Can you explain...

[AndyT]

Lets use a standard prop from a Cessna 172 as a baseline for this.

If the prop turns at more than X RPM (around 2500 I think) the tips go supersonic and that is very bad for the aircraft. If a 50 HP engine can turn the prop at its max speed, what good does more horse power do? The prop can not go any faster without causing damage to the plane. Its not going to be moving any more air because its size and or shape has not changed.

I know there MUST be a logical reasonable explaination for this but it escapes me right now.

[fweinrebe]

Andy, interesting topic.

If the HP is increased a different prop has to be used. To be able to limit the speeds of the tips you either have to increase the blade angle or move to a 3 bladed prop with shorter blades. If you make the prop more coarse, you will have a faster aircraft, but your take-off distance will be increased. Its like pulling away with your car in second gear instead of first.

The best solution will be to use a constant speed unit. You will notice this when you move to the C182 which has more hp.

The props on the C210's are interesting. On the C210 the tips are round and on the C210 Turbo the blades are cut-off straight. This is because the C210 max RPM is 2700 while the C210T is 2850. You can hear the difference when they take-off. The C210T is actually more quiet.

[AndyT]

Fritz,

Ok, I actually knew that much which is why I picked the C172 instead of the C182.

Here is my point:
If I build an airplane (C172ish styling and stuff) should'nt I be able to power it and get it flying with far less horse power? As long as the prop reaches its rated speed, the plane should fly with no differences due to changes in HP.
Why do I need 100 HP in my C172 if 50 will do the job?

I understand that lower HP engines will suffer more strain and such, but that is not germaine to my current line of thought.

I actually have an LSA design I've been working on for some time now. I know that a smaller engine without the PSRU can deliver the speed to the prop and use far less fuel and actually work the engine less, so I'm trying to figure out why I need such a large engine to get my plane off the ground. (Even though its still on paper.)

Case in point:
This bird flies well with a pair of 15 horse engines and has plenty of speed. Yes, it is small, but it still fits within the bounds of my inquiry.
http://en.wikipedia.org/wiki/Cricket_(airplane)

[fweinrebe]

Oh I see what you are getting at. A C172 would probably continue to fly with a 50hp engine. Just throttle back a bit and you will know what a 50hp engine will achieve.

The biggest problem is taking off. Most thrust is needed to accelerate the aircraft on the ground to a descent flying speed. Thereafter the available thrust determines your max rate of climb or your max airspeed you will be able to obtain in straight and level flight.

The weight of the aircraft also plays a big role since on take-off you will fight against Inertia. But once a heavy aircraft is moving, well you know...

[aviaparts]

Andy, if you would totally empty your plane and just one person, the pilot, you would have enough. But as your load, total weight of the plane goes up, the heavier your engine has to be.
Same goes for your rate of climb, if you are alone in the plane, with 50HP you would need a gentle rate of climb, double the HP and your rate of climb will increase remarcably.
If this is wat your mean ?

[No Longer Active]

Andy, you trying to hint to us that you going to build an aeroplane?

I dont mind test-flying it! Id fly anything me!

Alex

[fweinrebe]

Look at Page 11-4 in the FAA "Airplane Flying Handbook". I have the 2004 pdf book downloaded from the FAA website. File name is faa-h-8083-3a-5of7.pdf. See this post by Mike Powell: http://www.mycockpit.org/forums/show...90&postcount=4

There is a very nice explanation about the RPM and torque relationship on a constant-speed propeller.

[Geremy Britton]

Alex, if you're joining us in the simbuilding community please look up how to spell correctly that was confusing.

Plane (Aeroplane) - Is what you're building, not a plain ROFL

Excuse me if i'm being pedantic, but just felt for a commonly used word here it would help if you spelt it right!


Captain Alex strikes again eh?
Drilling holes through LCD screens
Building a plain

I dread to think on i realy do LOL!

[No Longer Active]

737 Gez......was just a simple spelling mistake, typing to fast! Chill out pal!

You all love Captain Alex's Posts, you'll soon miss em' if you didnt have em'

Excuse me if i'm being pedantic

Who uses a word 'pendantic' in their posts, is this countdown on TV?

Just so you dont get confused and lost ,ive changed the posting for you!

Alex

[AndyT]

Alex,

No hints about it. If I had the money, I'd already have it built. The only thing stopping me really is the money and the room to build in. This is probably the biggest reason my sims are so minimalist. Combine that with the current economic situation and... Well, you dont have to be a 'Jenius' to realize just how likely it is I'll ever get it finished.

Still, I keep working on it slowly and surely.