# Trouble with Chris Heintz design book

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#### WurlyBird

##### Well-Known Member
I have started reading Chris Heintz' book Flying on Your own Wings which I hoped would help me start getting a better understanding of design principles and get me back into math and science a bit since I dropped out of UCF's engineering program a few years back to join the Army. I have read Simplified Aircraft Design for Homebuilders by Raymer and Light Airplane Design by Pazmany and both of these are rather general in a lot of aspects and I wanted something with a little more meat. Here is my problem, and I want to make sure I am not crazy, I am only just into chapter 3 and Heintz has made few statements that I believe to be false. Very false. He is much more educated then me and has forgotten more about engineering then I ever learned but if I am correct about these statements it makes all the stuff I don't understand and am trying to learn questionable. Here is the real doosie that has be completely confused (if I am wrong someone please correct me)

He defines;
The unit of Force as grams and goes on to say that weight is grams, Kilograms, etc.
Mass=weight/earth's acceleration (this sounds good but then he goes backwards) so;
Mass= W/g = (kg.sec^2)/m

I learned that Mass is gram, kg, etc. and "weight" or the force we can measure due to gravity is measured in N, Newtons, and the correct unit for this is kg.m/sec^2.

This definition of Mass is used in all his following equations for inertia, kinetic energy, etc. which all look odd because of this. Am I crazy?

#### Himat

##### Well-Known Member
Without checking Wikipedia or the physics book (taking a risk):

Force = Acceleration * Weight, In this case Acceleration is the gravity constant.

Force (Newtons) = Acceleration (G) * Weight (kg)

If you look at the dimensions:

Force [(kg*m)/s^2] = Acceleration [m/s^2] * Weight [kg]

Think you are right.
And yes, it is common to confuse weight (mass) and gravitational force. Or mass and force. Some of the older systems of units did make this confusion even easier to do as the name of the weight (mass) unit tended to be more or less the same as the name of the force unit.

##### Well-Known Member
Both of you are right.

Note that quite a few of the "design/engineering" books really cut a hell of a lot of corners. Without understanding the underlying assumptions, it can be very risky to use them. Hollmann is a well-known example, don't know about Heintz.

#### WurlyBird

##### Well-Known Member
Heintz' book, from my understanding is supposed to be sort of a step by step how he designed the CH series. I have not gotten in to the actual meat of the book yet since the first three chapters are just a review/introduction to most of the math, science, and physics that the rest of the text should be based on. He has worked for a few noted aerospace companies and done some great things, not the least of which is design one of the most prolific kitplanes ever. I am just worried that if some of these simple explanations are wrong, and I know it, I am not going to invest the energy to learn anything else he teaches in the book. The faith will be lost. I am sure it is not a lack of knowledge on his part, but if the text goes out without proper QCing then what good is it?

#### Himat

##### Well-Known Member
Heintz' book, from my understanding is supposed to be sort of a step by step how he designed the CH series. I have not gotten in to the actual meat of the book yet since the first three chapters are just a review/introduction to most of the math, science, and physics that the rest of the text should be based on. He has worked for a few noted aerospace companies and done some great things, not the least of which is design one of the most prolific kitplanes ever. I am just worried that if some of these simple explanations are wrong, and I know it, I am not going to invest the energy to learn anything else he teaches in the book. The faith will be lost. I am sure it is not a lack of knowledge on his part, but if the text goes out without proper QCing then what good is it?
The good thing with paper books that have a wide margin on the side of the text is that it then space is provided for own notes. If you read and work trough the book, adding own notes and rectify mistakes as you go, you learn a lot. To rectify mistakes and extend explanations you have to search for other text, read these and realy understand the subject wich might make your wiser than read a "correct" book.

#### plncraze

##### Well-Known Member
HBA Supporter
I agree with Himat. One has to follow through with what is being written and compare it to other books and reports. Also as you read various things you will see who has the formulas you need so you can find them when you need them.

#### bmcj

##### Well-Known Member
HBA Supporter
He defines;
The unit of Force as grams and goes on to say that weight is grams, Kilograms, etc.
Mass=weight/earth's acceleration (this sounds good but then he goes backwards) so;
Mass= W/g = (kg.sec^2)/m

I learned that Mass is gram, kg, etc. and "weight" or the force we can measure due to gravity is measured in N, Newtons, and the correct unit for this is kg.m/sec^2.

This definition of Mass is used in all his following equations for inertia, kinetic energy, etc. which all look odd because of this. Am I crazy?
And one wonders why the wings fall off...

(For those of you who may be shaking your head and saying "He didn't just go there, did he?", I say to you, "Someone had to.")

#### Dana

##### Super Moderator
Staff member
It's important to use terms and units correctly.

WurlyBird, without seeing Heintz's book myself, it sounds like you are correct. Grams (and kilograms) are units of mass, not force. While it often convenient to speak of force in grams and mass in pounds (gram-force or lbm), it's necessary to remember that they are derived units which can't be directly used in equations that expect force in newtons or mass in slugs. I assume (hope!) that Heintz uses the appropriate conversion factors when he uses non standard units in his equations, but it is in fact non standard and thus bad practice due to the potential for confusion.

Himat, not quite correct. Weight is a unit of force, not mass; it's the force that gravity exerts on a mass. On another planet your weight would be different but your mass would remain the same.

-Dana

But do you trust the _government_ with semi-automatic assault rifles?

#### bmcj

##### Well-Known Member
HBA Supporter
Another area that gets confusing is some of the British variations, i.e. - pound-force, pound-mass, slug, and poundal.

#### Topaz

##### Super Moderator
Staff member
Log Member
+1 on Dana here. Dunno what Heintz is thinking if his book is as you say, but in the SI system, gram is a unit of mass and Newton is a unit of force. In SAE, pound is a unit of force and slug is the unit of mass. We don't often see slugs used, except here in aircraft design.

You must use the correct units or the equations won't work.

Be careful, take lots of marginal notes for yourself, and cross-check the formulas against other sources to make sure he has or hasn't (as appropriate) inserted constants to adjust for his poor use of correct units.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Simplest terms, as used in aero industries

Mass - SI uses kg, British uses Slugs

Force is derived - F=m*a

Force - SI uses N, British use Pounds

Weight - SI uses Newtons, which are kg*m/s^2, British use Pounds, which are slug*ft/s^2.

On Earth, a 1 Kg mass weights 9.802N, as the acceleration due to Earth's gravity is 9.802 M/s^2. A 1 slug mass weighs 32.2 pounds on this rock.

If you start trying to work in pounds mass, you are likely to get all tangled up in your undershorts. Get into the moments of inertia about the axes, and you will be way better served in slugs or kg...

Billski

#### Himat

##### Well-Known Member
It's important to use terms and units correctly.

WurlyBird, without seeing Heintz's book myself, it sounds like you are correct. Grams (and kilograms) are units of mass, not force. While it often convenient to speak of force in grams and mass in pounds (gram-force or lbm), it's necessary to remember that they are derived units which can't be directly used in equations that expect force in newtons or mass in slugs. I assume (hope!) that Heintz uses the appropriate conversion factors when he uses non standard units in his equations, but it is in fact non standard and thus bad practice due to the potential for confusion.

Himat, not quite correct. Weight is a unit of force, not mass; it's the force that gravity exerts on a mass. On another planet your weight would be different but your mass would remain the same.

-Dana

But do you trust the _government_ with semi-automatic assault rifles?
I stand corrected.:emb:
It is easy to mix this, and I mangaged to do it myself.
Could it be because we in the daily language usually not difffer between weight and mass?
Most of us only walk on this planet and never get to feel that even with less weight in a different gravitational field our mass and inertia would be the same.

#### Jan Carlsson

##### Well-Known Member
Usually using metric system is straight forward, but then you come to the conversion between units, but that is manageable, sort of frustrating if you are compeering data but after some rounds it is easier.

But what do snails have to do with airplanes? CH 601-701 is the answer.

Can you bring snails and slugs on a airplane

#### GarandOwner

##### Well-Known Member
kg-force IS a unit that is used in the engineering industry. It is the force one kilogram exerts at standard gravity (long number instead of rounded) The first time I came accross it I had the same question "Why isnt this in Newtons?" My understanding is it was created for real world use instead of "book work" as a way to have clean numbers for Forces and weights. A 1kg force produces 9.81N . 9.81N is a sloppy number compared to 1 Kgf. Since I work in the US aerospace industry we use imperial units, however I have seen kilogram force used to denote tension and thrust by others.

##### Well-Known Member
kg-force IS a unit that is used in the engineering industry.
French have a tendency to use kg for force. Germans more often use DaN, but then use 10 m/s^2 to the safe effect. An added benefit is that from kg to lbs the conversion is a bit cleaner as it would be for Newton (multiply by 2, add 10%).

In reality rarely a problem if discussing a real-world engineering issue.

#### Dana

##### Super Moderator
Staff member
kg-force IS a unit that is used in the engineering industry. It is the force one kilogram exerts at standard gravity (long number instead of rounded) The first time I came accross it I had the same question "Why isnt this in Newtons?" My understanding is it was created for real world use instead of "book work" as a way to have clean numbers for Forces and weights. A 1kg force produces 9.81N . 9.81N is a sloppy number compared to 1 Kgf. Since I work in the US aerospace industry we use imperial units, however I have seen kilogram force used to denote tension and thrust by others.
kgf, lbm, and other derived units are useful in many cases, but then you still need to use those "messy" conversion factors to use it in equations. F=ma doesn't work with kgf or lbm.

-Dana

Look, a UFO!!! Let me get my worst quality camera...

#### bmcj

##### Well-Known Member
HBA Supporter
French have a tendency to use kg for force. Germans more often use DaN, but then use 10 m/s^2 to the safe effect. An added benefit is that from kg to lbs the conversion is a bit cleaner as it would be for Newton (multiply by 2, add 10%).
kgf, lbm, and other derived units are useful in many cases, but then you still need to use those "messy" conversion factors to use it in equations. F=ma doesn't work with kgf or lbm.
I think we should petition to have gravity adjusted to 10 m/s^2. It would simplify many of our calculations.

#### Dana

##### Super Moderator
Staff member
And make pi equal 3.00...

-Dana

Exceeding the legal fun limit on a regular basis!

#### StarJar

##### Well-Known Member
I think we should petition to have gravity adjusted to 10 m/s^2. It would simplify many of our calculations.
And make pi equal 3.00...

-Dana
But then a lot of airplanes would be pink, because Madonna would be designing them.

But what do snails have to do with airplanes? CH 601-701 is the answer.

Can you bring snails and slugs on a airplane
Nice info about snails and slugs, Jan. And thanks for the link; At least we cleared that up!