Airspeed question

wjcalhoun

Member
Guys:

I bought an Eagle Tree system, with altimeter, 3-axis accelerometer, and GPS. Mounted the components on a 3x5" piece of ply, and installed adapter mounts in all my aircraft so i can move it about.

My question is this:
I was flying my Revolver 90 / DLE55RA gathering performance data, and I am unable to explain why the GPS speed (averaging upwind and downwind) exceeds the theoretical max.

I was out in 15 mph south winds , and ran some S/L speed runs. Averaging 2 upwind (86 mph ground speed) and 2 downwind runs (116 mph groundspeed), directly into, and directly with the wind (averaging 15 mph), I averaged 101 mph AIRSPEED. I had a week or so earlier, in different wind, gotten an estimate of 103 mph airspeed, so these two sets of data were pretty consistent.


I am missing something, however:

I turn 6800rpm on the ground with a Xoar 22x10 prop; that gives me a theoretical max airspeed of about 65 mph, not 101 mph.

The Eagle Tree GPS calibrates - when i take it for a ride in my truck, the GPS speed matchs my garmin gps speed, and matches my truck's calibrated spedometer, so I am pretty confident that I have accurate GPS data from the Eagle Tree system.


How then do I get airspeed above theoretical max? Yes, the engine unloads some, but doing the arithmetic, it would have to unload to >10,000 rpm to exceed 100 mph, and i doubt that a DLE55RA would unload that much.

What is the aerodynamic, or other, explanation?
 
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wjcalhoun

Member
No thoughts guys?
You folks, of all the forums out here, seem to be the experts, and have the experts that have left other forums.

I would appreciate any ideas you have.
 

BRUTUS

Plank Junky
Lifetime Supporter
I'm no expert- and we do have some smart guys on this forum, but it sounds like a math problem. You should gain a few hundred RPM in the air, but 10,000 seems a little high. If it were me, I'd just be happy with the extra speed!

I know a 150cc aerobat can hit 140 turning a 10 pitch prop. Not straight and level maybe, but it is possible.

What are you trying to accomplish? Just increasing your level of understanding? Is it for an engineering project? You building a UAV or something? Aerodynamics?
 

Bunky f. knuckle

Cover shot, MA 10/09!!!
I think he is over thinking it.............

All sorts of other factors equate into that. Drag coeffecients, etc.......... I'll let a aero dynamacist explain it. As, I'm far from one. I think Cody is spot on. Seems to be some sort of math error in your calc's...... I/he/we could be wrong tho..........

I've always been told, if you put alot of power on a turd, it will fly.

If I have an airplane that does 113 mph, I'm gonna be happy!!! I'm not gonna be concerned about getting 115. Will I notice that 2 extra mph??? Probably not.

Still not sure why you would be concerned about theory speed. Ahhhh!!! Real world vs book world.
 

BRUTUS

Plank Junky
Lifetime Supporter
Some guys on FG made good points about unloading the prop, and that the airfoil on the prop adds to the equation. I guess having 10 times the number of active members makes a difference. TFC is more like a laid back lounge. Someplace to cool your heels and connect with no BS people!
 

Tired Old Man

Staff member
The 55 can indeed unload in excess of 10k with the right prop but I doubt that's at issue here. Too bad you didn't add the tach fucntion to your Eagle tree because it would lay out exactly what's going on with RPM. I'll venture you are probably picking up a little over 1,000 RPM and just don't realize it.

Your GPS system may be getting interference from your radio system.
 
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jaredge

Well-Known Member
Those speeds sound about right. A friend has a radar gun and we used it on different planes. Suprisingly, they all seemed to be within about 20mph of each other. The forty-centers were about 95-110. A 33% yak with a BME 110 was about 115. The coolest part was getting the reading. For the most accurate reading, the plane has to be low and coming straight towards you. BAD ASS to have them rip right over your head at 15-20'!!!
 

Tired Old Man

Staff member
It's normal to see .40 size planes busting 100 mph. Scale wise they are much faster than their larger brethren. Then you have the specter of so many people that only fly at one speed (wot) until the tank runs dry.
 

TManiaci

Engineer
Lifetime Supporter
Everyone has pieces of the right answer.

The given prop at a given rpm will produce a fairly well defined thrust given air density and a reasonably accurate CF factor. In this case, the computation says the thrust is around 478 ozs (30 lbs). That rpm requires 5.0-5.2 hp to turn that speed, which agrees with the engine's power range.

So, this becomes a simple force problem, where the thrust is the force of the propeller and drag is the opposing force. The terminal (maximum) velocity is the point where the airframe drag equals the thrust.

It is indeed true that the prop rpm increases some as resistance is relieved from a static to a moving condition, but that change is not a huge influence on the physics, because the relative thrust remains somewhat constant. Relative thrust is basically the difference between the stationary airspeed of the thrust and the moving thrust airspeed minus the speed of the aircraft moving thru the air. So, as the airplane moves faster, the rpm will increase which results in a higher airspeed of the thrust relative to the airplane, but not to the air around the airplane. The difference is the air speed of the airplane.
 

TManiaci

Engineer
Lifetime Supporter
Oh, sorry... you can find a good Thrust and Power calculator HERE. Mess with the numbers, you'll see the airspeed is not part of this computation. The RPM can only increase if the power is there to make the thrust. So, if you assume there is an extra 1000 rpms "unloaded", it shows the increased HP required to make that thrust. So, don't be fooled by simply cranking up the rpm if you don't have the HP to do it. This is what you call "unloading", because the thrust is relative to airspeed.
 

wjcalhoun

Member
Gentlemen: thank you for your replies.

As to why i am asking, i just want to understand the apparent paradox of why recorded airspeed exceeds theoretical max; i am a science guy and would like to understand the physics. There are some at my field who assert that it is impossible to fly faster (S&L) than theoretical max, but the data are what they are.

To TOM's points, I am in the process of implementing RPM monitoring off the electronic ignition, but do not yet have the data. As a very crude (+/- 15%) check on the GPS ground speed, i did clock a downwind run over the 600' runway, and the time of flight was consistent with about 110 mph (downwind with 10-15mph tailwind).

To TMan's point - as i understand prop physics, the thrust produced depends on the AOA of the prop blade (and RPM), which may be stalled at WOT on the ground, so static thrust on the ground may not be a good measure of the max thrust produced. My conceptual issue is that prop blade AOA must decrease as airframe speed increases (just by geometry), such that when airframe velocity reaches 'theoretical max', the AOA of the relative wind the prop blade sees is zero.

Thanks again for thoughtful replies.

Bill
 

TManiaci

Engineer
Lifetime Supporter
My conceptual issue is that prop blade AOA must decrease as airframe speed increases (just by geometry), such that when airframe velocity reaches 'theoretical max', the AOA of the relative wind the prop blade sees is zero.
Precisely, well put.

The part that is hard to wrap your head around is the idea of relative speed. If you do the pure math, a 10-pitch prop advances 10 inches per revolution. At 6,800 rpm, that equates to 64.4 mph. Based on that, to go 100 mph with a 10-pitch prop requires 10,555 rpm. I don't think anyone will buy your prop "unloads" to nearly 4000 rpm higher, right?
 

wjcalhoun

Member
Exactly - maybe the prop/engine unloads 1000 rpm, maybe. Maybe the pitch is off by 10%, maybe. I have GPS data confirming airspaeed (upwind and downwind) in excess of theoretical max. Just to add another point, if the upwind and downwind runs are not exactly so, then it is possible that the crosswind component of each run could ADD to the measurements. That is why i specified that the up- and down-wind legs were truly oriented into and with the wind.
 

Tired Old Man

Staff member
I think we are all presuming that propeller pitch labeling is correct. The manufacturer that uses an A-B-C pith rating provides nothing for pitch information. I would be very surprised if some prop manufacturers were even close to the stated pitch dimension. As for unloading multi thousand RPM, there ain't no way. Perhaps the theoretical max is impacted by some Reynolds value due to smaller blade sizing. It does not make any sense that smaller props could be more efficient but stranger things have happened in this hobby.
 

gkamysz

Member
Oh, sorry... you can find a good Thrust and Power calculator HERE. Mess with the numbers, you'll see the airspeed is not part of this computation. The RPM can only increase if the power is there to make the thrust. So, if you assume there is an extra 1000 rpms "unloaded", it shows the increased HP required to make that thrust. So, don't be fooled by simply cranking up the rpm if you don't have the HP to do it. This is what you call "unloading", because the thrust is relative to airspeed.

You can't use the static calculations to determine crank power in flight. If you take this to the extreme a prop will absorb power from the air and turn the crank, like a windmill. This doesn't really happen in models except at very low throttle settings and very high airspeed. In electric models when props unload in the air, shaft power actually drops due to motor characteristics. This means less power but more RPM and speed. Of course all this is dependent on many variables. Unloading is really a decrease in local AoA of the prop airfoil, resulting in reduced torque required to spin the prop.

Airplanes routinely exceed "pitch speed". Even at pitch speed the prop makes thrust. If this thrust exceeds airframe drag, airspeed continues to increase until the local angle of attack at the prop blade drops and so thrust drops to match total drag. A prop designed to be most efficient at an air speed of pitch speed will drop to zero thrust approximately 30% above pitch speed.

The only thing this really means is that the prop is poorly matched to the airframe for efficiency at high speed. More pitch would get the airframe going faster without costing more fuel. But there are other tradeoffs.

Propeller performance/design texts will explain in detail.

Greg
 
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wjcalhoun

Member
Airplanes routinely exceed "pitch speed". Even at pitch speed the prop makes thrust. If this thrust exceeds airframe drag, airspeed continues to increase until the local angle of attack at the prop blade drops and so thrust drops to match total drag. A prop designed to be most efficient at an air speed of pitch speed will drop to zero thrust approximately 30% above pitch speed.

Greg
You sound like an engineer; thanks for this explanation. Could I ask you to expand a bit more?

My very basis understanding is that the effective AOA of the prop blade is determined by the rotational speed and the forward airspeed. Perhaps it is my misconception, but when airspeed reaches 'pitch speed', does that not make the AOA of the prop blade zero? If so, as you say above, how does the blade continue to make thurst. Your estimate of zero thrust 30% above pitch speed is consistent with my empiric number with three different airframes.

Thanks for your insights, Greg.
 

gkamysz

Member
Perhaps it is my misconception, but when airspeed reaches 'pitch speed', does that not make the AOA of the prop blade zero? If so, as you say above, how does the blade continue to make thurst.
I study topics that interest me.

AoA at pitch speed is dependent on how the prop designer decided to mark the prop. Many use the flat back of the blade which means the chord line is at a greater positive angle. The lift curve of the chosen airfoil usually means that the airfloil generates lift(thrust in this case) even to some negative AoA.

If model props were tested like full scale props all this data would be readily available in charts.

Greg
 
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