The shocking truth about speed or, Dustflyer finally gets a taste of reality!
10-22-2002, 12:03 AM
#1
The shocking truth about speed or, Dustflyer finally gets a taste of reality!
OK, my Jett FIRE 50 is SCREAMING, I'm turning a 7.4 X 8.25 carbon fiber prop at 21,400 on the ground. I launch. It unloads. Man this thing has got to be going....
Today I met my friend Sydney at the NE Philly RC field. Syd is no ordinary modeler. This guy was flying RC planes when most of us were still in liquid form. Today, retired with no worries other than where and when he is going flying, Syd sends humbled 30 year old hot-shots running home with fingers outstretched, itching to boot up that G2 sim for more practice.
Syd brought his radar gun along today, and no ordinary bargain model. This baby was 1500 bucks, just calibrated, and Syd could teach a New Jersey state trooper a thing or two about how to use it.
A while back, on a different forum, I remember other contributors, racers probably, telling me how all the charts, formulas, calculators, etc. were just so much nonsense for calculating aircraft speed and how I was obviously full of, well.. let's just say they were, uh, I was.. well, you know what I mean!
So here I am today. Syd is in position. Turning 20.4K, backed off from 21.4 I launch my Dust. Plenty of thrust! Off it goes like a slingshot, straight out. I take it up high, put it into a screaming dive, DOWNWIND, the doppler effect registers and the call comes through on the walkie-talkie to Frank, my spotter.
The speed? All of, are you ready? A whopping...150mph!
This is repeated over and over. The fastest I could get it to go was 150.7 mph.
So, what does this all mean? It means that all the charts, formulas, calculators, etc. are all NONSENSE, just like everyone told me they were!
Clearly, something more is at work here. You simply cannot take pitch x 10 x .000947 or whatever and calculate speed.
Let me ask you this. If you turn a 9X8 at 20K will you go the same speed as if you turn a 6X8 at 20K? How about a 5X8? If you could fly the airplane with a 3X8 turning 20K could anybody with any common sense assume the airplane would go the same speed as with a 9X8 turning 20K?
My point is there is more than rpm and pitch to the speed solution. There needs to be a certain amount of thrust. I don't know how that is figured out except by trial and error, maybe there is a way.
I am convinced the Diamond Dust has a lot of drag, induced drag mainly, plus a whole lot of wetted area. There is no way a teeny tiny prop is going to pull that thing through the air at 200 mph, I have proven it!
I know what I am going to do. I am going right back to where I started...long pipe, 9X10 prop turning 16K plus. That's where I am going to start with my Whiplash and to where I will return with my Dust.
Today I met my friend Sydney at the NE Philly RC field. Syd is no ordinary modeler. This guy was flying RC planes when most of us were still in liquid form. Today, retired with no worries other than where and when he is going flying, Syd sends humbled 30 year old hot-shots running home with fingers outstretched, itching to boot up that G2 sim for more practice.
Syd brought his radar gun along today, and no ordinary bargain model. This baby was 1500 bucks, just calibrated, and Syd could teach a New Jersey state trooper a thing or two about how to use it.
A while back, on a different forum, I remember other contributors, racers probably, telling me how all the charts, formulas, calculators, etc. were just so much nonsense for calculating aircraft speed and how I was obviously full of, well.. let's just say they were, uh, I was.. well, you know what I mean!
So here I am today. Syd is in position. Turning 20.4K, backed off from 21.4 I launch my Dust. Plenty of thrust! Off it goes like a slingshot, straight out. I take it up high, put it into a screaming dive, DOWNWIND, the doppler effect registers and the call comes through on the walkie-talkie to Frank, my spotter.
The speed? All of, are you ready? A whopping...150mph!
This is repeated over and over. The fastest I could get it to go was 150.7 mph.
So, what does this all mean? It means that all the charts, formulas, calculators, etc. are all NONSENSE, just like everyone told me they were!
Clearly, something more is at work here. You simply cannot take pitch x 10 x .000947 or whatever and calculate speed.
Let me ask you this. If you turn a 9X8 at 20K will you go the same speed as if you turn a 6X8 at 20K? How about a 5X8? If you could fly the airplane with a 3X8 turning 20K could anybody with any common sense assume the airplane would go the same speed as with a 9X8 turning 20K?
My point is there is more than rpm and pitch to the speed solution. There needs to be a certain amount of thrust. I don't know how that is figured out except by trial and error, maybe there is a way.
I am convinced the Diamond Dust has a lot of drag, induced drag mainly, plus a whole lot of wetted area. There is no way a teeny tiny prop is going to pull that thing through the air at 200 mph, I have proven it!
I know what I am going to do. I am going right back to where I started...long pipe, 9X10 prop turning 16K plus. That's where I am going to start with my Whiplash and to where I will return with my Dust.
10-22-2002, 12:50 AM
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Drop the same setup on a whiplash and I can almost guarantee a 15-20 mph increase with the exact same setup.. less drag + better airfoil = more speed. thats a formula that cant be disputed 
10-22-2002, 01:02 AM
#4
9X10
Chuck,
Unfortunately, no.
The next time Syd and I get together; however, he will have his Jett FIRE 50 powered Whiplash and I will have my mine along with my Dust. My planes will both be engined with Jett FIRE 50's and propped with 9X10's.
Should be interesting.
Unfortunately, no.
The next time Syd and I get together; however, he will have his Jett FIRE 50 powered Whiplash and I will have my mine along with my Dust. My planes will both be engined with Jett FIRE 50's and propped with 9X10's.
Should be interesting.
10-22-2002, 01:04 AM
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The shocking truth about speed or, Dustflyer finally gets a taste of reality!
Hey bago!!!! They are still trying to keep up with my os ducted fan 46.....I wonder how my prop size got scratched off...hmmmm
10-22-2002, 01:13 AM
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hmmmmmm
Gene I got $20.00 on you, any side bets out there? Hey Scott, I'll bet you're still trying to keep up with your 46DF!! thats gotta be some kinda quick... come down someday and I'll race you with my nitrous boosted JETT50 with turbo charger.... :stupid:
it unfortunately added 12 lbs to my whiplash, but thats ok the power exceeds the weight, but wow!! talk about some serious wing loading... :stupid:
it unfortunately added 12 lbs to my whiplash, but thats ok the power exceeds the weight, but wow!! talk about some serious wing loading... :stupid:
10-22-2002, 01:18 AM
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OS DF..
Yeah, I had a OS DF engine running the exact prop Dustflyer was running (I was turning just slightly higher RPM's at 3250' ) on a Dust..matter of fact I sent him that prop just to see how the Jett 50 would turn against it.
Unfortunately, My Whiplash won't balance with the OS, gotta go for a couple more HP...
And I still don't think they'll go 200 with a 9X7 or 9X8, just MHO.
Unfortunately, My Whiplash won't balance with the OS, gotta go for a couple more HP...
And I still don't think they'll go 200 with a 9X7 or 9X8, just MHO.
10-22-2002, 02:06 AM
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The shocking truth about speed or, Dustflyer finally gets a taste of reality!
The current FAI prop speed record listed HERE is only 215 mph (344 kph). Now granted they have to do it from level flight so diving into the trap is not an issue as far as I know but there you are.
If it was as easy as some here think to go that fast we'd all be setting a new FAI record with these models.
So I guess we shouldn't be so surprised that the speeds aren't as high as we think........
150 is still pretty damn fast. It must move from one horizon to the other pretty quickly.
If it was as easy as some here think to go that fast we'd all be setting a new FAI record with these models.
So I guess we shouldn't be so surprised that the speeds aren't as high as we think........
150 is still pretty damn fast. It must move from one horizon to the other pretty quickly.
10-22-2002, 11:52 AM
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The shocking truth about speed or, Dustflyer finally gets a taste of reality!
Dustflyer,
Ha ha! My old formula is still very close to real life. Pitch x RPM x effeciency (I use .85 here) x .00947 = 142.11 in your example. I've been trying to tell folk they aren't going as fast as they think they are. 200 mph is FAAAAAASSSSSTTT. When you see 200 go by, you will know the average Dust isn't doing it!
I can't stop laughing...he he :surprised
Mark M.
Ha ha! My old formula is still very close to real life. Pitch x RPM x effeciency (I use .85 here) x .00947 = 142.11 in your example. I've been trying to tell folk they aren't going as fast as they think they are. 200 mph is FAAAAAASSSSSTTT. When you see 200 go by, you will know the average Dust isn't doing it!
I can't stop laughing...he he :surprised
Mark M.
10-22-2002, 12:46 PM
#10
Bets, anyone?
BAGOSTIX,
Bad bet Bags!
Syds planes are ALWAYS faster than anyone elses, you can count on it! He was tuning racing engines before any of us even knew what a tuned pipe was.
The man has ALL they toys. Heck, the other day he brought out a glider with some kind of crazy engine and the wildest carbon fiber tuned pipe I ever saw and I swear that thing was faster than my Diamond Dust! It was one of wildest things I have ever witnessed in RC, next to the rudder only rolling circle he proceeded to do with it!
Bad bet Bags!
Syds planes are ALWAYS faster than anyone elses, you can count on it! He was tuning racing engines before any of us even knew what a tuned pipe was.
The man has ALL they toys. Heck, the other day he brought out a glider with some kind of crazy engine and the wildest carbon fiber tuned pipe I ever saw and I swear that thing was faster than my Diamond Dust! It was one of wildest things I have ever witnessed in RC, next to the rudder only rolling circle he proceeded to do with it!
10-22-2002, 02:17 PM
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The shocking truth about speed or, Dustflyer finally gets a taste of reality!
Dustflyer,
You raise a whole bunch of points. I'll offer a couple comments:
1) The 'formulas' can give some useful predictions, if you have accurate data to use in applying them. In particular, if you know the prop pitch and the best engine speed you can get, you can predict the best possible airspeed, just because you can't outrun your prop in level flights. This kind of prediction can't be expected to predict your actual speed, because it does not account for drag. Also, applying that 'formula' entirely ignores the possibility that your engine may not be able to spin that prop at that speed. This is why the speed planes that use high pitch to go fast must use smaller diameter. As the prop diameter increases, the power needed to spin it at a given speed increases really fast. If you choose the prop pitch needed to fly 200 MPH, and decide that it should be 15" in diameter, your engine will never be able to spin it fast enough. If you have already chosen your engine and have a target speed, you need to pick a pitch that is high enough to make that speed possible, and then pick a diameter that gives your engine a chance to spin it fast enough. If the engine simply can't make enough power, you won't make it. Larger diameter is not generally the way to get max speed though. Any 'formula' that doesn't include the CD for the airframe cannot be relied on for an accurate prediction of max. speed.
2) Forget about induced drag. Diamond Dust does have a fair amount of drag, but at high speed, induced drag is a very small component of it. Induced drag is high when the CL of the wing is high. When a plane is flying fast, the wing is operating at a low CL, and induced drag is small. Many modelers seem to assume that planes with low aspect ratio suffer from high induced drag, and it is not always the case. Diamond Dust has a lot of drag because the engine, pipe, servos, linkages, etc. are jutting out into the airflow. If you want to reduce the drag, get all that stuff inside fairings. Take a look at a control line speed plane. Even if you had to add a long 'bubble', or make the root of the wing thicker, it would be a huge win to get that stuff faired over. I would take a look at running the elevon linkages through a narrow fairing and through channels in the fins, or something like that. With all that junk smoothed over, Diamond Dust does not look all that draggy, overall. There is a fair amount of wetted area, which is hard to do much about. If you don't mind losing some of the turning performance, you could chop the wingspan down, but I would worry a bit about losing elevon area.
3) Some of the props being run may be too small in diameter, but I don't think 10 inches is going to be the answer. If you're really looking for top end, send Dub Jett an email about trying a single bladed prop. I bet he would have some good advice. If the balance problem can be solved, this is the most efficient solution.
If you get out your balsa and glue, and fair all the junk over, you can give Syd a nasty surprise the next time out.
banktoturn
You raise a whole bunch of points. I'll offer a couple comments:
1) The 'formulas' can give some useful predictions, if you have accurate data to use in applying them. In particular, if you know the prop pitch and the best engine speed you can get, you can predict the best possible airspeed, just because you can't outrun your prop in level flights. This kind of prediction can't be expected to predict your actual speed, because it does not account for drag. Also, applying that 'formula' entirely ignores the possibility that your engine may not be able to spin that prop at that speed. This is why the speed planes that use high pitch to go fast must use smaller diameter. As the prop diameter increases, the power needed to spin it at a given speed increases really fast. If you choose the prop pitch needed to fly 200 MPH, and decide that it should be 15" in diameter, your engine will never be able to spin it fast enough. If you have already chosen your engine and have a target speed, you need to pick a pitch that is high enough to make that speed possible, and then pick a diameter that gives your engine a chance to spin it fast enough. If the engine simply can't make enough power, you won't make it. Larger diameter is not generally the way to get max speed though. Any 'formula' that doesn't include the CD for the airframe cannot be relied on for an accurate prediction of max. speed.
2) Forget about induced drag. Diamond Dust does have a fair amount of drag, but at high speed, induced drag is a very small component of it. Induced drag is high when the CL of the wing is high. When a plane is flying fast, the wing is operating at a low CL, and induced drag is small. Many modelers seem to assume that planes with low aspect ratio suffer from high induced drag, and it is not always the case. Diamond Dust has a lot of drag because the engine, pipe, servos, linkages, etc. are jutting out into the airflow. If you want to reduce the drag, get all that stuff inside fairings. Take a look at a control line speed plane. Even if you had to add a long 'bubble', or make the root of the wing thicker, it would be a huge win to get that stuff faired over. I would take a look at running the elevon linkages through a narrow fairing and through channels in the fins, or something like that. With all that junk smoothed over, Diamond Dust does not look all that draggy, overall. There is a fair amount of wetted area, which is hard to do much about. If you don't mind losing some of the turning performance, you could chop the wingspan down, but I would worry a bit about losing elevon area.
3) Some of the props being run may be too small in diameter, but I don't think 10 inches is going to be the answer. If you're really looking for top end, send Dub Jett an email about trying a single bladed prop. I bet he would have some good advice. If the balance problem can be solved, this is the most efficient solution.
If you get out your balsa and glue, and fair all the junk over, you can give Syd a nasty surprise the next time out.
banktoturn
10-22-2002, 05:46 PM
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The shocking truth about speed or, Dustflyer finally gets a taste of reality!
And you said is was 150 mph, downwind, after a steep dive!
Again, take a look at that formula that MMoallory posted.
Like I "guessed" (with some reasearch into full-scale racing props), you will not exceed geometric pitch of the prop, period.
So a formula like pitch x rpm x efficiency = realistic. Now the question is for a Dust at a given pitch and given RPM, what is the efficiency (85% seems about right, but we do not know in-air RPM).
So, get an airframe with a wing at a better reynolds number, less draggy linkage, etc. and you work on efficiency.
200 MPH is going to be tough though.
Again, take a look at that formula that MMoallory posted.
Like I "guessed" (with some reasearch into full-scale racing props), you will not exceed geometric pitch of the prop, period.
So a formula like pitch x rpm x efficiency = realistic. Now the question is for a Dust at a given pitch and given RPM, what is the efficiency (85% seems about right, but we do not know in-air RPM).
So, get an airframe with a wing at a better reynolds number, less draggy linkage, etc. and you work on efficiency.
200 MPH is going to be tough though.
10-22-2002, 05:56 PM
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The shocking truth about speed or, Dustflyer finally gets a taste of reality!
Second thoughts...
Check out the sailplane Websites for control linkage fairings and the like. Most contest soaring ships little exposed linkages (all are covered with fiberglass or ABS covers).
Here are some sources of servo linkage covers I have:
http://www.multiplexrc.com/acc_servo.htm
http://www.hobby-lobby.com/servomount.htm
Check out the sailplane Websites for control linkage fairings and the like. Most contest soaring ships little exposed linkages (all are covered with fiberglass or ABS covers).
Here are some sources of servo linkage covers I have:
http://www.multiplexrc.com/acc_servo.htm
http://www.hobby-lobby.com/servomount.htm
10-22-2002, 10:25 PM
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Speed
I have posted my data with nelson whiplash. I have 7.4 x 8. tach at 21,300 on ground. It flew 163mph by 1000 dollar radar with internal calibration. The thrust formula and speed calculation from the web page i posted gave this 161 mph. It was darn close. Now i have new props comming. 7.2 x 8, 7.2x8.3, 7.2x8.6. I am shooting for 23000 on the ground. This should give me 190 mph with the 8.6. Testing this weekend if props arrive.
10-23-2002, 09:30 AM
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angle
No not straight at it. I dont have the guts to get out in the middle of the field. The DD i and get way out on the run so it is essentially straight on for all practical purposes. The whiplash i can only get at about 600 feet away so i do have small error with that measurement but i don think its that significant. I will do the calculation to see but its got to be less than 5mph off.
10-23-2002, 12:30 PM
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The shocking truth about speed or, Dustflyer finally gets a taste of reality!
200MPH? CAN YOU SAY ADVERTISING EXECUTIVES AT WORK?
DELTAS ARE DRAGGY-NOT GOOD FOR ULTIMATE SPEED!!
Jetts
DELTAS ARE DRAGGY-NOT GOOD FOR ULTIMATE SPEED!!
Jetts
10-23-2002, 01:33 PM
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The shocking truth about speed or, Dustflyer finally gets a taste of reality!
jettstarblue,
Well, I'm not sure whether deltas are necessarily all that draggy, all things considered. Generally, compared to a more conventional wing, for a given amount of lift, a delta wing is probably draggier. In this case, though, going with a delta wing allows the plane to be essentially a flying wing, which is a fairly low drag configuration. It seems like one of the bigger fundamental drawbacks, in terms of drag, is the wetted surface, which is pretty big for Whiplash, Diamond Dust, etc. One answer is a smaller delta, but that would degrade turning, which seems to be excellent for these planes. At high speed, for the same wetted surface, I don't think the delta is that bad, but I could be missing something. I do agree that the 200 mph claims are a lot like the claims of radio failure as a cause of crashes: not very likely.
banktoturn
Well, I'm not sure whether deltas are necessarily all that draggy, all things considered. Generally, compared to a more conventional wing, for a given amount of lift, a delta wing is probably draggier. In this case, though, going with a delta wing allows the plane to be essentially a flying wing, which is a fairly low drag configuration. It seems like one of the bigger fundamental drawbacks, in terms of drag, is the wetted surface, which is pretty big for Whiplash, Diamond Dust, etc. One answer is a smaller delta, but that would degrade turning, which seems to be excellent for these planes. At high speed, for the same wetted surface, I don't think the delta is that bad, but I could be missing something. I do agree that the 200 mph claims are a lot like the claims of radio failure as a cause of crashes: not very likely.
banktoturn
10-23-2002, 02:06 PM
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The 200 mph figure..
Came about years ago, when the Diamond Dust first came out.
A guy built one for a review (in RC Report, I think..) and said it went 200MPH. I don't remember what method he used to get this figure, it just became part of modeling folklore. Now this figure has become the most tossed about number in history. I remember one guy got 237
with a FX 46. Go figure.
A guy built one for a review (in RC Report, I think..) and said it went 200MPH. I don't remember what method he used to get this figure, it just became part of modeling folklore. Now this figure has become the most tossed about number in history. I remember one guy got 237
with a FX 46. Go figure.
10-23-2002, 06:20 PM
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Don't get me wrong!
Banktoturun;
Don't get me wrong, I am a delta worshiper myself! All of the advantages of the delta seem to outweigh any drawbacks. Plus you can build one at a fraction of the weight/cost/time of any other plane except a flying wing(ARGH!)
I personally haven't tried it with a radar gun, but have any of you built a negative aspect ratio "delta" to see how fast you can go? The one I built with a whooped Super Tigre .40 would easily outperform a low winged sport model a friend had- It had a FX .46 w/tuned pipe!! Used to get him really P.O.'ed!!
I also built a 2 hr. job that was a flat plate of 1" foam with all the goodies glued right in top-servoes, pushrods, tank, ect. and it had a repectable glide ratio-no kidding.
Keep me posted on the negative aspect ratio thing, if you will. I think I used a .7 width to lenght if I remember.
Jetts
Don't get me wrong, I am a delta worshiper myself! All of the advantages of the delta seem to outweigh any drawbacks. Plus you can build one at a fraction of the weight/cost/time of any other plane except a flying wing(ARGH!)
I personally haven't tried it with a radar gun, but have any of you built a negative aspect ratio "delta" to see how fast you can go? The one I built with a whooped Super Tigre .40 would easily outperform a low winged sport model a friend had- It had a FX .46 w/tuned pipe!! Used to get him really P.O.'ed!!
I also built a 2 hr. job that was a flat plate of 1" foam with all the goodies glued right in top-servoes, pushrods, tank, ect. and it had a repectable glide ratio-no kidding.
Keep me posted on the negative aspect ratio thing, if you will. I think I used a .7 width to lenght if I remember.
Jetts
10-23-2002, 06:44 PM
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The shocking truth about speed or, Dustflyer finally gets a taste of reality!
Jetts,
Don't worry, I wasn't assuming you were a delta-basher. Actually, even though I kind of like delta wings, I started out as kind of a delta-basher when the 'delta drag' thread started out. I had to think it through for a while to decide that they aren't all that bad for drag, if you get rid of the junk hanging out.
I assume that by negative aspect ratio you mean an AR less than one. My answer is that a plane like that would be plenty fast for straight, level flight, as long as there is enough wing area that it could fly at low angle of attack, and hence low drag. Turning would suffer, although the wing would be quite stall-resistant. Generally, if you want to go fast, make the wing as small as you can without requiring it to operate at high angle of attack. This applies whether the wing is a delta or not. I have been trying to decide whether you could get a wetted area advantage over the deltas by building a more conventional layout with a tail. I am inclined to think so, since you could pretty much cut a conventional planform out of a Whiplash's outline. It means having a fuselage to drag around, but I think you might come out ahead. The tradeoff is that as you reduce wetted area, you lose all those square inches that make Whiplashes turn on a dime.
Maybe this will be a project for a cold Minnesota winter.
banktoturn
Don't worry, I wasn't assuming you were a delta-basher. Actually, even though I kind of like delta wings, I started out as kind of a delta-basher when the 'delta drag' thread started out. I had to think it through for a while to decide that they aren't all that bad for drag, if you get rid of the junk hanging out.
I assume that by negative aspect ratio you mean an AR less than one. My answer is that a plane like that would be plenty fast for straight, level flight, as long as there is enough wing area that it could fly at low angle of attack, and hence low drag. Turning would suffer, although the wing would be quite stall-resistant. Generally, if you want to go fast, make the wing as small as you can without requiring it to operate at high angle of attack. This applies whether the wing is a delta or not. I have been trying to decide whether you could get a wetted area advantage over the deltas by building a more conventional layout with a tail. I am inclined to think so, since you could pretty much cut a conventional planform out of a Whiplash's outline. It means having a fuselage to drag around, but I think you might come out ahead. The tradeoff is that as you reduce wetted area, you lose all those square inches that make Whiplashes turn on a dime.
Maybe this will be a project for a cold Minnesota winter.
banktoturn
10-23-2002, 06:56 PM
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Deltas
Banktoturn,
Yes, or an Ohio winter! I'm thinking of building another with a better engine ( I have them now), paying close attention to all the draggy bits, and then I'll find someone with a radar gun.
At the time I built the last one I was trying to build a flying wing, and kept making it longer, and narrower- took four of them built of FANFOLD. I finally ended up with the .7 A/R and it flew like a trainer!- About 1/4" of ailerons, a LOT of elevator travel, I even did a touch and go on the (leading edge?) of the (wing?)- got that on video.
I didn't really know what I was doing at the time, but I sure did like to piss off the "Quikie Guys" with it!!!!!!!!!!!!!!
Jetts
Yes, or an Ohio winter! I'm thinking of building another with a better engine ( I have them now), paying close attention to all the draggy bits, and then I'll find someone with a radar gun.
At the time I built the last one I was trying to build a flying wing, and kept making it longer, and narrower- took four of them built of FANFOLD. I finally ended up with the .7 A/R and it flew like a trainer!- About 1/4" of ailerons, a LOT of elevator travel, I even did a touch and go on the (leading edge?) of the (wing?)- got that on video.
I didn't really know what I was doing at the time, but I sure did like to piss off the "Quikie Guys" with it!!!!!!!!!!!!!!
Jetts
10-23-2002, 11:20 PM
#24
Radar run
John,
Here's how it was done.
Syd positioned himself at the far end of the runway, crouching down on the centerline. I would take the Dust up high till I could barely see which end was up, then do a split S to start my dive, initially coming nearly straight down. I would then shallow it out to maybe a 5 or 10 degree dive, aiming just above Syd's head. (Yeah, the guy's got some major you know what's!)
We did a lot of runs. My best speeds were with the 7.4 X 8.25 carbon fiber or the 8.75 (cut down to 7.5) X 9NN. Both props yielded virtually the same top speed, 150 mph, over and over again.
Today I meticulously retuned the exhaust system of my Jett FIRE 50 for a 9X10 prop, achieving 16,700 rpm. One run gave me an instantaneous reading of 16,900 but that was probably an abberation.
The re-tuning was done by re-inserting sections of header and using two couplers to extend the system. I started with the system longer than the airplane, then hacked away initially 1/2", then 1/4" at a time. When I stopped getting rpm increases, I actually lost a bit on the final cut, I put that piece back in.
Now if I could just find a similar sized engine that could turn that prop at 20K!
Until someone proves otherwise, it really seems like the 200 mph figure with a Diamond Dust is somewhat of an RC myth. I believe the airplane has a great deal of drag, whether parasite or induced or whatever, it doesn't matter. It simply takes a lot of ponies to drag that thing through the air, even though looking at it you would not think that to be the case.
Looking at the photos of those Q40 racers in the latest Model Aviation the difference is striking. The Dust has a lot of wetted area, it rubs up against a whole lot of air molecules as it goes forward. Those Q40's are a different story, a couple of chef's knives for wings and a toothpick of a body slicing through the air. A huge difference.
One thing I've always wondered about was whether the covering on the Dust kind of balloons outward or resonates at high speed. The Whiplash would not have that problem, and as a result would probably have less drag than a Dust.
Maybe the key is to scale down the Dust and Whiplash. I haven't flown a Whiplash yet but the Dust practically glides like a kite. What if you put the same engine in a 2/3 sized Dust? Might land like a stone but you'd have a lot less drag. Hmm... Jeff Gilbert, are you reading?
Here's how it was done.
Syd positioned himself at the far end of the runway, crouching down on the centerline. I would take the Dust up high till I could barely see which end was up, then do a split S to start my dive, initially coming nearly straight down. I would then shallow it out to maybe a 5 or 10 degree dive, aiming just above Syd's head. (Yeah, the guy's got some major you know what's!)
We did a lot of runs. My best speeds were with the 7.4 X 8.25 carbon fiber or the 8.75 (cut down to 7.5) X 9NN. Both props yielded virtually the same top speed, 150 mph, over and over again.
Today I meticulously retuned the exhaust system of my Jett FIRE 50 for a 9X10 prop, achieving 16,700 rpm. One run gave me an instantaneous reading of 16,900 but that was probably an abberation.
The re-tuning was done by re-inserting sections of header and using two couplers to extend the system. I started with the system longer than the airplane, then hacked away initially 1/2", then 1/4" at a time. When I stopped getting rpm increases, I actually lost a bit on the final cut, I put that piece back in.
Now if I could just find a similar sized engine that could turn that prop at 20K!
Until someone proves otherwise, it really seems like the 200 mph figure with a Diamond Dust is somewhat of an RC myth. I believe the airplane has a great deal of drag, whether parasite or induced or whatever, it doesn't matter. It simply takes a lot of ponies to drag that thing through the air, even though looking at it you would not think that to be the case.
Looking at the photos of those Q40 racers in the latest Model Aviation the difference is striking. The Dust has a lot of wetted area, it rubs up against a whole lot of air molecules as it goes forward. Those Q40's are a different story, a couple of chef's knives for wings and a toothpick of a body slicing through the air. A huge difference.
One thing I've always wondered about was whether the covering on the Dust kind of balloons outward or resonates at high speed. The Whiplash would not have that problem, and as a result would probably have less drag than a Dust.
Maybe the key is to scale down the Dust and Whiplash. I haven't flown a Whiplash yet but the Dust practically glides like a kite. What if you put the same engine in a 2/3 sized Dust? Might land like a stone but you'd have a lot less drag. Hmm... Jeff Gilbert, are you reading?
10-24-2002, 12:41 AM
#25
Senior Member
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unloading
Do you think your engine "unloads" in the air and turns more rpm's? My nelson does not, at least yet to do so. Your 16,700 with a 10 pitch is only 150 mph. Thats why you only go 150 mph. The nelson does 23,000 on the ground, with 8 pitch. I am going to increase pitch and cut prop until i max out. If this thing "unloads" in the air watch out. If other people claim to get 27,000 in the air there is no reason a qm40 nelson wont. I didnt think anyone was going as fast as they thought. I think i was the first to question it here but now im not convinced i was right. My 163 mph on third run of engine has me thinking i can make at least 190.