asmund

Hello, I have a question for you. Could a Barbera Jean fitted with a Rossi .45 and APC 10-8 spinning 13400 rpm static, reach a speed of 142 miles pr hour? To me it seems impossible, but some guys in our rc-community has put a Garmin GPS in there and got these readings. The problem is that when I feed the data into the prop thrust calc of Pe Reivers, it says that the engine is only developing 1.19 hp and 101 miles pr hour inpitch speed. I know that the engine unloads a bit in the air, but still....

What do you think guys, plausible or false? in the spirit of Mythbusters

proptop

My Feedback: (8)

Maybe...

With a tail wind...slight dive...???

Was that just a peak reading? What's the average?

Back a few years ago, I flew some test flights for some local lab guys, with a GPS (amongst other equipment ) in a Tiger Trainer 60 that was powered by a .91FX and APC 13x8 on 15% Wildcat, and we saw 101 out of it...(that was down wind, admittedly, but still not bad for a trainer )

wjvail

My Feedback: (3)

We've been down this road MANY times. Taking 13.5K rpm, and being generous, you could say it unloads to 15K in the air. With 8" pitch that comes out to 113 mph. In a big ol' dive you can usually do about 10% over the prop speed or in this case 124 mph. Add a 15 knot tail wind gust and you might see 140.

You are correct to be sceptical!

If you've got time you might read here >> http://www.rcuniverse.com/forum/m_77...tm.htm#7781638


Bill

tIANci

Whatever RPM you are getting you calculate the PITCHspeed ... then the RCgurus will tell you to deduct about 25% for slippage. After that lose more speed for aircraft drag. The Barbera Jean +Rossi 45 doing 142 is a MYTH. I have run that engine before its fast but not enough to pull that plane to 142 mph. Even my GPViper 500 with a screaming Rossi 53 is only doing about 115 mph. The answer to speed is in the fuse ... Viper is so much more sleek than the Barbara Jean.

Look at what it takes to fly at 150 mph ... see the various planes such as the Revolver and the Kelly Racer ... they are using EP with very coarse pitch props.

asmund

We are having a rather big and heated discussion on our local Norwegian forum about this. The owner of the GPS is 100% sure that his instrument shows the correct speed, no matter what physics and math is saying about it.

He also claims that a friends 33% Yak 54 with a Da-85 and a  26x10 prop is doing 109 miles an hour, measured with the same GPS.

I use the Pe Rievers prop calc, and to get to that pitchspeed it would take almost 12 000 rpm. I wonder how that DA gas engine would like that  And I think that such a big 3-D plane would flutter and fall to pieces at that speed.

The strange thing is that when fitted in a helikopter the  speed readings are very plausible.

Anyone have any theories of why this Garmin GPS is showing the clearly wrong readings that it does when stuffed into a rc-plane??

Mike Connor

My Feedback: (5)

Downwind in a dive the GPS may not be that far off but I have read that abrupt maneuvers can make them give high readings. Do you have a video of a flight? I would be happy do do a Doppler speed check to confirm the claimed speed.

asmund

Sorry, I do not have a video of the flights. My guess is that there`s a good reason why everyone uses radarguns instead of GPS when they want to know how fast their plane goes.

asmund

There is a video of this plane beeing uploaded tonight, I will se if I can get it on here tomorrow

still4given

Since a GPS gets it's information from satalites, it can only register speed accurately parallel to the earth's surface. As far as it can tell, if you are diving straight down, you are standing still. The fastest reading it would give is at the moment that your plane levels out after a dive. The GPS in my car reads about 2 MPH slower than I am actually going, based on my speedometer and the speed detecting signs beside the road.

BGH

Hello, I have uploaded a video of the Barbera Jean here. (notready yet, but soon) http://www.vimeo.com/4934058

Mike Connor

My Feedback: (5)

Until then for comparison, check out this Bobcat video I just checked with Doppler. Maximum speeds of about 145.
http://www.rcuniverse.com/forum/m_8814477/tm.htm

Mike Connor

My Feedback: (5)

I had trouble viewing the video and the sound was broken in places. I did Doppler the first pass, the one at about 45 seconds and one other. I came up with speeds of 125 through just over 140 mph. Interesting. Wish I could have seen the video. Anyone else have problems with the video or do a Doppler check?

Edit - Inflight rpm was about 15,500 rpm

Mike Connor

My Feedback: (5)

Double post deleted

Mike Connor

My Feedback: (5)

I agree. But a shallow dive will help ground speed quite a lot, At least it does in full scale.

tIANci

Asmund ... tell them to get a proper speed trap ... that is best or time over a measured course. Again, is the Garmin a one for planes or the basic car GPS? Having owned the Rossi 45 before I have my doubts that she will haul the BJ to such speeds. Esp when your pitch speed is no where near it.

I measured the speed of my Micro Jet using a measured course and a radar gun, both came to about + or - 2 or 3 mph. So a radar gun is pretty accurate.

asmund

Hello, the GPS is designed for car use, so it might not be the best up in the air. Our club is considering buying a Bushnell speed gun to be available for all members.

Are these relatively inexpensive ( around 140$) Bushnell speed guns accurate enough or is it wasted money?

tIANci

Asmund ... only way to really know is to use it along side a measured course ... heheheehehee ...

asmund

Hmm..so are they no good then? I have seen numerous vids on youtube and it seems as Bushnell speedguns is very commonly used to measure speed of cars, boats and aircrafts

proptop

My Feedback: (8)

One of the guys in our club has a Bushnell gun...

We found that you have to fly practically down your throat to get a decent reading...the greater the angularity, the more difficult it was to get any kind of reading.

As far as accuracy is concerned, IMO I think they could be fairly accurate, but it was difficult to get a reading on something like a fast moving model airplane. (not much frontal area either..)

They were selling for about $75.00 around here last time I checked...that was a couple years ago.

RocketRob

My Feedback: (1)

Mike what doppler softwear are you using?

I've been using the wavoscope on .wav files.

BGH

Mike.! I have uploaded the video on Youtube, pleasetake a look here..http://www.youtube.com/watch?v=qkD-SLK1n14

Mike Connor

My Feedback: (5)

Yep, WavOScope

Mike Connor

My Feedback: (5)

Yep, that works much better. Anyway, it seems Doppler and the GPS both agree that 140+ mph was reached. So much for the prop pitch formulas.

proptop

My Feedback: (8)

It seems every now and then a certain airframe/engine/prop (etc. ) combination just works better than maybe it should?

Maybe the APC 10x8 is one of those props that is a little bit more that what it is labeled? (pitch wise I mean ) That could throw off your pitch speed calculations somewhat.

My WM Dago Red has a nice running Webra .50 in it...also has a Nelson/Ultrathrust muffler on it, an APC 10x8...and tachs just under 14K rich. It unloads quite a bit...

We radared it with the above mentioned Bushnell gun...flying straight at us (definitely NOT recommended, but occasionally some of us can get get a little [X(] huh? ) and got 127 MPH in a very shallow dive...est. 5 degrees or so with no wind to speak of.