20GPH reported by the Raptor Garmin. Is such a large error possible? Yes. However I haven't watched every single video so I am not sure whether PM has done any totalizer checks to verify the Motec or not. If not, then a 30% error in the indicated FF would explain a lot as we examine the lethargic acceleration.
 
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jet guy

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Its hard to argue with your acceleration logic when it takes so darn long for the Raptor to get to Vr.

However I dont agree that the Audi TDI BSFC is wildly out from the 0.38/lb/hr range, for reasons I laid out before.
The problem is Peter isn't using the 0.38 SFC. He's using 0.32, which is absolutely impossible at 4,000 rpm. That gives him his supposed 330 hp on 15 gallons.

If he was using 0.38 that same 15 gallons would give him 276 hp, which is still way more than his acceleration numbers support.

There's just no way around that acceleration number of 5 ft/s^2. Your own airplane on grass is nearly double that, and the SR22 is 1.5 times better, even by the conservative book values.

Many here may not be aware just how much problems that mismatched turbo system is causing. By running the exhaust into a second turbo that is the same size as the first is simply choking that engine. The flow DOUBLES in volume after going through that first turbo.

If he took the second turbo off his engine could breath and he might actually see 275 hp and 0.38 SFC as we expect from that kind of engine with 2 bar boost.

But he's making about 200 hp at the flywheel, at most about 225 [but I doubt it]. The acceleration doesn't lie.
 
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wsimpso1

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By the acceleration calcs, a rough number of 220HP at the prop seems likely. If we add a (generous) 20% losses for ancillaries and PSRU would see around 275HP at the flywheel (which BTW is very close to the max Audi rating for this engine).
So much speculation and guessing and linearizing accelerations... Well, lets's do it closer to reality. 23 years doing automatic transmission design, I can speak with some authority about how much power is lost in accessories and gearing systems. 20% is crazy high. 4% is more like it. Let's get to specifics:
  • An 80 amp alternator at full power is 80A*14.5V = 1160W, or 1.55 HP. Even if the alternator is only 85% efficient, that means at full power the alternator is pulling less than 2 hp from the engine, and at more nominal draws probably half that;
  • Water pump will draw no more power than the alternator and maybe quite a bit less;
  • The AC pulls based upon its size and running speed, but when the throttle is pushed full forward for takeoff power, the AC clutch should be disconnected, so no AC draw is likely. Yeah, maybe PM did not program the disconnect at WOT, but let's assume he is that smart ;
  • Gear systems running in oil on fully warmed up gearboxes rarely lose more than 1% per mesh, and belts have similar 1% losses. Even saying the mechanism for passing the governed oil flow to the controllable prop should not lose but a fraction of 1 HP;
Total losses to run the accessories, spin the belts, and power the prop are no more than about 6 hp or around 2-3%.

Just as a reference, a modern automatic transmission with an oil pump, several sets of gear meshes and around half of the clutches/brakes open and dragging with oil running through them, they routinely are better than 96% efficient on power in vs power out when the torque converter clutch is engaged.

Now as to what the power and BSFC really is for the Raptor, well, you are all guessing. Yes, the takeoff runs are taking too long and too much runway for him to be making claimed power as we can tell by comparison to surrogate airplane performance. Once we admit to ourselves that they are guesses, when will we stop talking about it?

Billski
 

231TC

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The AC pulls based upon its size and running speed, but when the throttle is pushed full forward for takeoff power, the AC clutch should be disconnected, so no AC draw is likely. Yeah, maybe PM did not program the disconnect at WOT, but let's assume he is that smart ;
Beside the point of your post, but I wouldn't bet on him being that smart. Somehow the AC managed to kill the engine at low throttle during taxi. I didn't follow it closely but I think it's an electric compressor rather than engine driven, and drew more juice than available when it kicked on. One would expect that to be an easy thing to make idiot proof, but nope, the solution is to immediately pull it back to idle when you notice it happening because the idle setting revs it up when the AC kicks on, but there is no such protection at slightly above idle. So anyway, it wouldn't surprise me at all if there's no AC cutoff at full throttle.

And while I'm going on about throttle, completely unrelated to the HP guessing or the AC, but the throttle by wire control is still a disaster waiting to happen. It failed during taxi a couple weeks ago (defaulted to idle) so he "solved" that problem by eliminating the comparison potentiometer, now running on a single pot with no check or redundancy. So when it does fail again, rather than defaulting to WOT as it should, it could potentially be some random value. But don't worry, he's "keeping an eye on it." I get the impression that means if it doesn't fail again soon, it's considered fixed.
 

Scheny

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As you say, the inclusion of a traditional flow meter would cross check the Motec calibration (injector time and injection pressure) however with a Diesel TDI, the supply line AND the injector return line need to be measured so that fuel consumption can be calculated. Not sure if Garmin can handle those calcs or not.
I didn't think so, but apparently it is possible (excerpt from G3X manual):
The GEA 24 and GSU 73 also have provisions for a second fuel flow input (FUEL FLOW 2) to use in aircraft that require a second fuel flow sensor for differential fuel flow measurement. If both fuel flow inputs are configured, the displayed fuel flow will be FUEL FLOW 1 (feed) minus FUEL FLOW 2 (return).

As for losses due to oil:
Just as a reference, a modern automatic transmission with an oil pump, several sets of gear meshes and around half of the clutches/brakes open and dragging with oil running through them, they routinely are better than 96% efficient on power in vs power out when the torque converter clutch is engaged.
Billski
Fun fact: the german dual clutch automatic transmissions are always marketed as 100% efficient (due to no torque converter losses), but the oil pump takes up to a few hp for a bigger engine. So, putting this in relation to a torque converter, there is no efficiency gain. There are also dry clutch versions, but they tend to get broken after 60.000km (~40k miles) if you have lots of stop and go and are not considered exchangable.
 

AdrianS

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So much speculation and guessing and linearizing accelerations... Well, lets's do it closer to reality. 23 years doing automatic transmission design, I can speak with some authority about how much power is lost in accessories and gearing systems. 20% is crazy high. 4% is more like it. Let's get to specifics:
  • An 80 amp alternator at full power is 80A*14.5V = 1160W, or 1.55 HP. Even if the alternator is only 85% efficient, that means at full power the alternator is pulling less than 2 hp from the engine, and at more nominal draws probably half that;
  • Water pump will draw no more power than the alternator and maybe quite a bit less;
  • The AC pulls based upon its size and running speed, but when the throttle is pushed full forward for takeoff power, the AC clutch should be disconnected, so no AC draw is likely. Yeah, maybe PM did not program the disconnect at WOT, but let's assume he is that smart ;
  • Gear systems running in oil on fully warmed up gearboxes rarely lose more than 1% per mesh, and belts have similar 1% losses. Even saying the mechanism for passing the governed oil flow to the controllable prop should not lose but a fraction of 1 HP;
Total losses to run the accessories, spin the belts, and power the prop are no more than about 6 hp or around 2-3%.

Just as a reference, a modern automatic transmission with an oil pump, several sets of gear meshes and around half of the clutches/brakes open and dragging with oil running through them, they routinely are better than 96% efficient on power in vs power out when the torque converter clutch is engaged.

Now as to what the power and BSFC really is for the Raptor, well, you are all guessing. Yes, the takeoff runs are taking too long and too much runway for him to be making claimed power as we can tell by comparison to surrogate airplane performance. Once we admit to ourselves that they are guesses, when will we stop talking about it?

Billski
Welcome to my world ;) We (dynamometers) measure power applied to the roller, but customers want us to tell them flywheel power. $@$#%#%

With Raptor, the big unknown is brake drag.
We know the brakes are dragging when taxiing, but not how much. So estimating power from acceleration has a big unknown variable.
It is just a guessing game at this point.
 

Rod Schneider

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Beside the point of your post, but I wouldn't bet on him being that smart. Somehow the AC managed to kill the engine at low throttle during taxi. I didn't follow it closely but I think it's an electric compressor rather than engine driven, and drew more juice than available when it kicked on. One would expect that to be an easy thing to make idiot proof, but nope, the solution is to immediately pull it back to idle when you notice it happening because the idle setting revs it up when the AC kicks on, but there is no such protection at slightly above idle. So anyway, it wouldn't surprise me at all if there's no AC cutoff at full throttle.

And while I'm going on about throttle, completely unrelated to the HP guessing or the AC, but the throttle by wire control is still a disaster waiting to happen. It failed during taxi a couple weeks ago (defaulted to idle) so he "solved" that problem by eliminating the comparison potentiometer, now running on a single pot with no check or redundancy. So when it does fail again, rather than defaulting to WOT as it should, it could potentially be some random value. But don't worry, he's "keeping an eye on it." I get the impression that means if it doesn't fail again soon, it's considered fixed.
The A/C compressor is belt driven by the engine..
 
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