# I need HELP

### Help Support Homebuilt Aircraft & Kit Plane Forum:

##### Well-Known Member
Supporting Member
I just assumed that 90 kg all-up meant tethered, with a pneumatic motor or similar. Simplifies stability quite a bit too.

#### Appowner

##### Well-Known Member
I just assumed that 90 kg all-up meant tethered, with a pneumatic motor or similar. Simplifies stability quite a bit too.
It would but the OPs comment in the 3rd post suggests he intends to man the beast.

#### Razorshark

##### Active Member
Thrust is lift. To fly up you have to make more thrust than gravity. If everything weighs 90 kg, you need to make 91 kg to float up. 180kg thrust means you can go up at will.

So what will i need to gain 91kg thrust in terms of blade size ☺

#### Razorshark

##### Active Member
My concern remains your 90 kilo weight. Is that supposed to be flying weight? That is complete with fuel, pilot, etc.? If so, that's a number I doubt you will ever achieve. It simply is not practical with current technology.

The numbers were just figurative, to understand how one would calculate blade/prop size from that (complete weight, including pilot and...etc)

#### Appowner

##### Well-Known Member
So what will i need to gain 91kg thrust in terms of blade size ☺

In theory. But with just one kilo advantage vertical performance would be lacking. Ideally you need to get somewhere around 120 kilos of thrust to start and work from there.

#### Sraight'nlevel

##### Well-Known Member
Lift = ½ x P x A x v^2 x Cl

Very roughly !

#### Razorshark

##### Active Member
Lift = ½ x P x A x v^2 x Cl

Very roughly !

Is their a simple app for that one to calculate
I am not a mathmatic i am merely just trying to get a easy way of simple facts.
Around lift etc...

And I really dont care about the security measurements, it is possible that I by the end of september have a prototype - of some artistic, green and innovative design.... Which can get you to the sky, and get you soaring down to the ground.
Only thing is if custom build blades or buy one of the ones around in shops
Noting big..

So all help and ideas are appreciated

#### wsimpso1

##### Super Moderator
Staff member
You guys are mixing up your units, which makes calcs tough. If the scale says your machine with payload has 90 kg of mass, it weighs 882 N or 198 lb, which are units of force. To just barely hover, you will need thrust of 882N or 198 lb. When you go to calculate thrust by any method, it will give you number in force units, which is why it is important to know how many force units you will need.

Next is that there are ways to calculate, which is to say "to estimate" the lift available from a well designed prop or rotor when stationary. The following is one derived from first principles, but I do not remember where I got it from, so remember that this advice may be worth no more than what you paid for it. Expression is in Excel:

Max Static Thrust = (PI()/2*D^2*rho*(nu*Power)^2)^(1/3)

Where PI() is Pi, 3.14159...
D is prop or rotor diameter in feet or m;
rho is air density in slug/ft^3 or kg/m^3;
Power is power available at the rotor in ft-lb/s or N-m/s (Watts);
nu is efficiency of the rotor turning shaft power into moving air, I guessed at 85%;

Use consistent units, all SI or all Brit. Start mixing units and nothing works.

This is the upper bound of prop or rotor at zero speed, and was worked out long ago. The sensitivity to rotor diameter is huge. Large rotors and modest power works. 200-300 hp helos have gross weights over 2000 pounds, with really large rotors. Opposite condition is the tail sitter fighters of similar weights and much high power because of the much smaller prop disc.

I just ran with D = 1.5m, rho of 1.225 kg/m^3, nu of 0.85, and adjusted power to just hold 90 kg in hover above our Earth is 18.5 hp. You will need 60-100% more power to make a useful helo. Now a 30 hp piston engine built to be light will somewhere between 20 and 30 kg, the gearbox and prop weigh something, so does your chair, is 90 kg still reasonable?

Triple the rotor diameter to 4.5m, and 6.1 hp is needed just for hover, the now 10 hp requirement is much lighter, but the gearbox/rotor will get more massive, and you can again adjust the overall mass to check on practical.

Billski

#### wsimpso1

##### Super Moderator
Staff member
Is their a simple app for that one to calculate
I am not a mathmatic i am merely just trying to get a easy way of simple facts.
Around lift etc...

And I really dont care about the security measurements, it is possible that I by the end of september have a prototype - of some artistic, green and innovative design.... Which can get you to the sky, and get you soaring down to the ground.
Only thing is if custom build blades or buy one of the ones around in shops
Noting big..
You are telling us you will design a helicopter without using math?

And you are planning to build your design with COTS (Commercial Off The Shelf) rotor blades?

Hard truth time:

It will either never lift off or it will fly uncontrollably. It also has a fair likelihood it will fly apart dangerously when you run it. Do yourself and everyone else a favor and do your test runs a long ways from everything of value and then run it by remote control.

Helo's, when designed by experts using tons of math and science, can lift off vertically, and fly at modest speeds compared to fixed wing airplanes of similar weight and power. These helos are carefully designed and evaluated to make sure excess mass is not being carried but that they are sturdy enough for their missions. You are proposing to skip math which means to skip making the parts sturdy enough (you will have no idea if they will break in use) and to skip optimization (you will have no idea of if the parts are too heavy for their jobs), so prediction of failure is easy. The only questions are if it will fly apart or if it will not lift off.

Building a flying machine in two months? One that works? Using rotor blades someone else optimized for their helo, its weight, power, rotor speeds, etc. I doubt you will find such blades that work for your machine. Building any flying machine in a couple months is over the top too. I want to see it, but am extremely skeptical it will happen.

BIllski

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