trottle

Thrust or HP, what's more important? and why? THX

Stick Jammer

Two totally different things. The prop creates thrust. The engine creates HP. Different prop designs and sizes will produce different amounts of thrust on the same engine. Is it possible that you meant torque and HP?

Ed_Moorman

My Feedback: (1)

In a non-racing RC engine horsepower is a totally meaningless figure. Advertised horsepower is double meaningless, like automobile horsepower.

If you measure thrust, you have static thrust, which is only good if you are going to hover a lot. Once you move, you have dynamic thrust where other factors come into play. Thrust has a lot to do with the prop used and ones that are great for static thrust you can measure may not be the best for flying thrust.

From my experience, and I have run most of the .46 engines around, there is more difference in a new engine and a well used one than there is in different brands of new ones.

There are no easy answers. Pick a reliable engine that has good parts support.

Ed Smith

To try and answer the "Why" of the question. As mentioned they are different things produced by different components. It is quite possible to have the same Max HP engine running different props and the result would be different thrusts. However HP is more important as it is a measure of the engines ability to do it's job. The accuracy of the manufacturers claims is a different discussion.

HP cannot be measured, it is a calculated figure, a product of rpm and torque both of which have to be measured. The two stroke engines we use produce max HP at a fairly high rpm with a fairly small prop. In most cases the prop required to produce the HP would not be suitable for general flying.

An example. The racing engine I use produces nearly 3 HP with a 7.4 x 8.0 prop at app 23K. it will pull my 4# racing airplane along at 170+ mph. If I was to put that same engine/prop on my 6# Piper Cub it would hardly get it off the ground. Yet my 1.25 hp 4stroke with a 12 x 6 prop at 10,000 rpm flies it beautifully. It is all a question of matching the prop/engine/airframe to the desired purpose.

As said before, manufacturers HP claims are meaningless, they provide a very, very, very rough guide.

ED S

ifixairplanes

My Feedback: (9)

Ed, that is a very well thought out and understandable explanation! good job.

sean

Jim Thomerson

To get to the thrust question, Ed's racing engine set in front of a big bulky front end airplane will not produce enough thrust at relatively low speeds to get it off the ground. On the other hand, if his Piper Cub were to be towed through the air at 170+ with engine at full throttle, his 12 x 6 would be a major source of drag. It is the propeller which produces the thrust, not the engine. You want in an engine which produces enough torque to turn the best prop for your application at the RPM you need.

Jim

J_R

There is a relationship between thrust and horsepower. If you are willing to use a tach and empirical formulas that are not absolutely perfect, but do approximate our needs very well, take advantage of limeybob, who is a member on RCU. He has a site at http://pages.sbcglobal.net/limeybob/

Look at the online manual for the use of his calculator and skip down to the technical explaination for an understanding of the relationship.

As Ed said, published horsepower is somewhat less than reliable. If you use a tach, and a known prop, you can then calculate the approximate hp of the engine. Once that is known, you can "what if" props for thrust or speed as required by your application.

The calculators on limeybob's page help, but, read the manual first to understand how it is all being done.

mar345

Simply put, whats most important is to have the optimum diameter and pitch prop turning at enough rpm to deliver the thrust necessary for optimal flight pervormance. So decide on what size prop you need and what rpm you want it to turn and find an engine that will do the job. Horsepower ratings are usually measured at higher impractical rpms and are used by manufacturers as selling features. Thrust is what is needed for flight. If you are building a kit or an arf pick an engine in the middle of the recommended range unless you want more power.

khodges

My Feedback: (1)

You have to have enough horsepower to turn a given prop at sufficient rpm to create adequate thrust to move the aircraft or provide power for climb. Think of a worst-case: a 65 horse J-3 engine trying to turn an eighteen foot diameter 4-blade prop off a B-29. Once started, the engine would turn such a prop, but would not make sufficient power to accelerate the prop at the neccessary rate when throttled up or to maintain the rpm needed to maintain flight or climb. Certainly, torque comes into play, but the ability of an engine to deliver work is rated in horsepower. If you compare torque and horsepower curves on any engine, torque usually peaks in the lower third to half of the power band while horsepower is almost always at the upper end of the rpm range. If your engine is loafing along at cruise and you want to climb, horsepower is what speeds the engine up to increase the prop's thrust to allow that climb. There's more to it, but I can't think of how to explain it. I'm tired. Wired a house the fist half of the day, flew my planes the second half. Didn't electrocute myself and didn't crash-- a pretty good day.