10/23 Raptor Video

[wsimpso1]

Technical issues I noticed in the latest video:

1. Post intercooler air inlet temps at 190* going into the engine. It's been my experience that anything over 160* is reducing power significantly
2. An ambient air temp probe was placed downstream of the radiator to see air temps coming out of the radiator. During the run air temps dropped post radiator. That seems to me that there is no air flow through the radiator at all.

Decreased charge air temp lowers EGT's pretty much linearly. Increasing charge air volume also reduces EGT. As high as the EGT's are, I would expect any of improvements to be chased...

The lack of temp rise behind the radiator goes right along with the rapid linear coolant temp rise - it corroborates the theory that there is very little air flow through the radiator. That also corroborates that anything else stacked with the radiator is not getting much air flow either. Basically this is solid evidence that pressure ducts are needed from the inlet to the radiator and any other air cooled heat exchangers.

Do we know why pilot fuel is being used? All of my history and research says pilot stream of fuel is for two things: Reduces oxides of nitrogen emissions; Reduces combustion noise. In an airplane, we do not care about either of them, but it does mean the primary fuel usually needs to be advanced some because the pilot does speed up ignition a little. It does raise BSFC, which reduces power calculated from fuel consumption.

Billski

 

[pictsidhe]

Coolant was only at 165F. Thermostat hadn't opened...
The radiator was probably cooling down the air from the intercooler during the test...
There is a delay in ignition after injection starts. That can lead to sharp increases in pressure very similar to detonation. That noise is violent combustion. That stresses out the engine more than if the air is preheated a bit with the pilot. The main shot then has far less delay and burns more smoothly. It's almost all win.

 

[wsimpso1]

I understand that the pilot shot makes for less noisy combustion (who cares in an airplane) and makes for modestly lower stress and strain on engine, but anytime you raise pressure before TDC it adds to compression work, which is negative power that is almost made up for on the power stroke - the process is almost reversible, but not quite. The fuel for the pilot shot was used but ends up doing no useful work directly.

For best power, timing of the main fuel shot is supposed to be set to achieve peak pressure at TDC. Remove the pilot stream and yeah, you have to start the main fuel charge earlier but you can still program for peak at TDC. The tuners out there talk online about this effect and how they use it.

How would you know that this engine needs the pilot shot to make durability? I do not expect VW would tell you much about it, but maybe they did. And how well could that be reflected without instrumentation? A bunch of the tuners turn down or turn off pilot streams as performance mods. Are they all wet? They tune with combustion chamber pressure sensors and compete with each other on making more power... Since they have tools for tuning the pilot stream, if it benefited power, I would expect that some of them would leave it in.

How does speeding up the main charge burn help power? It does allow later injection, but if you adjust timing to give peak at TDC, the energy has been converted to hot high pressure gases at the very top of the power stroke for maximum energy recovery to the crankshaft. Without combustion chamber instrumentation, this engine may or may not be approaching ideal timing during takeoff and climb. That instrumentation would allow a direct read on real power, then accurate tuning.

One other comment is that the alpha is showing greater than unity at low power and around unity during the accel. Hmm, I have always understood that diesels were predicated on excess air if they and their turbos were to last. That too correlates with high EGT's and corroborates the issue we have been talking about with induction air flow being too small for the power.

Is the intercooler air-to-air? IIRC, it is and is stacked on the radiator, then destacking and using pressure ducts will help both coolant temps and induction temps... New pressure ducts are easy to make using male molds of styrene foam and then laminate with graphite-epoxy.

Billski

 

[Mike0101]

Very generically, the greater expansion ratio (16+) of diesels, also has a significant impact on EGTs. The Raptor engine is listed as 17:1 compression ratio. A modern gasoline turbo 4 banger will be in 9.5 - 10.5 compression ratio range by comparison. Don't think it's the case anymore, but diesel turbo turbine used to be made of lower temperature alloy...

 

[Andy_RR]

On an SI, high IAT is bad news. Diesels don't care so much, if raised EGTs are not a consideration.

Pmax and piston crown temperatures are a very large consideration for diesel engines. If you lose intercooling or run too high IATs you can easily melt your pistons! I've seen that happen first-hand on a pair of MAN diesels which left us bobbing about in the ocean. Not so much fun but at least the landing was smooth...

 

[Jsample40]

Bravo to the moderators for re-instilling a sense of civility, and a requirement for treating each other with sincere respect... regardless of our different approaches to solving the challenges we face in our aviation quests...
In the history of aviation, empirical scientist's like the Wright brothers and DaVinci (Non Degreed trial and error "engineers") have contributed a great deal, in spite of the naysayers, detractors and "dooms dayers" they each contended with along the long, lonely roads of aviation experimentation. The more I learn, the more I realize how much there is to know, and therefore how little we know.
Jay Sample

 

[pictsidhe]

Pmax and piston crown temperatures are a very large consideration for diesel engines. If you lose intercooling or run too high IATs you can easily melt your pistons! I've seen that happen first-hand on a pair of MAN diesels which left us bobbing about in the ocean. Not so much fun but at least the landing was smooth...

That was a somewhat snarky comment about EGTs. Peter believes that EGTs are fine up to the turbo's limit. If they were way below an engines limit, they would not be a consideration. Yes, peak temps and pressures have a definite reliability limit in any given diesel engine.
The EGT is one of several aspects to the Raptor engine that give myself and other petrolheads here cold shivers.
The Raptor's Audi has a cylinder pressure sensor in one of its glow plugs. It is not being monitored, though. It is a reasonable assumption that Audi put that in so the ECU can actively manage peak pressure and maximise performance without compromising reliability. They limit EGT to 1640F. From what Peter has said he has done with his tuning, is likely that peak pressure is well above the stock limit that Audi has established for the engine. We know the EGT is going several hundred degrees above the Audi limit. The TIT is being measured, so we have to guess a bit.

 

[Mark Z]

What would be a best glide speed for a Raptor pilot to shoot for if the engine failed? What is similar on a canard airplane?

 

[Marc Zeitlin]

What would be a best glide speed for a Raptor pilot to shoot for if the engine failed? What is similar on a canard airplane?

I don't remember exactly what the wing loading was on the Raptor - I think it was somewhere in the 20 lb/ft^2 range, which was approximately similar to the COZY MKIV at MGW.

My COZY best glide speed is about 80 - 85 KIAS, although the curve is actually pretty flat between 80 - 90 KIAS, so it's not critical to hit an IAS within a couple of knots. My canard stall speed, even at MGW but with a rearmost CG, is about 63 KIAS, and at full forward CG, a few kts faster than that.

What we don't yet know (one of the MANY things) is what Raptor's actual canard stall speed it (or whether the canard will actually stall before the main wing - no deep stall analysis has been performed, to my knowledge). So even if the best glide speed is in the 80 - 90 KIAS range, we don't know what the stall (or deep stall) margin is at that speed.

 

[donjohnston]

What would be a best glide speed for a Raptor pilot to shoot for if the engine failed? What is similar on a canard airplane?

I don't know that there's a "similar" canard airplane.

My Velocity XL-RG showed the best glide at 70kts. But that's too close to canard stall for me so I plan for 80kts.

 

[Mark Z]

My biggest fear is PM’s powerplant failure or the connection between the engine and the psru turning loose during his testing. The over temping alone is frightful. Hopefully we don’t see him come to a catastrophic failure that puts him into a dead stick (or worse) situation.
Being unfamiliar with the canards flying dynamics, I do understand the idea is that the canard is supposed to stall first to prevent the wing from stalling.
I have seen the F-16 do deep stall maneuvers and power out of them. Can one hold a canard in a deep stall and recover or will a properly rigged/designed not stall the wing at all?

 

[Marc Zeitlin]

... My Velocity XL-RG showed the best glide at 70kts. But that's too close to canard stall for me so I plan for 80kts.

That seems a bit low - I’d expect the XL to be about the same as the COZY. I’d be interested to see the Phase 1 glide test data for your plane. Plus, I should go back and look at mine to make sure I’m not misremembering

 

[Marc Zeitlin]

... Can one hold a canard in a deep stall and recover or will a properly rigged/designed not stall the wing at all?

if the plane is designed correctly, then as long as you are in the approved CG range, a deep stall can not occur. But the answer is GENERALLY no - if you get into a deep stall, it’s a VERY stable condition and pitch, roll and power do almost nothing to get out of it.

My hangar mate, Mike Melvill, is the only person I know who has actively recovered from a deep stall in a Rutan derivative canard aircraft (his Long-EZ - deep stalled on purpose with a VERY aft CG for testing purposes). He recovered by pumping the rudders is phase with the roll-yaw coupling frequency of the plane until it rolled over nose down, at which point he could fly out of it (at 1000 ft. Above the supermarket in Mojave, after a 12k ft descent).

Basically, if you deep stall, you’re going to ride it down at about 3K - 4K rpm in a flattish attitude.

 

[Mark Z]

 

[pictsidhe]

if the plane is designed correctly, then as long as you are in the approved CG range, a deep stall can not occur. But the answer is GENERALLY no - if you get into a deep stall, it’s a VERY stable condition and pitch, roll and power do almost nothing to get out of it.

My hangar mate, Mike Melvill, is the only person I know who has actively recovered from a deep stall in a Rutan derivative canard aircraft (his Long-EZ - deep stalled on purpose with a VERY aft CG for testing purposes). He recovered by pumping the rudders is phase with the roll-yaw coupling frequency of the plane until it rolled over nose down, at which point he could fly out of it (at 1000 ft. Above the supermarket in Mojave, after a 12k ft descent).

Basically, if you deep stall, you’re going to ride it down at about 3K - 4K rpm in a flattish attitude.

I've read enough horror stories about aft cg testing to think that having jetisonable water ballast in the tail is a good idea.

 

[HomeBuilt101]

>What we don't yet know (one of the MANY things) is what Raptor's actual canard stall speed it (or whether the canard will actually stall before the main wing - no deep stall analysis has been performed, to my knowledge). So even if the best glide speed is in the 80 - 90 KIAS range, we don't know what the stall (or deep stall) margin is at that speed.

So watching both the Raptor first flight and the tower cam videos it looks really scary...

The tower cam showed a huge amount of runway being burned up and a VERY shallow climb rate...plus a great deal of wing wagging too and fro...yikes to say the least.

At the time he did not have any tuffs on the canard nor did he have a camera with a view of the tuffs.

Does this look to you all that the Raptor is just so dang heavy that not only is the takeoff roll so long but it looks like the canard was riding the canard stall and the nose surged up and down.

Marc...what would your airplane do if loaded up to 3600 pounds?

Why is he moving the CG forward by taking out the weights from under the pilot seat...wait...scratch that...the copilot seat since the pilot is seated in the right side seat and moving the weight forward? That is counter productive as the CG moving forward will add more loading to the canard wing...

 

[Voidhawk9]

I've read enough horror stories about aft cg testing to think that having jetisonable water ballast in the tail is a good idea.

If you have a tail to put ballast in to begin with... I believe Nat Puffer installed a movable weight in the prototype Cozy (IV? Pretty sure it was the IV) to be able to move weight forward if needed. I don't recall it being used beyond perhaps an easy way to vary CG between tests.

Does this look to you all that the Raptor is just so dang heavy that not only is the takeoff roll so long but it looks like the canard was riding the canard stall and the nose surged up and down.

The elevator appeared to be fairly neutral through most of the flight, so it looks to me more like instability, rather than a canard stall. But, there are so many unknowns here...

 

[pictsidhe]

>What we don't yet know (one of the MANY things) is what Raptor's actual canard stall speed it (or whether the canard will actually stall before the main wing - no deep stall analysis has been performed, to my knowledge). So even if the best glide speed is in the 80 - 90 KIAS range, we don't know what the stall (or deep stall) margin is at that speed.

So watching both the Raptor first flight and the tower cam videos it looks really scary...

The tower cam showed a huge amount of runway being burned up and a VERY shallow climb rate...plus a great deal of wing wagging too and fro...yikes to say the least.

At the time he did not have any tuffs on the canard nor did he have a camera with a view of the tuffs.

Does this look to you all that the Raptor is just so dang heavy that not only is the takeoff roll so long but it looks like the canard was riding the canard stall and the nose surged up and down.

Marc...what would your airplane do if loaded up to 3600 pounds?

Why is he moving the CG forward by taking out the weights from under the pilot seat...wait...scratch that...the copilot seat since the pilot is seated in the right side seat and moving the weight forward? That is counter productive as the CG moving forward will add more loading to the canard wing...

If he loads the canard too much, it will refuse to fly. If it makes it much worse, it will likely show up straight away, he'll have several thousand feet of runway left to put it down on. Flying it after the drunken ground effect flights seemed unwise.
Covering up the wheel wells AND moving the cg for the same test isn't going to give a clear result.

 

[bmcj]

It would be cosmetically and structurally messy, but could you put an emergency variable incidence adjust or on the canard as a means to break a deep stall?

 

[Marc Zeitlin]

Marc...what would your airplane do if loaded up to 3600 pounds?

Take an awful lot of runway to get off the ground, even at SL on a standard day, assuming it could do so, and then climb like crap and fly slowly.

But I have about 100 ft^2 of wing, and the Raptor has a lot more, so the wing loading is approximately the same. And I have a 180 HP engine, so at 2175 lb (MY MGW) I've got a power loading of 12 lb/HP. The Raptor, at 3600 lb and say, 250 HP (what folks here seem to be estimating it has for actual output) is 14.4 lb/HP - not substantially lower. And if he is actually getting 275 - 300 HP, then the power loading is essentially the same as the COZY MKIV.

So I'd expect the performance, based on just these two ratios, to be vaguely similar to the COZY MKIV for takeoff roll, rotation speed and climb rate. But these are gross estimates, just to get in the ballpark. It's impossible to say what's actually going on here from the paucity of information that we actually have.