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SuperCharging a RV-7 = Speed & Power

G3i Ignition

G3i Ignition

Well Known Member
Well Russ was out putting time on his supercharged 7 today. We have the blower drive ratio set-up fairly mild @ 33.5? MP for take-off with a field elevation of 5020MSL. Boost pressure being totally controllable. Just some quick numbers recorded through his GRT-4000/ EFIS. At 7500 MSL, 8400 DA, OAT 56f, Throttle control set at 85%, 2450 RPM, 30?MP, FF @ 14.3gph, GRT showing 100% calculated engine power, 186kt true. He was up @ 10,000 MSL, 2450RPM, 29.7?MP truing out @ 194kts. Same conditions when the 7 was aspirated, the average was, throttle control all the way in at 100%, 2450 RPM, FF10.8, GRT showing 76% calculated engine power, 165kt true @ 7500MSL. By the way?.G3i Series?2 ignition system is firing Slick 4300 series mags. More to come.
 
Impressive numbers for sure...buuut..

...do you have any estimate of the impact on engine life running this much power over a long period of time will have. After all, we're now running -7's at Rocket speeds:)

Any concerns about beating the crank bearings out?

BTW...your engineering and work look top notch!

Thanks,
 
Am I correct in assuming running the "boost" at 29.7" would be same as operating the engine at sea level?
The induction air temperature may be slightly higher from the blower compression process but other than that,
would you consider this to be more of a "normalizing" process as we occasionally see barometric pressures near 32"?
 
dpansier said:
Am I correct in assuming running the "boost" at 29.7" would be same as operating the engine at sea level?
The induction air temperature may be slightly higher from the blower compression process but other than that,
would you consider this to be more of a "normalizing" process as we occasionally see barometric pressures near 32"?
Click to expand...

don't forget about the "thickness" of the air affecting propeller as well, just normalizing the manifold pressure to sea level doesn't affect the power change there
 
bkthomps said:
don't forget about the "thickness" of the air affecting propeller as well, just normalizing the manifold pressure to sea level doesn't affect the power change there
Click to expand...

If you have a CS prop, I think the governor compensates for that. Of course, there will be an altitude above which the prop hits the coarse pitch stop. But my guess is that altitude is high enough that other limits come into play, so it'd be a moot point.
 
dpansier said:
Am I correct in assuming running the "boost" at 29.7" would be same as operating the engine at sea level?
The induction air temperature may be slightly higher from the blower compression process but other than that,
would you consider this to be more of a "normalizing" process as we occasionally see barometric pressures near 32"?
Click to expand...

Keep in mind that at the same MP/RPM, engine power increases as you climb due to the decreased exhaust backpressure. Also consider that the temperature decreases as you climb, also increasing engine output.

So, to paraphrase the Sacramento Sky Ranch manual, to keep power constant while climbing you need to reduce MP by 1" per 3K feet of increased altitude.

Mark Olson N407V RV-7A N16XV F1-EVO Rocket
 
Supercharged and Turbocharged Power is not Equal

molson309 said:
Keep in mind that at the same MP/RPM, engine power increases as you climb due to the decreased exhaust backpressure. Also consider that the temperature decreases as you climb, also increasing engine output.

So, to paraphrase the Sacramento Sky Ranch manual, to keep power constant while climbing you need to reduce MP by 1" per 3K feet of increased altitude.

Mark Olson N407V RV-7A N16XV F1-EVO Rocket
Click to expand...

Also keep in mind that this engine is supercharged, not turbocharged. It requires significant power from the engine to drive a supercharger and the power required increases as altitude hence the pressure ratio across the supercharger increases. Because of this, the net power output of a supercharged engine operating at see level manifold pressure while flying at 10,000 feet will be considerably less than the engine's rated see level power.

The main advantage of turbocharging over supercharging is the fact that the power needed to drive a turbocharger comes mostly from wasted energy in the exhaust gasses, while the power to drive a supercharger comes directly from the engine's crankshaft. You can, of course, make up for this power loss by increasing the engine's manifold pressure, but then you would also be increasing fuel consumption and the overall stress on the engine.

Skylor
 
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I don't think that this is strictly correct as the increased back pressure caused by the turbine reduces the available crankshaft horsepower, too.

- John
 
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dpansier said:
Am I correct in assuming running the "boost" at 29.7" would be same as operating the engine at sea level?
The induction air temperature may be slightly higher from the blower compression process but other than that,
would you consider this to be more of a "normalizing" process as we occasionally see barometric pressures near 32"?
Click to expand...

I should have worded the question slightly different:

Am I correct in assuming running the "boost" at 29.7" the engine would see the same stress levels as operating at sea level?
 
G3i Ignition said:
Well Russ was out putting time on his supercharged 7 today. We have the blower drive ratio set-up fairly mild @ 33.5? MP for take-off with a field elevation of 5020MSL. Boost pressure being totally controllable. Just some quick numbers recorded through his GRT-4000/ EFIS. At 7500 MSL, 8400 DA, OAT 56f, Throttle control set at 85%, 2450 RPM, 30?MP, FF @ 14.3gph, GRT showing 100% calculated engine power, 186kt true. He was up @ 10,000 MSL, 2450RPM, 29.7?MP truing out @ 194kts. Same conditions when the 7 was aspirated, the average was, throttle control all the way in at 100%, 2450 RPM, FF10.8, GRT showing 76% calculated engine power, 165kt true @ 7500MSL. By the way?.G3i Series?2 ignition system is firing Slick 4300 series mags. More to come.
Click to expand...

I hope that Russ keeps in mind that at 194 Knots TAS he is cruising within 6 knots of VNE, in the context of control system flutter margin.

More details on the subject available Here
 
rvbuilder2002 said:
I hope that Russ keeps in mind that at 194 Knots TAS he is cruising within 6 knots of VNE, in the context of control system flutter margin.

More details on the subject available Here
Click to expand...

I hope that Steve Smith chimes in on this. We've discussed this in the past in the context of gliders, for which flutter is a very real concern, and which sometimes operate at very high altitudes.

While it is clear that determining Vne based on IAS an unconservative approach, some think that determining Vne based on TAS might be too conservative. One approach I've heard of is to split the difference between TAS and IAS, and use that to determine Vne.

Thanks, Bob K.
 
John R. Graham said:
I don't think that this is strictly correct as the increased back pressure caused by the turbine reduces the available crankshaft horsepower, too.

- John
Click to expand...

Turbos rule at high altitude as Delta P across the turbine increases with increasing altitude which helps offset the increasing input hp demands of the compressor. Additionally, modern centrifugal compressors are considerably more efficient than any lobe type supercharger so there is less heating of the charge air. Boost control is easier and more efficient through a wastegate than bleeding air from the compressor with a fixed ratio.
Modern turbos are also capable of 4 to 1 pressure ratios with a single stage.
Finally, they take the bark out of the exhaust which is easier on the ears.

This being said, some people prefer superchargers and I must say this is a clean looking installation presented here. Very cool.:cool::)
 
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That all sounds sensible. I was just reacting to the notion that spinning the turbine was somehow free but spinning the blower cost real horsepower.

- John
 
John R. Graham said:
That all sounds sensible. I was just reacting to the notion that spinning the turbine was somehow free but spinning the blower cost real horsepower.

- John
Click to expand...

That notion is pretty much true. There is typically a small amount of engine efficiency loss due to turbocharger installation, but that loss is much smaller than the the actual power needed to turn the compressor. The energy used to spin the turbine really does come from exhaust energy that is otherwise wasted. The concept of utilizing wasted exhaust energy was best demonstrated by the Wright R-3350 Turbo-Compound Engine that had 3 power recovery turbines that were driven by engine exhaust gases and coupled to the crankshaft. They provided an additional 600 horsepower for takeoff.

Skylor
 
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Really?

skylor said:
Also keep in mind that this engine is supercharged, not turbocharged. It requires significant power from the engine to drive a supercharger and the power required increases as altitude hence the pressure ratio across the supercharger increases. Because of this, the net power output of a supercharged engine operating at see level manifold pressure while flying at 10,000 feet will be considerably less than the engine's rated see level power.

snip

Skylor
Click to expand...

Get thee a look at ANY radial engine power chart to see that your assumption is not correct. Almost all radials have blowers, and spec a lower MP for a certain percentage of power or HP output as altitude increases. It is widely known that the reduced exh back pressure at higher altitudes is the reason for this.

Example, using the good ol' 985: setting 220HP output as a data point, at 5000'PA, ambient temp 10C, 1700RPM, the engine needs 24.7"MP to produce this power. Move to 10000'PA, 0C, 1700RPM, the MP drops to 23". At 10000'PA/10C, it is 23.5". 24.7"MP @10000'PA/0C/1700RPM = 240HP, a gain of 20HP.

BTW the 985 is power rated using a constant 32-38C induction temp, so in most cases, this requires carb heat. Seems this might take the ambient temp out of the equation if used as directed by the folks at Pratt.

As for that power robbing blower, the fuel specifics on the blown 540s seems to equal the normally aspirated ones -- determined by side-by-side flights of more or less equal airframes and comparing fuel flows during those flights. So, while the blower (either exh or mechanically driven) must take some power to operate, it makes the engine think it has a higher compression ratio, which is more efficient at converting refined dinosaurs into speed.

The nice thing about superchargers is that they have no TIT temp restriction, which causes turbos to be VERY thirsty in the ROP regime for a given power output.

To confirm or deny what I have said, I'm sure Tom will have a NA engined RV fly next to the supercharged one to run the comparison. I'll bet the results will be positive.

Carry on!
Mark
 
F1Boss said:
Get thee a look at ANY radial engine power chart to see that your assumption is not correct. Almost all radials have blowers, and spec a lower MP for a certain percentage of power or HP output as altitude increases. It is widely known that the reduced exh back pressure at higher altitudes is the reason for this.

Example, using the good ol' 985: setting 220HP output as a data point, at 5000'PA, ambient temp 10C, 1700RPM, the engine needs 24.7"MP to produce this power. Move to 10000'PA, 0C, 1700RPM, the MP drops to 23". At 10000'PA/10C, it is 23.5". 24.7"MP @10000'PA/0C/1700RPM = 240HP, a gain of 20HP.

BTW the 985 is power rated using a constant 32-38C induction temp, so in most cases, this requires carb heat. Seems this might take the ambient temp out of the equation if used as directed by the folks at Pratt.

As for that power robbing blower, the fuel specifics on the blown 540s seems to equal the normally aspirated ones -- determined by side-by-side flights of more or less equal airframes and comparing fuel flows during those flights. So, while the blower (either exh or mechanically driven) must take some power to operate, it makes the engine think it has a higher compression ratio, which is more efficient at converting refined dinosaurs into speed.

The nice thing about superchargers is that they have no TIT temp restriction, which causes turbos to be VERY thirsty in the ROP regime for a given power output.

To confirm or deny what I have said, I'm sure Tom will have a NA engined RV fly next to the supercharged one to run the comparison. I'll bet the results will be positive.

Carry on!
Mark
Click to expand...

Mark,

That's very good data that you gave for the 985, except that those manifold pressures require very little boost, hence not much power is being used to drive the blower. In this case, the increased engine efficiency from reduced exhaust back pressure does outweigh the power required to drive the blower. What are the numbers for a supercharged engine operating at 35 or 40 inches?

Skylor
 
skylor said:
Mark,

That's very good data that you gave for the 985, except that those manifold pressures require very little boost, hence not much power is being used to drive the blower. In this case, the increased engine efficiency from reduced exhaust back pressure does outweigh the power required to drive the blower. What are the numbers for a supercharged engine operating at 35 or 40 inches?

Skylor
Click to expand...

OK, I probably should have looked at some numbers before posting. It does look like engine efficiency gains at altitude will have a greater effect on power output than the increased drive power requirements of a blower operating at increased pressure ratios, even at moderate boost levels. What I have't had time to look at yet is the power reduction due to increased intake temps in a non-inter cooled configuration. Just some quick back of the envelope calculations show that a typical Eaton blower requires nearly 10 horsepower to drive it if operating at 15000 feet with a discharge pressure of 30" and producing O-360 flowrates. However, the discharge temperature will approach 50C at these conditions.

Skylor
 
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My point was..

skylor said:
Mark,

That's very good data that you gave for the 985, except that those manifold pressures require very little boost, hence not much power is being used to drive the blower. In this case, the increased engine efficiency from reduced exhaust back pressure does outweigh the power required to drive the blower. What are the numbers for a supercharged engine operating at 35 or 40 inches?

Skylor
Click to expand...

..that altitude affects the MP requirement to produce a certain HP number, specifically, lowering the required MP as altitude increases. The numbers quoted are real-world cruise numbers. This in response to your statement: "the net power output of a supercharged engine operating at see level manifold pressure while flying at 10,000 feet will be considerably less than the engine's rated see level power."

I'm not sure there are well-documented charts that can be used to compare output @ higher output numbers using a specific engine that has versions that are both turbo-supercharged and straight mechanically supercharged, which seems to be your focus here. There might be charts that tell what the BSFC is for certain turbosupercharged and mechanically supercharged engines - would those numbers be considered definitive for your inquiry? In that case, Google is your friend - let us know what you find.

BTW I heard a Beech twin fly over the other day - this one was the 18 follow-on that has the supercharged Lycs on it: the Queen Air, precursor to the King Air. GEEZ that thing was LOUD as it passed over - I think this decibel level tells us how much energy is going out the Main Drain. Yep, those supercharged motors have a bark that tells everyone how far the Big Lever is pushed forward! I will tell you that the B-25 sound/cockpit vibration level changes from cannon fire 3 feet away to 50 cal fire 3 feet away when we reduce power to slow to come into the pattern...I never knew that 50 cal fire could be so soothing.:D

Carry on!
Mark
 
Encountering moderate or extreme turbulence at 194 knots when VA is 120 knots or there about is a bit disturbing. In turbo or supercharging you are probably looking at cruising altitudes up to 18,000 feet and perhaps above where clear air turbulence is often encountered. Not sure I want to do that. Also bear in mind that on longer cross country flights at those altitudes you will invariably encounter IFR or perhaps VFR on top conditions. RV?s are not good instrument platforms and we tend to rely on our autopilots for sustained flight. Our RV?s are not P-51?s but rather in the weight class of a Piper PA-18. Surely they are better stressed but no match for Mother Natures nasty moods. Have you ever felt like a leaf in a wind storm? Don?t go there....
J Wright
 
Supercharged Power

I know some of you guys have seen my blown AcroDuster-2. It definitely has vertical penetration, when I’m competing (IAC) or just goof’ in off. What is nice, is when I do have a cross-country trip and going up to 13,000+ I still have an acceptable manifold pressure to make speed. Believe me, I don’t fly at 100% throttle all the time, however, having the upstairs power available is nice, or out running a Lear jet on take-off on parallel runways (KAPA) to pattern alt. is cool to. The look on there faces cracks me up.
 
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Climb

With all of the tech talk about turbochargers and superchargers I almost (almost) forgot how bad I want one after watching Tom's plane climb faster than anything on the field (RVs included) then turn to a speck and disappear in no time. Sounds cool too.
 
discussion on TAS would be nice flying high

the indicated Vne given by Van's is an indicated airspeed that being said up at high altitudes there could be an area where the indicated airspeed Vne should be lowered
most high altitude aircraft jets have different airspeed limits based upon the altitude in which they are flying again do to flutter

I am quite sure that Van's has not done Vne flutter test at all altitudes up into the mid 20,000 feet
there is a requirement under part 23 where you have to be able to pitch the air craft over to a preset angle and not experience flutter
a lot of high performance aircraft have mach limiting airspeed indicators

like it or not in the mid 20's indicating 200 mph your covering the ground with a TAS approaching 250 mph

just food for thought
jack
 
I wouldn't speculate about the validity or conservatism of Van's published Vne. Flutter can kill you instantly. Unless you do you own testing (pack a chute and a prayer), best stick to the established limits.
 
Speed Limit

Most of the new EFIS units have TAS inidcated in plane sight so staying within the TAS VNE is a no brainer and it should certainly be respected.

Of course you would not want to go near that speed limit in rough conditions, but cruising around up high at close to 200 knots is, well, desireable, is it knot?

I had that kind of capability before I fried a turbo bearing and let me tell you it is quite addictive to have a knob that you can turn to increase MAP. I think of this when I see all the effort that goes into getting an extra 1" of MAP via ram air and all the drag reduction efforts I see going on. Not to say those are not all very worthwhile improvements, but they just don't compare to having adjustable MAP...

It must be addictive or I would not be going through all this work to get that capability again. I am in the process of converting back to a turbo engine, one little step at a time.

FYI, I am referring to my Subaru STI motor. Normally aspirated it is a slug. Turbo charged it absolutley kicks ---, with capability to go through VNE at 8000', with plenty of cooling drag I presume.

I used to depart at around 4400 engine RPMs and 42" MAP which was a very mild take off power setting. As I went up in altitude the boost would gradually fall off. If I wanted to get some back I just turned the little knob to the right and the MAP would come right back up to where I wanted it. I could easily cruise at 4400 RPMs and 40" MAP up at 15,500'. ( I am referring to a bleeder valve that was connected to the diaphragm on the waste gate controller)

Having experienced this engine running an Eaton M45 roots type blower, and then switching to a turbocharger, I would never even consider going back to use of a belt driven roots blower agian. (It did however give me 52" MAP for take off power if I wanted it!) When I first flew with the turbo after having the supercharger, I was amazed at how free the engine was turning. I could easily feel quite a difference and the performance went up accordingly. With the drag of the blower gone but still having the boost to use was like alot of free horsepower on tap. I also really appreciated the automatic waste gate operation. Just one throttle control and boost was controlled automatically according to the spring installed in the waste gate controller. I did alot of flying at partial throttle settings.

Well, just sharing some of my previous experience with boost. Can't wait to be able to report back with some current boost performance results but it takes time and money to make these changes:)

Randall Crothers
 
MauiLvrs said:
That's what the guy in Canada probably thought, until parts of the tail departed the aircraft due to flutter... :eek:
Click to expand...

Transport Canada said the plane was over the weight limit for aerobatics, and said it was either flutter OR overstress:

"The aircraft encountered either flutter or overstress of some rudder components. Subsequently, the vertical stabilizer and parts of the rudder separated from the empennage during flight. Consequently, the aircraft became uncontrollable resulting in the impact with terrain."
 
Danny7 said:
Transport Canada said the plane was over the weight limit for aerobatics, and said it was either flutter OR overstress:

"The aircraft encountered either flutter or overstress of some rudder components. Subsequently, the vertical stabilizer and parts of the rudder separated from the empennage during flight. Consequently, the aircraft became uncontrollable resulting in the impact with terrain."
Click to expand...

Based on a full reading of the report, I believe that this event was in fact an instance of flutter. This is borne out by the stress reversals evidenced by the fracture surfaces, and by the distribution of damage. I think that TC is just being a bit cautious and hedging their bets by saying that it might also have been overstress.

However, I also believe that the flutter was a relatively direct result of just plain going way too fast. I do not think that this is a case of someone determining using IAS instead of TAS, or of some similar miscalculation. I think it more a case of just not paying attention to airspeed.

Thanks, Bob K.
 
However said:
That may be the case that it could have been inattention as opposed to misunderstanding, but still clearly demonstrates the exceeding Vne can cause flutter and the folks that designed the airplane say that Vne is to be measured using TAS.

The designer said it. At least two cases demonstrate it. Good enough for me.

In the case of C-GNDY, the control surfaces may not have been properly balanced contributing to the issue. However, that issue didn't seem to become a structural problem until beyond Vne.
Click to expand...
 
MauiLvrs said:
That may be the case that it could have been inattention as opposed to misunderstanding, but still clearly demonstrates the exceeding Vne can cause flutter and the folks that designed the airplane say that Vne is to be measured using TAS.

The designer said it. At least two cases demonstrate it. Good enough for me...
Click to expand...

To revisit, my original argument is that VNE, in certain situations where it is limited by flutter, can safely be pegged to a speed that splits the difference between IAS and TAS. This is not a new idea. I heard about it from Steve Smith, an aeronautic engineer and also the designer behind the RV-10 wing.

The flutter cases we've seen so far are completely moot in regard to this idea. The evidence at hand shows that they occurred well above the split-the-difference VNE and even above the IAS VNE. So they do not necessarily prove anything we don't already know: If you go too fast you will encounter flutter.

Thanks, Bob K.
 
Bob Kuykendall said:
To revisit, my original argument is that VNE, in certain situations where it is limited by flutter, can safely be pegged to a speed that splits the difference between IAS and TAS.
Click to expand...
In some cases that may be true. BUT in THIS case of the RVs, the designer referenced Vne to TAS. They had good reasons and have been doing it for a LONG time. Think you have a better idea ... good luck with that :rolleyes:
 
can you guys take this Vne discussion elsewhere? I was looking for climb and high altitude gains from the supercharger, but really i'd like to see pricing and more installed pictures
 
I looked at the G3 installation at Airventure

Might have some pictures, I'll check. It was a very very clean, well engineered installation.

Spoke with Russ (whose last name I can't recall). Very cooperative and I'm sure the G3 guys will put you in touch. He was fighting some cooling issues but was reporting impressive performance numbers.
 
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