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Jun 28, 2002, 06:36 AM

static thrust vs in flight thrust

I am trying to figure out this matter please help. According to P-cal when I use a 7x4 MAS prop on my mega I have 26oz static thrust, 21 oz in flight thrust. But if I change to 7x5 APC prop, I will have 32oz static thrust, 17os in flight thrust. I don't know what is the difference between those 2 kinds of thrust? What are those good for? Please help. Thank you.
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Jun 28, 2002, 08:34 AM
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My non-technical take on static vs. flight thrust is that it's like comparing torque and horsepower in a car. Torque is the twisting force that launches a car from rest. Maximum torque is produced at lower rpm than maximum horsepower. It's what gives neck-snapping performance off the line. Maximum horsepower is produced at higher rpm than maximum torque. It's what gives high top speeds. So you want high static thrust to give you power to launch and pull through low-speed maneuvers, while you want high flight thrust for high-speed flying.

If you tune for too much flight thrust so that there is insufficient static thrust, you may not have enough acceleration power to get the aircraft up above its stall speed. If you tune for too much static thrust so that there is insufficent flight thrust, you may get a powerful launch but not be able to fly fast enough to keep the aircraft from stalling. The reason why people experiment so much with different props is to achieve the best balance of static and flight thrust for a specific aircraft design and their personal flying style.
Jun 28, 2002, 08:45 AM
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Dick Huang's Avatar
Dear Dave Hederich:
Thrust varies as speed;where static thrust (0 speed) is usually the greast thrust and decreases as speed is increased. If you go to MotoCal or ElecCal and plot speed vs. thrust you will see the effects.
Dick Huang
Jun 28, 2002, 10:04 AM
Balsa Flies Better!

What does in flight thrust mean?

I'm not sure in flight thrust means anything. AFAIK, as a propellor approaches its pitch speed, thrust decays to zero.

There are two parameters to really worry about- pitch speed and static thrust (less important.) An airplane with insufficient pitch speed falls out of the sky in a turn, an airplane with inadequate static thrust never leaves the ground.

I use the highly scientific Brauer Jerk Test (BJT) for static thrust testing. (Developed by a Ph.D. if that makes you feel better.) If the airplane does not jerk forward (on a smooth surface) when full throttle is applied for one second, then static thrust is insufficient. (If you don't want to shut down the power in a hurry in limited space like a driveway.) This test is somewhat inconclusive with old timer models or other high lift configurations, but does work on e-conversions of rubber powered scale aircraft.

Pitch speed should be calculated using a tach, and pitch of the prop. Sample calculation- 4.7 inch pitch x 5.3 (rpm/1000) yields a pitch speed of 25 mph. (It's not exact, but close enough.) Pitch speed of 2.5x stall speed is recommended as a minimum.

Sam (who doesn't trust calc programs much)Brauer
Stamford, CT
Jun 28, 2002, 10:46 AM
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Sparky Paul's Avatar
Jun 28, 2002, 03:18 PM
jrb's Avatar

Not the limit

Been waiting Paul for something more than “Facinating!”. More akin to your other aero postings.

In the meantime here are some of my thoughts:

Have written before that I don’t think pitch speed limits the maximum airspeed, and that a prop adds velocity to the incoming flow. Found the following reference that indicates the same: .

Also Ecalc indicates the same thing, in that flight speed is where thrust equals drag and that can be greater than pitch speed.
Jun 28, 2002, 04:08 PM
Registered User

Re: Not the limit

Originally posted by jrb
Have written before that I don?t think pitch speed limits the maximum airspeed,
One problem here:

The geometric pitch (GP), as printed on the prop, is probably not the same as the prop's mean aerodynamic pitch (MAP), which is really what determines pitch speed. The MAP is usually larger, so a 6x3 may really be a 6x3.5 or 6x4 in practice. The more camber the blade airfoils have, the larger the MAP (and the pitch speed) is relative to the GP. I don't know how Motocalc deals with this, if at all.

Th difference is because GP is usually measured from the bottom of the blade, while MAP is measured from the blade airfoil zero-lift lines. Tha latter have a larger "bite" angle and hence more pitch.
Jun 28, 2002, 05:48 PM
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Sparky Paul's Avatar
An illustration from my aero text.. "Technical Aerodynamics", K.D. Wood, 1955..
What Mark said...
And I doubt than anyone other than a propellor manufacturer has real information on model airplane propellors and thrust.
Inflight airspeed and rpm are required at the very least..
The forces involved are so small that a precision device would be needed to determine the numbers..
Jun 28, 2002, 09:44 PM
Registered User
Tim, now see what you've started -- all of the math and science nerds from high school are all grown up and they still remember all of the equations that I never learned.

I can't argue with any of the technical stuff. But I can tell Dick Huang that I have run billions and billions of computations in P-Calc, and I have found many combinations that have virtually identical thrust figures for both static and in-flight. I accept no blame for any incorrect data that may come out of any *Calc program.
Jun 28, 2002, 09:56 PM
Hi Dave, :-) I just love Ezone and Ezoners for whatever it is... It is very educational and highly interesting. I love to learn what I don't know and review what I forgot... So the fun is there and many nice people in the zone... What more do I want? Oh yeah... more time for my planes... is that sensible? :-)
Jun 28, 2002, 11:40 PM
Ascended Master
Sparky Paul's Avatar
If you -really- wanna get into prop design, you need information like this..
Frankly, it's way too much effort for me, on a toy airplane, to do this much work.
Let the geniuses at Landing Products, or Graupner scratch their heads bald trying to get this information at model scales and build the props.
Which they do quite nicely however they develop them.
As models at the sport level of flying are far from precision instruments, it's easiest to try it and fly it.. take a bucket full of props to the field, and fly them. Your plane will necessarily be different from all the other planes, including those from the same kit. What works on your plane may not work as well on next guy's "identical" plane.
Jun 29, 2002, 09:40 AM
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Dick Huang's Avatar
Dave Hederich
"I can't argue with any of the technical stuff. But I can tell Dick Huang that I have run billions and billions of computations in P-Calc, and I have found many combinations that have virtually identical thrust figures for both static and in-flight. I accept no blame for any incorrect data that may come out of any *Calc program."
Really,show me on sample and the velocity for the in flight thrust.
Dick Huang
Jun 29, 2002, 09:06 PM
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Larry Dudeck's Avatar
The inflight thrust must exactly equal the aero drag of the air frame (at constant velocity) and is a function of of the frontal area of the air frame. Larger frontal area, more drag.

A plane with 100 of frontal area may have a maximum velocity of 25 mph. Use the same power setup (including the prop) on an aircraft that has half the frontal area. The top speed will be higher as there is less drag. Since drag must equal thrust at constant velocity the inflight thrust is now lower than with the 100 frontal area ship.

The difference between static thrust and inflight thrust is what allows your airplane to accelerate to its top speed. The mass of the airplane will determine how long it takes to reach that top speed. Heavier ships accelerate slower, need longer take off runs.

The only accurate method of determining inflight thrust (aero drag) is to measure the constant velocity of the plane in level flight, zero wind (radar gun, etc,) and the frontal area. Knowing the ambient air temperature and relative humidity, the dynamic pressure due to velocity can be calculated, hence the the drag is known. This value is the inflight thrust.
Jun 29, 2002, 09:16 PM
Registered User
Dick, most people probably haven't seen the kind of numbers that I'm getting because I'm doing some pretty odd combinations. Basically, I was looking to overpower a lightweight (~9 oz.), high wing area (>500 sq. in.) IFO. The combination that I looked at was a high Kv Johnson 250 (EMPS MM1 in P-Calc) on 7 AAA NiMH 720mAh cells in a 6.6:1 GWS 280-size gearbox with a 10x4 APC prop.

The numbers that P-Calc spit out are: 5.819 motor amps (close to the limit on the AAA NiMH cells), 21.4 mph pitch speed (the IFO can get away with a much lower pitch speed than most aircraft), 7:25 full-throttle duration, 12.5 oz. static thrust and 12 oz. in-flight thrust.

As I said, I'm only telling you what P-Calc told me. I'd be very interested if anyone with a technical background has any thoughts on these numbers and what they mean in real life.
Jun 30, 2002, 09:30 AM
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Dick Huang's Avatar
Dear Dave,
I ran your numbers through motocal and got the attached numbers. I tried the plot of thrust vs. velocity but could not load it due to the wrong file extension.
Dick Huang

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