Dogdewd

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This is probably a very simple/stupid question...but I have to ask.

I've been reading a number of threads on this forum and the speed numbers are very impressive. That being said, I'm having trouble with some of the stated speed numbers based on what is probably my limited understanding of estimating speed.

Here's my question.....lets assume:

A prop on a given plane is a 9 x 8
It's turning at 21000 rpms
Assume 100% efficiency

Based on my calculations the maximum speed this imaginary aircraft could obtain is 159.09 mph.

Here's how I come up with this number:

21000 x 8" = 168000 inches per minute
168000/63360 = 2.6515 miles per minute (63360 = number of inches in a mile)
2.6515 x 60 = 159.09 mph

Where have I gone wrong? Remember, we're assuming 100% prop efficiency. If we're assuming 100% efficiency, it is my understanding that the pitch is not an issue since it only lets is "grab" more air.

Can someone explain? The reason I ask is that I'm seeing some numbers here that are hard to believe (no offense). I'm just trying to further my understanding of props speed.

Thanks in advance.

ChuckAuger

My Feedback: (12)

Nope, you got it right..that's why I try to never estimate how fast my plane is going. I just cite other people's claims as to how fast their plane is going, then try to use the same prop and airframe. That's also why you will notice in my post (I assume you are talking about me, since I'm about the only one turning a 9X8 @21K) that I too question the speeds reported and also concede the fact that I am not going as fast.

You will get a vast array of claims as to speed. The only way you will ever know is to time your plane.

Dogdewd

My Feedback: (6)

Hey, thanks a bunch Chuck. You are correct, I was referring to your thread (with regard to 21k). However, I noticed that any claims to your speed, by you....were not present (applause). What prompted the question was somebody else estimating your speed at at least 225 during the dive.

As I'm sure you know, there are some pretty outlandish claims at nearly every flying field with regard to speed. I got to thinking that perhaps I was the one that was confused. So, I thought I'd ask some of you folks here who are much more expirenced than I.

Also, am I correct in assuming that that if same 9 x 8 prop unloaded at approximately 26400 in a dive that the tips of the prop would exceed the speed of sound...or at least try. If this is the case...wouldn't we hear the loud popping that we hear on large radial engines with big props?

(I guess you can tell that we've got somebody at our field that is having trouble with reality).

Thanks again...and sorry to read about your Whiplash!

ChuckAuger

My Feedback: (12)

I think 21K static is a tad high for a 9" diameter prop, that's why I was in Amarillo on that fateful day, picking up some 9X9's so I could load the engine a bit more.

I don't thgink it unloads to nearly 26.4K..maybe 23K or so. And yes if the prop goes supersonic you will hear it. The tips don't have to be the first thing to go supersonic..I think that portions of the blade will go first due to the airflow over the airfoil of the blade.

And this opens a whole different set of arguments. Can the prop actually go faster than it's theoretical pitch X RPM speed due to the lift of the blades?? I don't know.

I do plan to do a few timed runs if I can keep one in the air long enough. I know that the folks that have done Radar and GPS tests on their speed are not just a bunch of misguided speed freaks, they are reputable and honest flyers and I don't question their integrity. I'm just like you, trying to put the pieces of the puzzle together. I hope that this next Whiplash will last long enough for me to get a few answers on my own.

Blue Skyy

My Feedback: (3)

The only real way to check model plane speeds are with stop watches, radar and GPS. The present method I prefer is the use of GPS. It's handy and seems to be pretty darn accurate.

The size of a plane can also decieve the eye. How many times have you seen airliners coming in for a landing looking like they were barely crawling along when in reality they are coming in at 175-200mph?


Blue Skyy

Dustflyer

My Feedback: (13)

Fred,

It's because we measure our speed out of a dive, not straight and level or on a closed course.

Rudeboy

I like to fly as close to the ground as my nerves can take...
That way, even when I'm doing only 150 mph on the flat, it feels much faster.
Try an entire flight not exceeding 50 ft of height...lol... things always seem to happen much faster when you are flying low...

Now about those hilarious speed claims... I used to have a hard time with that too...but now I just think to myself: "Oh well...", and don't react to it...

I get enough satisfaction out of working on my plane, test flying it, and then seeing a noticeable speed increase... I don't even care if it is doing 175 or 185 mph; as long as I can get a kick out of it, I'm happy.

Dogdewd

My Feedback: (6)

Dustfly,

Dive or level doesn't matter. The prop cannot exceed it's pitch times the rpm. That means that a 9 x 8 prop has an approximate maximum speed of 200 mph based on 100% efficiency. Based on my understanding, it cannot exceed that speed no matter what the AOA. Further, 100% efficiency is a "pipe" dream, but it gives us a best case benchmark.

Dustflyer

My Feedback: (13)

Fred,

The engine picks up rpm in a dive.

daven

My Feedback: (1)

I race some of the fastest prop planes available, and speeds very rarely exceed 200 mph. In most cases, an extremely fast Q40 (look at my avatar picture) doesn't exceed 190 mph.

Most quickees with Nelson .40s will turn a 8.8" x 9 prop at over 19,000 on the ground. Do the math for me, but I've seen many on the radar gun in the mid to upper 160's mph.

This is much faster than most people imagine. Unless the claimers have seen a Q40 or Q500 race, I don't think that many truely understand the perception of speed.

I had a Whiplash with Jett .50, and although I didn't get to fly it much, it was very close to Q40 speeds. I wish I could have trimmed it out a bit better, to get a feel for the speed.

Cactus.

My Feedback: (1)

at least the days where you'd hear OS46FX powered Dusts will go 240mph

Dogdewd

My Feedback: (6)

I realize that the engine unloads in a dive....and in flight for that matter; but the efficiency also decreases. The reason a 9 x 8 prop can't exceed 200 mph is that the tips of the prop exceed the speed of sound at 26400. ChuckAuger brings up a very valid point (in a reply above) when he states that the airflow over the prop would reach supersonic speeds before the prop tips reach 700+ mph.

Interesting stuff, thanks for all the education you folks have given me. You all have done a fine job of answering my question. I'm a little sharper now because of it.

Mike Connor

My Feedback: (5)

Fred H,

First you ask a question and then you tell us how it works, Hmmm. A typical prop airfoil will generate lift a -3* AOA and would have a forward movment with a 0* pitch. That makes the above quote wrong.

(Prop tip speed)
After finding the tip speed of a prop at a particular RPM you must apply the formula Vtip = SQR( Vr ^ 2 + V ^ 2 ) to consider the forward movement of the aircraft. Vr is the prop tip speed and V is the aircraft speed. A master Airscrew prop will reach critical mach at about .7 mach and make a bunch of noise. APC critical mach is higher.

I use video cameras and timers to calculate speed. If my calculations are correct I am close to the "pipe" dream of 100% efficiency.

No proof of anything. Just my thoughts.

FlooredCOBRA

prop tip airspeed formula is H2-o x's E=MC2

Therefor the engine slows down time and it goes into warp drive and catapulting the machine into the future.

Common guys just basic math now

My Dust goes how ever fast you think it goes. I have no clue so I just let others guess for me. People ask me how "fast is it going"

I reply "about 100 and something"

Thats because of the warp drive I have in it. lol

mr_matt

My Feedback: (10)

If it looks like I am only going 100 mph faster than you, then you must be doing 200 mph :-)

ChuckAuger

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Good looking plane..where's the prop? Maybe I'll pop into the Jets Forum and discuss some paint stirring techniques.. Only kidding, I wouldn't do that.

And I agree..your plane cost more than my house..it should absolutely go faster!

vanguard

My Feedback: (25)

Daven is right about how fast 190 mph looks. The racers used to come to our club to practice during the nationals at Muncie. I had an opportunity to watch several in the 180 to 195 mph range and they look unbelievably fast. Several guys , at our club, who thought they had fast planes got to see these and quickly admitted that maybe they had over estimated how fast their planes were flying.

Rudeboy

We have a guy in our club who finished in the top ten at the F3D world champs a couple of times...

He was invited to a couple of shows in our country. All this was before turbine engines were around.
So one time he went to a "jet" show. These guys were thinking they had really fast DF planes... doing vertical power dives from 500 feet up, and then passing the speed traps on the field.
So then our club member pulled his F3D plane out of the trunk of his car and fired it up. All I can say is that he ran the **** out of those DF things. We didn't hear a single 200 mph claim after his flight that day...

mr_matt

My Feedback: (10)

Oops I better slink back over to the snobs :-)

mr_matt

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I did have a question, hope I did not ruffle any feathers...

Does anyone know the width and depth of the motor mount opening on a Whiplash?


TIA

ChuckAuger

My Feedback: (12)

Naw..all in good fun. If I had a runway I'd prolly be a "snob" too.

Best I can tell the Whiplash (wow,,you really ARE slumming!) is about 1.25" wide and a little over 2.75" deep. I have carved mine out, but I'm pretty sure that was what it started out at.

Don't know what you are mounting, but there is some pics Here Page 3 where I mounted a Rossi .65 disk in one. I had a buddy who wanted to mount a Rossi .81, but it won't fit without raising the engine about .375" due to the different carb setup..

mr_matt

My Feedback: (10)

Believe me in terms of fun per dollar, jets are definitely at the low end of the scale.

As for the whiplash, I am thinking about using a KBV 82 I have lying around. These are pretty much collectors items but what the heck.

It will fit on the width, but the depth with the lower carb looks like a problem. To say nothing of fuel consumption. When I used to run these, they used about 4 ounces per minute.

Also, I would need a prop that would load it to about 23.5K on the ground, and let it unload quite a bit (to maybe 24.5 to 25K). Maybe 11 inch pitch.

That seems like the only way to get the power out of it, I would guess maybe 4+ hp on 10% fuel.

daven

My Feedback: (1)

Matt,

You will probably have issues with carrying enough fuel for that engine. The standard set - up allows for a 6 oz tank, and you could probably get an additional 2 oz hopper in the radio area. Other than that, theres not much room anywhere to add another tank.

Then again, 2 minutes of flight with a Whip can seem like an awfully long time

ChuckAuger

My Feedback: (12)

I get 2 1/2 minutes on 8oz...but I've never drained the main tank completely, always about 1oz or so left. I suppose a KBV 82 would do a little worse.